quadriceps to hamstrings coactivation ratios during … · q:h coactivation ratios during exercise...

Q:H COACTIVATION RATIOS DURING EXERCISE

QUADRICEPS TO HAMSTRINGS COACTIVATION RATIOS DURING CLOSED

CHAIN, HIGH VELOCITY EXERCISE IN HEALTHY, RECREATIONALLY

ACTIVE ADULTS

________________________________________________________________________

Independent Research

Presented to

The Faculty of the College of Health Professions and Social Work

Florida Gulf Coast University

In Partial Fulfillment

Of the Requirement for the Degree of

Doctor of Physical Therapy

________________________________________________________________________

By

Maci Hatch

Keisha Sollie

2015


APPROVAL SHEET

This independent research is submitted in partial fulfillment of

the requirements for the degree of

Doctor of Physical Therapy

_____________________________

Maci Hatch

_____________________________

Keisha Sollie

Approved: May 2015

_____________________________

Dr. Arie van Duijn, EdD, PT, OCS

Committee Chair/Advisor

_____________________________

Dr. Eric Shamus, PhD, DPT, CSCS

Committee Member

The final copy of this independent research has been examined by the signatories, and we

find that both the content and the form meet acceptable presentation standards of scholarly

work in the above mentioned discipline.


Acknowledgements

We would like to express the appreciation to our committee chair Dr. Arie van

Duijn and committee member, Dr. Eric Shamus for their time, support, and

encouragement throughout the duration of our Independent Research Study. In addition,

we would like to thank Dr. Shawn Felton from the Human Performance Department for

his time and help with our study. Lastly, we would like to thank all of the participants

who voluntarily participated in our research. Without the help of each of these

individuals, we would not have been able to successfully complete this study.

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Table of Contents

Abstract ................................................................................................................................6

Introduction ........................................................................................................................10

Coactivation Ratios ........................................................................................................12

Mechanism of Injury ......................................................................................................19

Plyometric Training ........................................................................................................20

Purpose of the Study ..........................................................................................................22

Research Questions ............................................................................................................23

Methodology ......................................................................................................................23

Study Design ..................................................................................................................23

Participants .....................................................................................................................24

Inclusion Criteria ............................................................................................................24

Equipment and Preparation ............................................................................................24

Procedures ......................................................................................................................26

High Velocity, Closed Chain Exercise ...........................................................................28

Barrier jump front to back ..........................................................................................29

Barrier jump side to side.............................................................................................29

Lateral bounding .........................................................................................................30

Scissor jump ...............................................................................................................30

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Squat jump ..................................................................................................................30

Data Sampling and Reduction/Data Analysis ................................................................31

Statistical Analysis .........................................................................................................32

Results ................................................................................................................................33

Q:H Coactivation Ratios ................................................................................................33

Peak EMG Flexion Angles for Each Muscle During All Jumps ....................................36

Peak EMG Flexion Angles for Each Muscle Within Each Exercise .............................45

Discussion ..........................................................................................................................54

Q:H Coactivation Ratios ................................................................................................54

Peak EMG Flexion Angles for Each Muscle During All Jumps ....................................56

Peak EMG Flexion Angles for Each Muscle Within Each Exercise .............................57

Recommendations and Limitations ................................................................................60

Conclusions ........................................................................................................................62

References ..........................................................................................................................63

3


List of Figures and Tables

Figure 1. Qualisys© soft marker placement for biomechanical assessment .....................25

Figure 2. Noraxon© SEMG dual electrodes .....................................................................26

Figure 3. Noraxon© SEMG Electrode Placement Figure .................................................27

Figure 4. Barrier jump front to back ..................................................................................29

Figure 5. Barrier jump side to side.....................................................................................29

Figure 6. Lateral bounding .................................................................................................30

Figure 7. Scissor jump .......................................................................................................30

Figure 8. Squat jump ..........................................................................................................31

Table 1. Calculated Quadriceps: Hamstrings Coactivation Ratios for Each Plyometric

Exercise (Max+SD) ...........................................................................................................34

Table 2. Pairwise Comparisons Between Exercises ..........................................................35

Table 3. Multivariate Tests with Jump as the Within Subject Variable and Gender as the

Between Subject Variable ..................................................................................................36

Table 4. EMG Channel 1 (VM) Peak Flexion Angle Differences Among Exercises .......37

Table 5. Effect of Jump on Peak Flexion Angle VM Muscle Activation ..........................38

Table 6. EMG Channel 2 (VL) Peak Flexion Angle Differences Among Exercises ........38

Table 7. Effect of Jump on Peak Flexion Angle VL Muscle Activation ...........................39

Table 8. Pairwise Comparisons of VL Muscle Activation for Each Exercise ...................40

Table 9. EMG Channel 3 (MH) Peak Flexion Angle Differences Among Exercises .......41

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Table 10. Effect of Jump on Peak Flexion Angle MH Muscle Activation ........................41

Table 11. Pairwise Comparisons of MH Muscle Activation for Each Exercise ................42

Table 12. EMG Channel 4 (BF) Peak Flexion Angle Differences Among Exercises .......43

Table 13. Effect of Jump on Peak Flexion Angle BF Muscle Activation .........................43

Table 14. Pairwise Comparisons of BF Muscle Activation for Each Exercise .................44

Table 15.Peak EMG Flexion Angles of all Muscles During Barrier Jump Front to Back

............................................................................................................................................46

Table 16. Effect of Barrier Jump Front to Back on Peak EMG Flexion Angle for Each

Muscle ................................................................................................................................46

Table 17. Pairwise Comparisons of Peak EMG Flexion Angle of All Muscles During

Barrier Jump Front to Back ................................................................................................47

Table 18. Peak EMG Flexion Angles of all Muscles During Barrier Jump Side to Side .48

Table 19. Effect of Barrier Jump Side to Side on Peak EMG Flexion Angle for Each

Muscle ................................................................................................................................48


Barrier Jump Side to Side ..................................................................................................49

Table 21. Peak EMG Flexion Angles of all Muscles During Lateral Bounding ...............50

Table 22. Effect of Lateral Bounding on Peak EMG Flexion Angle for Each Muscle .....50


Lateral Bounding ...............................................................................................................51

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Table 24. Peak EMG Flexion Angles of all Muscles During Scissor Jump ......................52

Table 25. Effect of Scissor Jump on Peak EMG Flexion Angle for Each Muscle ............52


Scissor Jump ......................................................................................................................53

Table 27. Peak EMG Flexion Angles of all Muscles During Squat Jump ........................54

Table 28. Effect of Squat Jump on Peak EMG Flexion Angle for Each Muscle ..............54

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Abstract

Purpose

The anterior cruciate ligament (ACL) has been reported as one of the most

commonly injured ligaments of the knee. A high incidence of ACL injuries are non-

contact injuries that occur during high velocity, closed chain movements and quick

changes in motion, such as accelerating, decelerating, cutting, and pivoting (Noyes &

Barber-Westin, 2012). There is paucity in the current literature regarding quadriceps to

hamstrings (Q:H) coactivation ratios during closed chain, high velocity exercises. These

exercises may be useful to prevent future knee injury by increasing the dynamic stability

of the knee joint and its surrounding structures.

The primary purpose of this study was to determine the functional Q:H

coactivation ratios during high velocity, closed chain knee movements in healthy,

recreationally active adults. A secondary purpose of this research was to determine the

knee flexion angles at which the maximum EMG activity occurred for each muscle

examined. Previous research has focused on the Q:H coactivation ratios during open

chain isokinetic knee motion, as well as low velocity, closed chain knee motion. This

study investigated the following research questions: What are the Q:H coactivation ratios

during closed chain, high velocity exercises including squat jump, barrier jump side to

side, barrier jump front to back, scissor jump, and lateral bounding in recreationally

active adults? At what angle of knee flexion does the maximum EMG activity occur of

the vastus medialis (VM), vastus lateralis (VL), medial hamstrings (MH), and biceps

femoris (BF)?

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Number of Subjects

Convenience sampling was utilized to recruit 20 healthy, recreationally active

college students (12 men, 8 women) between the ages of 18-30 years old within the

Department of Physical Therapy and Human Performance at Florida Gulf Coast

University.

Materials/Methods

This was a descriptive study of cross-sectional design with repeated measures in

which the participants performed 8 repetitions of 5 high velocity, closed chain exercises

on the selected lower extremity. Data collection was performed utilizing Noraxon©

surface electromyography (EMG) measurements of the vastus medialis, vastus lateralis,

medial hamstrings, and biceps femoris, in addition to Qualisys© Motion Capture System

to measure the joint angles and planes of motion during the exercises. Normalized EMG

amplitude levels were used to derive Q:H coactivation ratios for each of the exercises.

Ratios were calculated by dividing the sum of the peak quadriceps EMG activity (VM,

VL) by the sum of the peak hamstrings EMG activity (MH, BF):

(VM + VL)/(MH + BF) = Q:H coactivation ratio.

A one way repeated measures analysis of variance (ANOVA) to identify

differences in Q:H coactivation ratios among exercises. A multivariate analysis was used

to identify the effect of the jump between subjects. In addition, a one way repeated

measures analysis of variance (ANOVA) was used to identify differences in peak muscle

activity for each of the four muscles during all five exercises and to identify differences

in peak muscle activity for each of the five exercises. A multivariate analysis was used to

identify the effect of jump on peak EMG flexion angle for each EMG channel (each

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muscle) and to identify the effect of jump on peak muscle activity within each exercise.

SPSS was used to perform all statistical analysis.

Results

Statistically significant differences (p<0.05) were found between the Q:H ratios of

lateral bounding and the scissor jump (mean=-1.069), 95% CI [-2.135, -0.004]) and

between lateral bounding and the squat jump (mean=-0.694), 95%CI [-1.288, -0.100). In

addition, there was a statistically significant difference (F4,14=37.963, p<0.001) in vastus

lateralis activation during lateral bounding when compared to the other four exercises.

There was a statistically significant difference (F4,14=3.22, p<0.05) in peak flexion medial

hamstrings activation during bounding when compared to the barrier jump front to back,

barrier jump side to side, and the scissor jump. There was also a statistically significant

difference (F4,14=5.728, p<0.05) in peak flexion biceps femoris activation for lateral

bounding when compared to barrier jump side to side, scissor jump, and squat jump.

Furthermore, there were statistically significant differences found during the barrier jump

front to back (F3,15=10.561, p<0.001), barrier jump side to side (F3,15=14.810, p<0.001),

lateral bounding (F3,15=3.533, p<0.05, and scissor jump (F3,15=13.216, p<0.001).

Conclusion

We evaluated the Q:H coactivation ratios among five high velocity, closed chain

plyometric exercises, as well the knee flexion angles that coincide with peak muscle

activity. Results of our study identified that the barrier jump front to back, barrier jump

side to side, and scissor jump facilitated earlier activation of the hamstrings in relation to

the quadriceps suggesting that these exercises provide the most stability to the posterior

aspect of the knee, thus protecting the ACL. In contrast, lateral bounding facilitates

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earlier quadriceps activation and therefore should be used with caution in the early stages

of ACL rehabilitation due to the anterior shear force placed on the ACL from the

quadriceps. In conclusion, having knowledge of both the overall Q:H ratios as well as

the timing of peak muscle contraction allows for better exercise prescription and

progression and could also be used in injury prevention programs to decrease the

likelihood of ACL injury or re-injury.

Clinical Relevance

This study identified exercises that facilitate hamstring activation and

stabilization, as well as exercises that should be used with caution during ACL

rehabilitation. Clinicians can use the results of this study to guide their exercise

prescription with the ACL rehabilitation and prevention population.

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Introduction

Anterior cruciate ligament (ACL) tears are the most common ligamentous injuries

that occur in the knee joint with over one million injuries occurring worldwide every year

(Noyes & Barber-Westin, 2012). Of these injuries, it is estimated that approximately

250,000 physically active young adults sustain ACL tears annually (Salmon, Russel,

Musgrove, Pinezewski, & Refshauge, 2005). The majority of those who sustain ACL

injuries are athletes under 25 years of age who are involved in high school, collegiate, or

league sports (Noyes & Barber-Westin, 2012). At least two-thirds of ACL injuries occur

during non-contact situations while an athlete is accelerating, decelerating, cutting, or

pivoting, all motions which occur frequently in athletes who play sports or are

recreationally active (Noyes & Barber-Westin; Begalle, DiStefano, Blackburn, & Padua

2012). In addition, it is common for an ACL injury to be paired with a meniscus tear,

which is a risk factor for tibiofemoral osteoarthritis in later years (Meunier, Odensten, &

Good, 2007). ACL injuries result in both physical impairments as well as high economic

costs for athletes who undergo ACL reconstructive surgery (Gottlob & Baker, 2000).

Salmon et al. (2005) found that 12% of patients who underwent ACL reconstruction

suffered a recurrent ACL injury within 5 years of surgery. Furthermore, young, active

adults who return to activities that require lateral side stepping, cutting, and jumping have

up to a ten-fold increased likelihood of repeated ACL injury (Salmon et al., 2005).

When examining the overall stability of the knee, it is essential to understand the

antagonistic relationship between the hamstrings and the quadriceps. To provide

dynamic stability to the knee joint, the actions of the quadriceps and hamstrings must be

coordinated and coactivated to assist in protecting the joint. Thus, the coactivation of

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both muscle groups is necessary to prevent motions that may predispose the ACL to

injury (Withrow, Huston, Wojtys, & Ashton-Miller, 2008). During knee extension the

quadriceps contract concentrically while the hamstrings contract eccentrically.

Conversely, the hamstrings contract concentrically and the quadriceps contract

eccentrically during knee flexion (Coombs & Garbutt, 2002). The hamstrings function

synergistically with the ACL to counteract the force of the quadriceps contraction, and

ultimately the anterior translation of the tibia on the femur (Draganich, Jaeger, & Kralj,

1989, Aagaard et al., 2000). Moreover, the hamstring muscles are activated by

mechanoreceptors in the ACL when the ligament is placed under stress (Myer, Ford, &

Hewett, 2005). The ACL provides 87% of restraining force at 30 degrees of knee flexion

and 85% at 90 degrees of knee flexion making it the main restraint against anterior tibial

translation during knee movements (Noyes & Barber-Westin, 2012).

In order to understand the agonist-antagonist relationship between the quadriceps

and hamstrings during knee movements, specific muscle activation ratios are utilized.

When describing knee extension, the ratio used is concentric quadriceps to eccentric

hamstrings contraction (Qcon/Hecc). Conversely, during knee flexion, the ratio is

described as eccentric quadriceps contraction to concentric hamstrings contraction

(Qecc/Hcon) (Coombs & Garbutt, 2002). A significant over activation of either muscle

group during knee flexion or knee extension can result in excessive anterior or posterior

translation of the tibia on the femur causing added stress on the knee ligaments (Beynnon

& Fleming, 1998). An imbalanced Q:H ratio, particularly a quadriceps dominant ratio,

causes an anterior translation of the tibia on the femur(Cheung, Smith, & Wong, 2012).

Because the primary function of the ACL is to limit this anterior translation of the tibia,

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an individual’s ACL may be predisposed to injury as the result of an unbalanced Q:H

ratio, with an excessive anterior tibial shear force from the over-activity of the quadriceps

(Markolf et al., 1995; Renstrom et al., 1986). One study found that a vigorous

quadriceps contraction has actually been shown to cause ACL rupture in cadavers

(DeMorat, Weinhold, Blackburn, Chudik, & Garret, 2004). The hamstrings provide

posterior stabilization to the knee by counteracting the anterior shear force produced by

the quadriceps, thus reducing the amount of force placed on the ACL (Noyes & Barber-

Westin, 2012; Renstrom et al., 1986). Reduced activation of the hamstrings relative to

the quadriceps causes an increase in restraint force to be placed on the ACL (Beynnon &

Fleming, 1998; Croisier et al., 2008). In addition, weakness of the hamstrings may

increase the risk of an ACL rupture by contributing to a greater ground reaction force

being transmitted to the knee joint upon landing (Hewett, Myer, & Ford, 2006).

Therefore, decreased activation and strength of the hamstrings relative to the quadriceps

may increase one’s susceptibility to ACL injury (Boden, Griffin, & Garret, 2000, Myer et

al., 2009, and Chappel, Creighton, Giuliani, Yu, & Garret, 2007). Consequently, a more

balanced ratio between the hamstrings and the quadriceps assists in stabilizing the knee,

as well as prevents the likelihood of ligamentous instability and injury.

Coactivation Ratios

The quadriceps to hamstrings (Q:H) coactivation ratio has been evaluated to

determine the muscle balance surrounding the knee joint during various activities. This

ratio is not only position dependent, but can vary greatly based on the velocity of the

motion (Rosene, Fogarty, & Mahaffey, 2001). The conventional concentric H:Q strength

ratio (Hcon/Qcon) is frequently described as the hamstrings to quadriceps peak moment

13


ratio. It is calculated by dividing the maximal concentric hamstrings moment by the

maximal concentric quadriceps moment during a specific joint angular velocity. There is

a lack of consensus of a normative value for this H:Q ratio; however, 0.6 has frequently

been cited in previous research (Baltzopoulos & Brodie, 1989; Kannus, 1994). The

limitation to using this conventional ratio is that concentric muscle contraction cannot

occur simultaneously in antagonistic muscle groups (Coombs & Garbutt, 2002). It has

recently been suggested that the agonist-antagonist relationship for knee extension and

flexion may be better described by a more functional Q:H ratio of concentric quadriceps

to eccentric hamstrings muscle activation (Qcon/Hecc) (Aagaard, Simonsen, Magnusson,

Larsson, & Dyhre-Poulson, 1998). Functional knee joint movement only allows

eccentric hamstring muscle contraction to be paired with concentric quadriceps muscle

contraction during knee extension or concentric hamstring muscle contraction paired with

eccentric quadriceps muscle contraction during knee flexion. This functional Qcon/Hecc

ratio may be used to indicate the extent to which the hamstring muscles are capable of

activating to counteract the anterior tibial shear force produced by maximal concentric

quadriceps contraction. Aagaard, Simonsen, Magnusson, Larsson, and Dyhre-Poulson

(1998) found a functional H:Q ratio of 1.00 for fast isokinetic open-chain knee extension,

indicating a significant capacity of the hamstrings to provide dynamic joint stabilization

during active, open-chain knee extension. On the other hand, lower values of 0.30 have

been reported for functional H:Q ratios representative of fast isokinetic open-chain knee

flexion. This suggests that that hamstrings have a reduced capacity for dynamic knee

joint stabilization during forceful open-chain knee flexion movements paired with

simultaneous eccentric quadriceps contraction (Aagaard, Simonsen, Magnusson, Larsson

14


& Dyhre-Poulson, 1998). Although these values are useful, it is necessary to evaluate the

Q:H ratio at various velocities as the velocity of the movement has a large impact on the

overall ratio at the knee.

An issue that arises when trying to quantify the Q:H ratio is that the ratio is

velocity and position dependent. Therefore, it is imperative that Q:H ratios are evaluated

at multiple velocities to determine what those differences are. Since there is a lack of

research on high velocity closed chain Q:H ratios, previous literature regarding open

chain isokinetic Q:H ratios at varying velocities will be examined. Evaluation of

isokinetic eccentric antagonistic hamstring strength relative to concentric agonist

quadriceps strength may provide a relationship of value in describing the maximal

potential of the hamstring muscle group (Coombs & Garbutt, 2002). Hence, isokinetic

measurements are commonly used to measure the hamstrings to quadriceps strength

ratios, and can provide a quantitative measurement of the agonist and antagonist

contraction at the knee joint (Rosene et al., 2001). These values can then be used to

determine the moment-velocity patterns for both muscle groups, and allows the muscle

balance and functional ability at the knee to be derived. Rosene, Fogarty, & Mahaffey

(2001) assessed isokinetic Q:H ratios at three different speeds for both male and female

intercollegiate athletes and found that as the velocity increased, the Q:H ratio also

increased. The researchers found the Q:H ratios of 49.8% at 60°·s-1

, 53.6% at 120°·s-1

,

and 58.6% at 180°·s-1

for men and 50.3% at 60°·s-1

, 56.1% at 120°·s-1

, and 58.9% at

180°·s-1

for women. An additional research study also examined the ratio with varying

velocities and joint angles, and found that as the leg is extended, the ratio changes from

0.48 to 1.29, indicating a greater ability of the hamstrings to counteract the overactive

15


quadriceps near terminal extension (Coombs & Garbutt, 2002). The aforementioned

research is able to provide evidence that Q:H ratios are velocity dependent when

performing open-chain knee flexion and extension. In addition, instability of the knee is

well compensated for at slow speeds, but compensation decreases as the speed increases

(Rosene et al., 2001). However, there is a gap in the current literature examining high

velocity closed chain Q:H ratios. Thus, Q:H ratios must be measured during high

velocity closed chain movements to examine the impact of velocity on hamstring and

quadriceps activity. Once determined, these ratios can be utilized to determine the

functional stability of the knee at higher velocities, which more closely mimic athletic or

recreational activities.

In order to understand the coactivation ratios of the hamstrings during maximal

knee extension, electromyography (EMG) readings are taken during active knee

movements. Although closed chain Q:H coactivation ratios are the focus of this research,

current literature has reported surface EMG data during open-chain isokinetic knee

movements as well as low velocity closed chain exercises. Aagaard et al (2000) had their

subjects perform two types of knee extension moments, maximal concentric quadriceps

contractions and maximal eccentric hamstrings contractions. Based on the moment and

EMG recorded, the relationship between hamstring EMG and the consequent flexor

moment was established for all joint angles through the ROM. The researchers then used

the hamstring antagonist EMG values and converted them into antagonist moment, based

on the EMG-moment relationships determined in both trials (Aagaard et al., 2000). From

this data the researchers found substantial hamstring coactivation during quadriceps

(agonist) contraction. When they examined the range of 30-10° from full knee extension,

16


the antagonist hamstring moment corresponded to 30-75% of the measured knee extensor

moment (Aagaard et al). The researchers presented the hamstring coactivation data,

averaged in 10° intervals through the active ROM. They determined that hamstring

coactivation is greater toward full knee extension (10-30°) than in midrange of joint

movement (40-60°), and conversely the quadriceps are less active toward full knee

extension (Aagaard et al., 2000). This research study found higher coactivation ratios

than previous studies based on the way that the EMG data was normalized. In previous

research, antagonist hamstring EMG was normalized relative to the EMG of the

concentric hamstring agonist contraction (Aagaard et al., 2000). As a result, eccentric

antagonist hamstring moments were estimated from concentric EMG-moment

relationships, resulting in a significant underestimation of the antagonist hamstring

moments. However, this study normalized the antagonist EMG relative to the EMG

recorded during agonist contraction of the exact same type, eccentric hamstrings and

concentric quadriceps, which resulted in much higher coactivation ratios. These methods

are advantageous due to the greater ability to predict the antagonist moments because it is

based on real measurements rather than using mathematical assumptions to predict the

coactivation data. This study found that there is substantial antagonist coactivation of the

hamstring muscles during slow isokinetic knee extension, causing a flexor moment at the

knee (Aagaard et al., 2000). These moments have the ability to counteract the anterior

tibial translation induced by the quadriceps near full knee extension. This further supports

the notion that coactivation of the hamstrings during knee extension can act

synergistically to assist the ACL in preventing the anterior translation of the tibia (Baratta

et al., 1988).

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Although the isokinetic assessments provide clear data for the Q:H ratios, these

isolated contractions are not functional motions for athletes participating in a sport or

recreational activities. Therefore, the state in which athletes are commonly found playing

and practicing in must also be evaluated to determine the Q:H ratios for active play.

Closed kinetic chain (CKC) exercise is commonly used during ACL rehabilitation and

lower extremity injury prevention due to the added joint compression and stability that

they provide to the knee. Closed chain exercises vary greatly from open chain exercise as

they reduce the amount of strain on the ligaments of the knee, namely the ACL, and they

encourage the coactivation of the quadriceps and hamstrings to provide stability to the

knee during movement (Beynnon et al., 1995). In addition, closed chain exercises

closely resemble functional activities of daily living as well as recreational activities.

Open chain exercises also encourage the over-activity of the quadriceps, an issue that is

to be avoided to decrease the risk of ligamentous injury to the knee (Begalle et al., 2012).

An issue that arises when examining closed chain exercises is that the Q:H ratio can vary

greatly depending on the exercise performed. As stated prior, a Q:H ratio of 1.0 indicates

a perfect coactivation of the hamstrings and quadriceps, which is optimal for dynamic

joint stabilization to protect the knee during athletic movements. The differences in ratios

are important for clinicians to be aware of, as the risk of injuring the knee increases

greatly with specific exercises.

Begalle et al. (2012) set out to determine the coactivation ratios of commonly

prescribed lower extremity rehabilitation exercises as well as those used in injury

prevention programs. Rather than looking specifically at strength ratios, they presented

the data in coactivation ratios, to guide rehabilitation specialists with exercise

18


prescription and progression. They determined the exercises with the most balanced

ratios to be the single-limb deadlift, lateral hop-to-balance, transverse hop-to-balance,

and lateral band-walk exercises. These exercises all had the lowest levels of quadriceps

activation (45-68% MVIC), with midrange hamstrings activation (10-18% MVIC).

Although these exercises have the most balanced ratios, they are all still quadriceps

dominant and do not produce enough hamstring activation to actually strengthen the

muscles (Begalle et al., 2012). Thus, when examining closed chain exercise for lower

extremity rehabilitation and injury prevention it is necessary to be mindful of the over-

activity of the quadriceps and to isolate the strengthening of the hamstrings in order to

achieve the desired functional muscle balance at the knee.

The Q:H ratio has also been examined as a possible screening tool for

predisposition to injury. When the knee is injured, the Q:H ratio is often used as a

rehabilitative goal due to the importance of the flexor-extensor strength balance in overall

knee stabilization. Reduced function of the antagonist hamstrings due to activities that

emphasize loads on the knee extensors may result in muscular imbalances between the

hamstrings and quadriceps, thereby possibly predisposing recreationally athletes to injury

(Begalle et al., 2012; Rosene et al., 2001). This predisposition may be due to the

surrounding ligamentous structures supporting most of the imposed load and decreased

antagonist hamstrings coactivation during extension loads (Rosene et al., 2001). One

research study determined that six weeks of hamstring strength training was sufficient to

increase the functional Q:H ratio to greater than 1.0, which is recommended for

prevention of non-contact ACL injuries (Holcomb, Rubley, Lee, & Guadagnoli, 2007).

As physical therapists, it is necessary to use the Q:H ratio data from all sources of

19


measurement including isokinetic, low velocity closed chain, and high velocity closed

chain, to determine the best type of injury prevention as well as rehabilitation program

for athletes. Although isokinetic and low velocity closed chain exercises have been

heavily researched, high velocity closed chain activity has yet to be examined.

Mechanism of Injury

The quadriceps, as anterior cruciate ligament (ACL) antagonists, may contribute

to ACL injury. Numerous investigators have reported that quadriceps contraction

increases ACL strain between 10° and 30° of knee flexion (Renstrom et al., 1986; Arms,

Pope, & Johnson, 1984). Because most noncontact ACL injuries occur with the knee

close to full extension, it is likely that the quadriceps play an important role in ACL

disruption (Boden, Griffin, & Garret, 2000).

Previous research has found that most non-contact ACL tears occur during quick,

high velocity movements when the knee angle is moving towards full extension (Cheung,

Smith, & Wong, 2012; Noyes & Barber-Westin, 2012). Several studies show that ACL

loading increases as the knee flexion angle decreases. Arms et al. (1984) studied the

biomechanics of ACL rehabilitation and found that quadriceps muscle contraction

significantly strains the ACL from 0-45 degrees of knee flexion, but did not strain the

ACL when knee flexion is greater than 60 degrees. Similarly, Beynnon et al. (1995)

measured ACL strain during rehabilitation exercises and found that isometric quadriceps

muscle contraction resulted in a significant increase in ACL strain at 15 and 30 degrees

of knee flexion, while it resulted in no change in ACL strain relative to the relaxed

muscle condition at 60 and 90 degrees of knee flexion. Furthermore, Li et al. (1999)

investigated the quadriceps and hamstrings muscle loading in relation to ACL loading

20


and showed that ACL loading increased as knee flexion angle decreased, while the

quadriceps were loaded regardless of the hamstring muscle loading conditions.

A study done by Colby et al. (2000) used surface EMG to measure hamstring and

quadriceps muscle activation in male and female collegiate athletes during eccentric

motions including sidestep cutting, crosscutting, single leg stopping, landing, and

pivoting at various knee flexion angles. The percentage of muscle activation during these

motions was determined based off of a maximum isometric contraction of both the

quadriceps and the hamstrings. The results found that peak quadriceps muscle activation

occurred between 39-53 degrees of knee flexion and averaged 161% of maximum

isometric quadriceps contraction during quick, stopping motions. In contrast, the

minimal hamstring muscle activation occurred between 21-34 degrees of knee flexion

and averaged between 14-40% of maximum isometric hamstring contraction during

stopping and cutting motions. The high level of quadriceps activity paired with the low

level of hamstring activity along with the low angles of knee flexion during these motions

can result in significant anterior translation of the tibia on the femur (Noyes & Barber-

Westin, 2012).

Plyometric Training

Plyometric training consists of high velocity eccentric to concentric muscle

loading, reflexive reactions, and functional movements. Movements are characterized by

rapid eccentric contraction in which the muscle lengthens immediately followed by a

concentric contraction of the same muscle in which it shortens (de Villarreal, Requena, &

Newton, 2010). Plyometric training has frequently been utilized in sport-specific training

as well as in ACL rehabilitation and prevention programs due to its ability to train the

21


muscles, connective tissue, and nervous system to react quickly, while maintaining

proper technique and body mechanics (Hewett, 2007). Ultimately, this prepares the

athlete for high velocity situations that require rapid starting and stopping movements or

quick changes in direction (Kisner & Colby, 2012). This dynamic neuromuscular

training has been demonstrated to reduce gender related differences in force absorption,

active joint stabilization, muscle imbalances, and functional biomechanics (Rahimi,

Arshadi, Behpur, Boroujerdi, & Rahimi, 2006). Researchers have concluded that

plyometric training has been shown to be one of the most effective tools to reduce non-

contact ACL injuries when compared to other prevention programs that solely focus on

resistance or balance training (Alentorn-Geli et al., 2009).

A study by Hewett, Stroupe, Nance, & Noyes (1996) focused on the effectiveness

of plyometrics to increase hamstring strength as well as decrease landing forces by

teaching neuromuscular control of the lower limb in both male and female athletes.

Researchers found that after a six week plyometric training program, both male and

female participants experienced decreased landing forces, which translates to less force

being placed on the knee joint and associated ligaments. Additionally, the jump training

program brought the female athletes from a Q:H ratio that was significantly lower than

the male subjects up to an equivalent value (Hewett, Stroupe, Nance, & Noyes, 1996).

Similarly, Myer, Ford, Palumbo, and Hewett (2005) conducted a study to examine the

effects of a comprehensive neuromuscular training program on measures of performance

and lower limb movement biomechanics in female athletes. The subjects participated in

a six week training program that included plyometrics, resistance training, and speed

training. The trained group demonstrated increased lower extremity strength, single leg

22


hop distance, and speed as well as decreased knee valgus and varus torques during

landing when compared to the untrained group. In addition, Heidt et al (2000)

implemented a seven week preseason training program that consisted of strength training,

plyometrics, and sport specific cardiovascular exercise for 300 female soccer players.

Results of this study found that the trained group of females experienced a lower

percentage (2.4%) of ACL injuries compared to the untrained group (3.1%), suggesting

that preseason conditioning that includes plyometric training can have an influence in

preventing ACL injury.

Purpose

The primary purpose of this study was to determine the functional Q:H

coactivation ratios during high velocity, closed chain knee movements in healthy,

recreationally active adults. A secondary purpose of this research was to determine the

knee flexion angles at which the maximum EMG activity occurred for each muscle

examined. Previous research has focused on the Q:H coactivation ratios during open-

chain isokinetic knee motion, as well as low velocity, closed chain knee motion.

However, the high incidence of ACL injuries in the recreationally active population occur

during high velocity, closed chain movements and quick changes in motion, such as

accelerating, decelerating, cutting, and pivoting (Noyes & Barber-Westin, 2012). Due to

paucity in the current literature, high velocity, closed chain motions require further

examination.

Prior literature has found that plyometrics are effective in reducing torque on the

knee joint during landing, as well as strengthening and increasing activation of the

quadriceps and hamstrings (Potteiger et al., 2005). Therefore, these exercises may be

23


useful to prevent future knee injury by increasing the dynamic stability of the knee joint

and its surrounding structures. Previous researchers have noted that a more balanced

functional Qcon/Hecc ratio (1.0) is ideal because it puts less anterior tibial shear force on

the knee joint and thus, places less strain on the ACL (Coombs & Garbutt, 2002). One

research study determined that six weeks of hamstring strength training was sufficient to

increase the functional Q:H ratio to greater than 1.0, which is recommended for

prevention of non-contact ACL injuries. However, a ratio of 1.0 which constitutes no net

movement is not possible during active knee movements.

Research Questions

Due to paucity in the current literature regarding Q:H coactivation ratios during

closed chain, high velocity exercise, this study investigated the following research

questions: What are the Q:H coactivation ratios during closed chain, high velocity

exercises including squat jump, barrier jump side to side, barrier jump front to back,

scissor jump, and lateral bounding in recreationally active adults? At what angle of knee

flexion does the maximum EMG activity occur of the vastus medialis, vastus lateralis,

medial hamstrings, and biceps femoris?

Methods

Study Design

This research was a descriptive study of cross-sectional design with repeated

measures, in which all participants performed the selected exercises. Data collection was

performed utilizing Noraxon© surface electromyography (EMG) measurements for the

quadriceps and hamstrings in addition to Qualisys© Motion Capture System to measure

the joint angles and planes of motion during the exercises.

24


Participants

The participants initially included 20 healthy men and women (12 men, 8 women)

between 18-30 years old who were recreationally active, which was defined as 60

minutes of physical activity at least three days per week. However, due to technical issues

during data collection, only 18 subjects were included in the final data analysis. Subjects

were gathered using convenience sampling from the student body of Florida Gulf Coast

University's Department of Physical Therapy and Human Performance. All subjects

demonstrated the ability to perform the required exercises without pain and with proper

form. The data was collected during a single testing session. All participants provided

written informed consent, and the study was approved by the Institutional Review Board

of Florida Gulf Coast University.

Inclusion Criteria

The inclusion criteria included: participants must fall within the age range 18-30

years old, be currently enrolled in the Department of Physical Therapy and Human

Performance at Florida Gulf Coast University, have no history of surgery to the tested

knee within the last year, and no history of knee injury to the tested knee within the last

six months at the time of testing. All subjects were required to perform the selected

exercises without pain and with proper form.

Equipment and Preparation

The laboratory utilized for data collection was equipped with a 10-camera Oqus

300 1.3MP infrared motion capture system (Qualisys© Gothenburg, Sweden) and a

portable Noraxon© surface EMG system (Scottsdale, Arizona). Reflective markers as

well as rigid marker sets (RMS) were placed on bilateral landmarks throughout the body,

25


with the exception of the RMS placed on the center of the lower back, to document the

spatial position of the ankle, knee, hip, and pelvis throughout the motion (see Figure 1).

Each RMS consisted of a rigid black shell with four reflective markers attached. Elastic

wraps (SuperWrap; Fabrifoam Products, Exton, PA) were utilized to provide an

attachment point for each RMS. The individual reflective markers were placed on the

appropriate reference points through anatomic palpation. The system was calibrated to

each individual with the subject in a static position as well as while performing a

dynamic movement prior to each subject’s performance of the exercises. The EMG

signals were recorded on the participant’s tested limb throughout all exercises and MVIC

collection. The selected muscles included vastus lateralis, vastus medialis, medial

hamstrings, and biceps femoris.

Figure 1. Qualisys© soft marker placement for biomechanical assessment.

26


Procedures

Before any measurements were recorded, participants performed a five minute

jogging warm-up at submaximal speed. The participants wore standard workout attire

consisting of athletic shorts, shirts, and tennis shoes. Prior to testing, the participants

were verbally and physically taught the chosen test exercises and given time to practice

the movements until they felt comfortable performing them correctly.

After the participants completed the five minute jogging warm-up, the surface

EMG dual electrodes (see Figure 2) were placed on the skin over the quadriceps and

hamstrings.

Figure 2. Noraxon© SEMG dual electrodes.

Source: Noraxon USA | Noraxon Dual EMG Electrode. Retrieved from

http://www.noraxon.com/products/accessories/noraxon-dual-emg-electrode/

The electrodes were taped onto the skin and reinforced with fitted spandex shorts to

ensure optimal attachment during the exercises. The skin was prepped with alcohol prior

to placement. The EMG electrodes were positioned in parallel along the appropriate

muscle bellies to record the muscle activity during the selected exercises. The specific

electrode placement was based on the Noraxon suggestions as follows: vastus lateralis:

lateral, anterior surface of the distal one-third of the thigh; vastus medialis: medial,

anterior surface of the distal one-third of the thigh; medial hamstrings: medial, posterior

surface of the thigh approximately midway between the ischial tuberosity and the

27


popliteal fossa; biceps femoris: lateral, posterior surface of thigh approximately midway

between the popliteal fossa and the ischial tuberosity.

Figure 3. Noraxon© SEMG Electrode Placement Figure. The muscles utilized in the

study include vastus medialis, vastus lateralis, semitendinosus (medial hamstrings), and

biceps femoris. Source: Noraxon© Muscle Map: Guide for Electrode Placement. Noraxon© USA Inc. Retrieved from

http://www.health.uottawa.ca/biomech/courses/apa4311/applications.html

Once the electrodes were in place, three separate maximum voluntary isometric

contractions (MVIC’s) were performed against manual resistance provided by a single

investigator for muscles of the quadriceps and the hamstrings, including vastus lateralis,

vastus medialis, medial hamstrings, and biceps femoris to normalize muscle activation

data recorded during the exercises. Vastus medialis MVIC testing was performed with

the participant seated at the edge of a mat table with the hips and knees flexed to 90

degrees while the investigator manually resisted knee extension, with external rotation of

28


the tibia. Vastus lateralis MVIC testing was performed in the same position as the vastus

medialis testing, with the exception that the tibia was placed in internal rotation during

resisted knee extension. Medial hamstrings MVIC testing was performed with the

participant lying in prone with the hip positioned in neutral, tibia in external rotation,

knee flexed to 90 degrees, and the investigator manually resisted knee flexion. Biceps

femoris MVIC testing was performed in the same position as the medial hamstrings

testing, with the exception that the tibia was placed in internal rotation during resisted

knee flexion.

The procedures of the study followed the protocol described by Begalle et al.

(2012) in which EMG readings for closed chain kinetic exercises were recorded. They

followed the manuscript of DiStefano et al. (2009) that looked at the gluteal muscle

activation for common therapeutic exercises. Begalle et al. (2012) focused on quadriceps

and hamstring coactivation in low velocity, closed chain exercises only.

The focus of this study was on closed chain exercises at higher velocities to

mimic the motions in which ACL tears occur. The EMG data was collected while

participants completed eight repetitions of each of the exercises in a randomized order

with a two minute rest period between exercises.

High Velocity, Closed Chain Exercise

According to Nyland et al. (1999), knee rehabilitation and injury prevention

programs should be focused on lower extremity closed kinetic chain tasks, which would

include mini squats, single leg vertical and horizontal hopping, lateral shuffles in a mini

squat position, back pedaling, and quick multidirectional movement responses.

29


The following exercises were chosen based on their prevalence in injury prevention and

sport specific training for athletes. All exercises are adapted from Noyes & Barber-

Westin (2012) and are described as follows:

Barrier jump front to back. A cone approximately 6-8” in height was placed on

the floor. The participants started in an upright position with the knees deeply flexed and

were instructed to jump in front of the cone to behind the cone, keeping the feet together.

They were instructed to land on both feet at the same time with the same amount of knee

flexion as the starting position. Participants were instructed to land softly on the balls of

the feet and rock back to the heels to control the landing.

Figure 4. Barrier jump front to back Figure 5. Barrier jump side to side

Barrier jump side to side. A cone approximately 6-8” in height was placed on

the floor. The participants started in an upright position with the knees deeply flexed and

were instructed to jump from one side of the cone to the other, keeping the feet together.

They were instructed to land on both feet at the same time with the same amount of knee

30


flexion as the starting position. Participants were instructed to land softly on the balls of

the feet and rock back to the heels to control the landing.

Lateral bounding. The participants began by assuming a half squat position leg

with the knees slightly flexed, and eyes looking forward. The participants were

instructed to shift weight onto the outside leg and immediately push off and extend

through the outside leg attempting to bound to the opposite side, landing on the opposite

leg and remaining in this position for 3 seconds.

Figure 6. Lateral bounding Figure 7. Scissor jump

Scissor jump. Participants began in a lunge position with the non-dominant knee

bent directly over the ankle. They were instructed to push off with the front leg and jump

straight up in the air while landing with the opposite leg bent in front.

Squat jump. Participants started in a fully crouched position with the hands

touching the ground on the outside of the heels. The participants were instructed to point

the knees and feet forward while keeping the upper body upright with the chest open.

31


They were also instructed to keep the knees under the hips and to keep the knees and

ankles shoulder width apart. Participants were instructed to jump up and raise the arms

as high as possible and return to the starting position with the hands reaching back

towards the heels.

Figure 8. Squat jump

Participants utilized a metronome to perform each exercise and they were

required to stabilize in the landing position for three seconds (equivalent of three beats of

the metronome). They were observed during all practice and recorded repetitions to

ensure correct performance of the exercise.

Data Sampling and Reduction/Data Analysis

Preamplified active surface EMG electrodes with an interelectrode distance of

10mm, and amplification factor of 10,000 (20-500 Hz), and a common mode rejection

ratio of more than 80 dB at 60 Hz was used to measure activation of the quadriceps and

hamstrings. Data was collected and processed utilizing Qualisys© and Visual 3D

32


software (C-motion, Germantown, MD, USA). Electromyographic signals were pre-

amplified at the interface and sampled at 1500 Hz. Utilizing Visual3D software, the EMG

signals were band-pass filtered at 20 to 500 Hz and full-wave rectified before a linear

envelope was created with a 10-Hz low-pass, phase-corrected Butterworth filter.

Qualisys© software was utilized to identify the beginning and end of the middle 4

repetitions for each exercise, and the peak EMG signal amplitudes for the quadriceps and

hamstrings were calculated and averaged.

One MVIC value was obtained for each muscle by averaging the three means.

The mean EMG amplitudes for each exercise were normalized to these reference values

and expressed as percentages of MVIC’s.

Normalized EMG amplitude levels were used to derive Q:H coactivation ratios

for each of the exercises. Ratios were calculated by dividing the sum of the peak

quadriceps EMG activity (VM, VL) by the sum of the peak hamstrings EMG activity

(MH, BF).

(VM + VL)/(MH + BF) = Q:H coactivation ratio

Balanced or equal coactivation calculated by this method resulted in a

coactivation ratio of 1.0, whereas ratios greater than 1.0 indicated greater quadriceps than

hamstrings activation. Similarly, ratios less than 1.0 indicated greater hamstrings than

quadriceps activation.

Statistical Analysis

Although the participants performed eight repetitions of each exercise, only the

middle four repetitions were used for data analysis. Descriptive statistics of the Q:H

33


coactivation ratios were generated by dividing the sum of the peak quadriceps EMG

activity (VM, VL) by the sum of the peak hamstrings EMG activity (MH, BF).

(VM + VL)/(MH + BF) = Q:H coactivation ratio

A one way repeated measures analysis of variance (ANOVA) to identify

differences in Q:H coactivation ratios among exercises. A multivariate analysis with

jump as the within subject variable and gender as the between subject variable was used

to identify the effect of the jump between subjects. A one way repeated measures

analysis of variance (ANOVA) was used to identify differences in peak muscle activity

for each of the four muscles during all five exercises and to identify differences in peak

muscle activity for each of the five exercises. A multivariate analysis was used to

identify the effect of jump on peak EMG flexion angle for each EMG channel (each

muscle) and to identify the effect of jump on peak muscle activity within each exercise.

SPSS version 22 was used to perform all statistical analysis.

Results

Q:H Coactivation Ratios

Calculated Q:H coactivation ratios with standard deviations are displayed in Table

1 for each plyometric exercise. The Q:H coactivation ratios were greatest (quadriceps

dominant activation pattern) during the barrier jump front to back and the barrier jump

side to side, displaying approximately five times more quadriceps than hamstrings

activation in these plyometric exercises. The Q:H ratios were smaller during the lateral

bounding exercise and the squat jump. The lateral bounding exercise (3.08+2.61)

resulted in the smallest Q:H coactivation ratio (most balanced quadriceps and hamstrings

activation). Pairwise comparisons between jumps are displayed in Table 2. Statistically

34


significant differences (p<0.05) were found between the Q:H ratios of lateral bounding

and the scissor jump (mean=-1.069), 95% CI [-2.135, -0.004]) and between lateral

bounding and the squat jump (mean=-0.694), 95%CI [-1.288, -0.100). No between

subjects effect was noted. A statistically significant difference (F4,13=3.651, p<0.05) was

noted with the overall multivariate test of jump (see Table 3). Between subjects analysis

was not significant, so no post hoc analysis was run.

Table 1

Calculated Quadriceps: Hamstrings Coactivation Ratios for Each Plyometric Exercise

(Max + SD)

Exercise Q:H Coactivation Ratio

Barrier Front to Back: Max 5.05+7.33

Barrier Side to Side: Max 4.51+5.29

Lateral Bounding: Max 3.08+2.61

Scissor: Max 4.10+3.24

Squat Jump: Max 3.70+3.02

Note. SD=standard deviation; Q:H=quadriceps to hamstrings

35


Table 2

Pairwise Comparisons Between Exercises

(I) jump (J) jump

Mean

Difference

(I-J)

Std.

Error Sig.b

95% Confidence Interval for

Differenceb

Lower

Bound Upper Bound

1 Barrier

jump front

to back

2 .481 .565 .407 -.716 1.678

3 1.862 1.773 .309 -1.896 5.620

4 .792 1.358 .568 -2.086 3.671

5 1.168 1.742 .512 -2.525 4.861

2 Barrier

jump side

to side

1 -.481 .565 .407 -1.678 .716

3 1.381 1.256 .288 -1.283 4.044

4 .311 .826 .711 -1.440 2.062

5 .687 1.214 .579 -1.886 3.260

3 Lateral

bounding

1 -1.862 1.773 .309 -5.620 1.896

2 -1.381 1.256 .288 -4.044 1.283

4 -1.069* .503 .049* -2.135 -.004

5 -.694* .280 .025* -1.288 -.100

4 Scissor

jump

1 -.792 1.358 .568 -3.671 2.086

2 -.311 .826 .711 -2.062 1.440

3 1.069* .503 .049* .004 2.135

5 .376 .500 .464 -.685 1.437

5 Squat

jump

1 -1.168 1.742 .512 -4.861 2.525

2 -.687 1.214 .579 -3.260 1.886

3 .694* .280 .025* .100 1.288

4 -.376 .500 .464 -1.437 .685

Note. Based on estimated marginal means; 1=barrier jump front to back; 2=barrier jump

side to side; 3=lateral bounding; 4=scissor jump; 5=squat jump

bAdjustment for multiple comparisons: Least Significant Difference (equivalent to no

adjustments).

*p<0.05. **p<0.01. ***p<0.001

36


Table 3

Multivariate Tests with Jump as the Within Subject Variable and Gender as the Between

Subject Variable

Value F

Hypothesis

df Error df Sig.

Partial Eta

Squared

Pillai's trace .529 3.651a 4.000 13.000 .033* .529

Wilks' lambda .471 3.651a 4.000 13.000 .033* .529

Hotelling's trace 1.123 3.651a 4.000 13.000 .033* .529

Roy's largest

root 1.123 3.651

a 4.000 13.000 .033* .529

Peak EMG Flexion Angles for Each Muscle During All Jumps

EMG signals for each individual muscle were examined to determine if there

were statistically significant differences in peak EMG flexion angles during each of the

five exercises. Vastus medialis peak flexion angles were examined and the values are

listed in Table 4. There were no statistically significant differences for vastus medialis in

peak EMG flexion angles when comparing all five exercises (see Table 5). Vastus

lateralis peak flexion angle values are listed in Table 6. There was a statistically

significant difference (F4,14=37.963, p<0.001) in vastus lateralis activation during lateral

bounding when compared to the other four exercises (see Tables 7 & 8). Medial

hamstrings peak flexion angle values are listed in Table 9. There was a statistically

significant difference (F4,14=3.22, p<0.05) in peak flexion medial hamstrings activation

during bounding when compared to the barrier jump front to back, barrier jump side to

side, and the scissor jump (see Tables 10 & 11). In addition, when examining lateral

Note. Each F tests the multivariate effect of jump. These tests are based on the linearly

independent pairwise comparisons among the estimated marginal means. aExact statistic

*p<0.05. **p<0.01. ***p<0.001

37


bounding, the peak medial hamstrings muscle activity occurred at a greater flexion angle

(58.94˚) when compared to the peak medial hamstrings EMG during the other exercises.

Biceps femoris peak flexion angle values are listed in Table 12. There was a statistically

significant difference (F4,14=5.728, p<0.05) in peak flexion biceps femoris activation for

lateral bounding when compared to barrier jump side to side, scissor jump, and squat

jump (see Tables 13 & 14). Overall, the peak EMG for all muscle groups occur at

smaller flexion angles for lateral bounding than all other exercises examined. The one

exception to this pattern is the medial hamstrings peak flexion angles during the lateral

bounding exercise.

Table 4

EMG Channel 1 (VM) Peak Flexion Angle Differences Among Exercises

Jump Mean

Std.

Error

95% Confidence Interval

Lower

Bound

Upper

Bound

Barrier jump front to back 63.453 4.621 73.203 53.704

Barrier jump side to side 66.591 4.041 75.116 58.066

Lateral bounding 53.470 2.403 58.541 48.399

Scissor jump 64.018 4.981 74.528 53.508

Squat jump 61.038 6.358 74.452 47.625

Note. Mean flexion angle values are measured in degrees

38


Table 5

Effect of Jump on Peak Flexion Angle VM Muscle Activation

Effect Value F

Hypothesis

df Error df Sig.

jump Pillai's Trace .416 2.495b 4.000 14.000 .090

Wilks' Lambda .584 2.495b 4.000 14.000 .090

Hotelling's Trace .713 2.495b 4.000 14.000 .090

Roy's Largest

Root .713 2.495

b 4.000 14.000 .090


independent pairwise comparisons among the estimated marginal means.

bExact statistic

*p<0.05. **p<0.01. ***p<0.001

Table 6

EMG Channel 2 (VL) Peak Flexion Angle Differences Among Exercises

Jump Mean

Std.

Error


Lower

Bound

Upper

Bound




Scissor jump 64.592 3.563 72.108 57.076

Squat jump 66.270 4.710 76.207 56.333


39


Table 7

Effect of Jump on Peak Flexion Angle VL Muscle Activation

Value F

Hypothesis

df Error df Sig.

Pillai's trace .916 37.963a 4.000 14.000 .000*

Wilks' lambda .084 37.963a 4.000 14.000 .000*

Hotelling's trace 10.847 37.963a 4.000 14.000 .000*

Roy's largest

root 10.847 37.963

a 4.000 14.000 .000*



*p<0.05. **p<0.01. ***p<0.001

40


Table 8

Pairwise Comparisons of VL Muscle Activation for Each Exercise

(I) jump (J) jump

Mean

Difference

(I-J)

Std.

Error Sig.b


Differenceb

Lower

Bound Upper Bound

1 Barrier

jump

front to

back

2 1.635 5.364 .764 9.681 12.952

3 16.904* 2.467 .000*** 22.109 11.698

4 2.110 3.114 .507 8.680 4.460

5 .432 5.285 .936 11.581 10.718

2 Barrier

jump side

to side

1 1.635 5.364 .764 12.952 9.681

3 18.539* 3.592 .000*** 26.118 10.960

4 3.745 4.799 .446 13.870 6.380

5 2.067 3.897 .603 10.289 6.154

3 Lateral

bounding

1 16.904* 2.467 .000*** 11.698 22.109

2 18.539* 3.592 .000*** 10.960 26.118

4 14.794* 2.664 .000*** 9.174 20.414

5 16.472* 3.929 .001** 8.182 24.761

4 Scissor

jump

1 2.110 3.114 .507 4.460 8.680

2 3.745 4.799 .446 6.380 13.870

3 14.794* 2.664 .000*** 20.414 9.174

5 1.678 4.611 .720 8.050 11.405

5 Squat

jump

1 .432 5.285 .936 10.718 11.581

2 2.067 3.897 .603 6.154 10.289

3 16.472* 3.929 .001** 24.761 8.182

4 1.678 4.611 .720 11.405 8.050


side to side; 3=lateral bounding; 4=scissor jump; 5=squat jump; mean flexion angle

values are measured in degrees bAdjustment for multiple comparisons: Least Significant Difference (equivalent to no

adjustments).

*p<0.05. **p<0.01. ***p<0.001

41


Table 9

EMG Channel 3 (MH) Peak Flexion Angle Differences Among Exercises

Jump Mean

Std.

Error


Lower

Bound

Upper

Bound




Scissor jump 34.976 5.934 47.496 22.455

Squat jump 48.123 8.173 65.366 30.879


Table 10

Effect of Jump on Peak Flexion Angle MH Muscle Activation

Value F

Hypothesis

df Error df Sig.

Pillai's trace .479 3.220a 4.000 14.000 .045*

Wilks' lambda .521 3.220a 4.000 14.000 .045*

Hotelling's trace .920 3.220a 4.000 14.000 .045*

Roy's largest

root .920 3.220

a 4.000 14.000 .045*



*p<0.05. **p<0.01. ***p<0.001

42


Table 11

Pairwise Comparisons of MH Muscle Activation for Each Exercise

(I) jump (J) jump

Mean

Difference

(I-J)

Std.

Error Sig.b


Differenceb

Lower

Bound Upper Bound

1 Barrier

jump

front to

back

2 16.768 9.377 .092 3.017 36.553

3 32.145* 9.411 .003** 12.289 52.000

4 8.186 8.175 .331 9.061 25.433

5 21.333 10.465 .057 .747 43.412

2 Barrier

jump side

to side

1 16.768 9.377 .092 36.553 3.017

3 15.377* 6.908 .040* .803 29.951

4 8.582 6.807 .224 22.944 5.779

5 4.564 8.916 .615 14.246 23.375

3 Lateral

bounding

1 32.145* 9.411 .003** 52.000 12.289

2 15.377* 6.908 .040* 29.951 .803

4 23.959* 7.104 .004** 38.947 8.971

5 10.812 9.233 .258 30.291 8.667

4 Scissor

jump

1 8.186 8.175 .331 25.433 9.061

2 8.582 6.807 .224 5.779 22.944

3 23.959* 7.104 .004** 8.971 38.947

5 13.147 8.864 .156 5.555 31.849

5 Squat

jump

1 21.333 10.465 .057 43.412 .747

2 4.564 8.916 .615 23.375 14.246

3 10.812 9.233 .258 8.667 30.291

4 13.147 8.864 .156 31.849 5.555




adjustments).

*p<0.05. **p<0.01. ***p<0.001

43


Table 12

EMG Channel 4 (BF) Peak Flexion Angle Differences Among Exercises

Jump Mean

Std.

Error


Lower

Bound

Upper

Bound




Scissor jump 56.256 5.381 67.609 44.903

Squat jump 61.761 5.267 72.874 50.648


Table 13

Effect of Jump on Peak Flexion Angle BF Muscle Activation

Value F

Hypothesis

df Error df Sig.

Pillai's trace .621 5.728a 4.000 14.000 .006**

Wilks' lambda .379 5.728a 4.000 14.000 .006**

Hotelling's trace 1.636 5.728a 4.000 14.000 .006**

Roy's largest

root 1.636 5.728

a 4.000 14.000 .006**



*p<0.05. **p<0.01. ***p<0.001

44


Table 14

Pairwise Comparisons of BF Muscle Activation for Each Exercise

(I) jump (J) jump

Mean

Difference

(I-J)

Std.

Error Sig.b


Differenceb

Lower

Bound Upper Bound

1 Barrier

jump

front to

back

2 14.655 7.369 .063 .893 30.202

3 12.047 7.893 .145 28.700 4.605

4 8.120 6.991 .262 6.630 22.869

5 13.625 7.480 .086 2.157 29.406

2 Barrier

jump side

to side

1 14.655 7.369 .063 30.202 .893

3 26.702* 5.172 .000*** 37.615 15.789

4 6.535 5.017 .210 17.119 4.049

5 1.030 3.961 .798 9.388 7.328

3 Lateral

bounding

1 12.047 7.893 .145 4.605 28.700

2 26.702* 5.172 .000*** 15.789 37.615

4 20.167* 6.738 .008** 5.950 34.383

5 25.672* 6.131 .001** 12.737 38.607

4 Scissor

jump

1 8.120 6.991 .262 22.869 6.630

2 6.535 5.017 .210 4.049 17.119

3 20.167* 6.738 .008** 34.383 5.950

5 5.505 5.130 .298 5.318 16.328

5 Squat

jump

1 13.625 7.480 .086 29.406 2.157

2 1.030 3.961 .798 7.328 9.388

3 25.672* 6.131 .001** 38.607 12.737

4 5.505 5.130 .298 16.328 5.318




adjustments).

*p<0.05. **p<0.01. ***p<0.001

45


Peak EMG Flexion Angles for Each Muscle Within Each Exercise

EMG signals for each muscle were examined to determine if there were

statistically significant differences in peak EMG flexion angles of the four muscles

during barrier jump front to back, barrier jump side to side, lateral bounding, scissor

jump, and squat jump. Peak EMG flexion angles for the barrier jump front to back are

listed in Table 15. There was a statistically significant difference (F3,15=10.561, p<0.001)

between the flexion angles of the combined hamstrings when compared to the combined

quadriceps. The peak EMG flexion angles for the hamstrings occurred at a smaller

flexion angle than the quadriceps (see Tables 16 & 17). Peak EMG flexion angles for the

barrier jump side to side are listed in Table 18. There was a statistically significant

difference (F3,15=14.810, p<0.001) between the flexion angle of the medial hamstrings

compared to the other muscles. The peak EMG flexion angle (43.56˚) for the medial

hamstrings was significantly smaller than the biceps femoris, vastus lateralis, and vastus

medialis (see Tables 19 & 20). Peak EMG flexion angles for lateral bounding are listed

in Table 21. There was a statistically significant difference (F3,15=3.533, p<0.05)

between the flexion angle of the medial hamstrings compared to the other muscles. The

peak EMG flexion angle (58.94˚) for the medial hamstrings was significantly larger than

the biceps femoris, vastus lateralis, and vastus medialis (see Tables 22 & 23). Peak EMG

flexion angles for the scissor jump are listed in Table 24. There was a statistically

significant difference (F3,15=13.216, p<0.001) between the flexion angles of the combined

hamstrings compared to the combined quadriceps (see Tables 25 & 26). The peak EMG

flexion angles for the combined hamstrings were significantly smaller than the peak

EMG flexion angles for the combined quadriceps. Peak EMG flexion angles for the

46


squat jump are listed in Table 27. There were no statistically significant differences

between peak EMG flexion angles amongst the muscles during this exercise (see Table

28).

Table 15

Peak EMG Flexion Angles of all Muscles During Barrier Jump Front to Back

Flexion angle Mean

Std.

Error


Lower

Bound

Upper

Bound

Vastus medialis 63.453 4.621 73.203 53.704

Vastus lateralis 66.702 3.345 73.759 59.645

Medial hamstrings 26.790 6.999 41.557 12.023

Biceps femoris 48.136 6.516 61.884 34.388


Table 16

Effect of Barrier Jump Front to Back on Peak EMG Flexion Angle for Each Muscle

Value F

Hypothesis

df Error df Sig.

Pillai's trace .679 10.561a 3.000 15.000 .001**

Wilks' lambda .321 10.561a 3.000 15.000 .001**

Hotelling's trace 2.112 10.561a 3.000 15.000 .001**

Roy's largest

root 2.112 10.561

a 3.000 15.000 .001**

Note. Each F tests the multivariate effect of flexion angle. These tests are based on the

linearly independent pairwise comparisons among the estimated marginal means. aExact statistic

*p<0.05. **p<0.01. ***p<0.001

47


Table 17

Pairwise Comparisons of Peak EMG Flexion Angle of All Muscles During Barrier Jump

Front to Back

(I) flexion angle

(J) flexion

angle

Mean

Difference

(I-J)

Std.

Error Sig.b

95%

Confidence

Interval for

Differenceb

Lower

Bound

1 Vastus medialis 2 3.248 4.955 .521 7.205

3 36.663* 8.972 .001** 55.593

4 15.317* 6.465 .030* 28.957

2 Vastus lateralis 1 3.248 4.955 .521 13.702

3 39.912* 7.720 .000*** 56.199

4 18.566* 6.783 .014* 32.877

3 Medial

hamstrings

1 36.663* 8.972 .001** 17.734

2 39.912* 7.720 .000*** 23.624

4 21.346 10.422 .056 .642

4 Biceps femoris 1 15.317* 6.465 .030* 1.678

2 18.566* 6.783 .014* 4.254

3 21.346 10.422 .056 43.334

Note. Based on estimated marginal means;1=vastus medialis; 2=vastus lateralis;

3=medial hamstrings; 4=biceps femoris; mean flexion angle values are measured in

degrees bAdjustment for multiple comparisons: Least Significant Difference (equivalent to no

adjustments).

*p<0.05. **p<0.01. ***p<0.001

48


Table 18

Peak EMG Flexion Angles of all Muscles During Barrier Jump Side to Side

Flexion angle Mean

Std.

Error


Lower

Bound

Upper

Bound

Vastus medialis 66.591 4.041 75.116 58.066



Biceps femoris 62.791 3.486 70.147 55.436


Table 19

Effect of Barrier Jump Side to Side on Peak EMG Flexion Angle for Each Muscle

Value F

Hypothesis

df Error df Sig.

Pillai's trace .748 14.810a 3.000 15.000 .000***

Wilks' lambda .252 14.810a 3.000 15.000 .000***

Hotelling's trace 2.962 14.810a 3.000 15.000 .000***

Roy's largest

root 2.962 14.810

a 3.000 15.000 .000***



*p<0.05. **p<0.01. ***p<0.001

49


Table 20

Pairwise Comparisons of Peak EMG Flexion Angle of All Muscles During Barrier Jump

Side to Side

(I) flexion angle

(J) flexion

angle

Mean

Difference

(I-J)

Std.

Error Sig.b

95%

Confidence

Interval for

Differenceb

Lower

Bound

1 Vastus medialis 2 1.747 3.200 .592 5.005

3 23.032* 3.330 .000*** 30.057

4 3.800 4.384 .398 13.048


3 24.779* 4.953 .000*** 35.229

4 5.546 4.222 .206 14.453

3 Medial

hamstrings

1 23.032* 3.330 .000*** 16.008

2 24.779* 4.953 .000*** 14.329

4 19.233* 5.766 .004** 7.067

4 Biceps femoris 1 3.800 4.384 .398 5.449

2 5.546 4.222 .206 3.360

3 19.233* 5.766 .004** 31.399




adjustments).

*p<0.05. **p<0.01. ***p<0.001

50


Table 21

Peak EMG Flexion Angles of all Muscles During Lateral Bounding

Flexion angle Mean

Std.

Error


Lower

Bound

Upper

Bound

Vastus medialis 53.470 2.403 58.541 48.399



Biceps femoris 36.089 6.066 48.888 23.291


Table 22

Effect of Lateral Bounding on Peak EMG Flexion Angle for Each Muscle

Value F

Hypothesis

df Error df Sig.

Pillai's trace .414 3.533a 3.000 15.000 .041*

Wilks' lambda .586 3.533a 3.000 15.000 .041*

Hotelling's trace .707 3.533a 3.000 15.000 .041*

Roy's largest

root .707 3.533

a 3.000 15.000 .041*



*p<0.05. **p<0.01. ***p<0.001

51


Table 23

Pairwise Comparisons of Peak EMG Flexion Angle of All Muscles During Lateral

Bounding

(I) flexion angle

(J) flexion

angle

Mean

Difference

(I-J)

Std.

Error Sig.b

95%

Confidence

Interval for

Differenceb

Lower

Bound

1 Vastus medialis 2 3.672 2.262 .123 8.445

3 5.465 6.898 .439 9.089

4 17.381* 5.993 .010* 30.024


3 9.137 7.304 .228 6.274

4 13.709* 6.418 .048* 27.250

3 Medial

hamstrings

1 5.465 6.898 .439 20.019

2 9.137 7.304 .228 24.548

4 22.846* 8.833 .019* 41.482

4 Biceps femoris 1 17.381* 5.993 .010* 4.738

2 13.709* 6.418 .048* .167

3 22.846* 8.833 .019* 4.209




adjustments).

*p<0.05. **p<0.01. ***p<0.001

52


Table 24

Peak EMG Flexion Angles of all Muscles During Scissor Jump

Flexion angle Mean

Std.

Error


Lower

Bound

Upper

Bound

Vastus medialis 64.018 4.981 74.528 53.508



Biceps femoris 56.256 5.381 67.609 44.903


Table 25

Effect of Scissor Jump on Peak EMG Flexion Angle for Each Muscle

Value F

Hypothesis

df Error df Sig.

Pillai's trace .726 13.216a 3.000 15.000 .000***

Wilks' lambda .274 13.216a 3.000 15.000 .000***

Hotelling's trace 2.643 13.216a 3.000 15.000 .000***

Roy's largest

root 2.643 13.216

a 3.000 15.000 .000***



*p<0.05. **p<0.01. ***p<0.001

53


Table 26

Pairwise Comparisons of Peak EMG Flexion Angle of All Muscles During Scissor Jump

(I) flexion angle

(J) flexion

angle

Mean

Difference

(I-J)

Std.

Error Sig.b

95%

Confidence

Interval for

Differenceb

Lower

Bound

1 Vastus medialis 2 .574 5.594 .919 11.228

3 29.042* 5.009 .000*** 39.611

4 7.762 6.818 .271 22.146

2 Vastus lateralis 1 .574 5.594 .919 12.377

3 29.616* 5.738 .000*** 41.722

4 8.336 5.756 .166 20.481

3 Medial

hamstrings

1 29.042* 5.009 .000*** 18.473

2 29.616* 5.738 .000*** 17.510

4 21.280* 7.617 .012* 5.210

4 Biceps femoris 1 7.762 6.818 .271 6.623

2 8.336 5.756 .166 3.809

3 21.280* 7.617 .012* 37.350




adjustments).

*p<0.05. **p<0.01. ***p<0.001

54


Table 27

Peak EMG Flexion Angles of all Muscles During Squat Jump

Flexion angle Mean

Std.

Error


Lower

Bound

Upper

Bound

Vastus medialis 61.038 6.358 74.452 47.625



Biceps femoris 61.761 5.267 72.874 50.648


Table 28

Effect of Squat Jump on Peak EMG Flexion Angle for Each Muscle

Value F

Hypothesis

df Error df Sig.

Pillai's trace .161 .963a 3.000 15.000 .436

Wilks' lambda .839 .963a 3.000 15.000 .436

Hotelling's trace .193 .963a 3.000 15.000 .436

Roy's largest

root .193 .963

a 3.000 15.000 .436



*p<0.05. **p<0.01. ***p<0.001

Discussion

Q:H Coactivation Ratios

The primary purpose of this study was to assess Q:H coactivation ratios during

high velocity, closed chain exercises in healthy, recreationally active adults. We found

differences in the levels of Q:H coactivation during the five selected exercises.

Interpretation of our results may offer insight into the potential effectiveness of these

55


exercises in terms of muscle activation between the quadriceps and hamstrings. Our

study provides information that may assist clinicians in selecting high velocity, closed

chain exercises that are most appropriate for their current rehabilitation goals and

throughout their exercise progression. Exercises and their implications for rehabilitation

will be discussed in the order of the smallest (most balanced) Q:H coactivation ratios to

the largest coactivation ratios (most quadriceps dominant).

The two most balanced Q:H coactivation ratio was observed during the lateral

bounding exercise (3.08+2.61) and the squat jump (3.70+3.02). Of the five selected

exercises, these two exercises appear to be driven by less quadriceps activity than the

scissor jump, barrier jump front to back, and barrier jump side to side. This data implies

that these exercises may be safer to utilize in ACL rehabilitation due to more balanced

activation between the quadriceps and hamstrings.

The largest Q:H coactivation ratio was observed during the barrier jump front to

back (5.05+7.33). This exercise appears to be driven primarily by quadriceps activity

with minimal hamstring activation to counteract the quadriceps. Previous research has

shown that an imbalance in muscle activation between the quadriceps and hamstrings,

particularly overactive quadriceps muscle activation, causes an anterior translation of the

tibia on the femur (Cheung, Smith, & Wong, 2012). Because the primary function of the

ACL is to limit this anterior translation of the tibia, an individual’s ACL may be

predisposed to injury as the result of muscular imbalance between the quadriceps and

hamstrings, with an excessive anterior tibial shear force from the over-activity of the

quadriceps (Markolf et al., 1995; Renstrom et al., 1986). These results suggest that the

large Q:H ratio seen during the barrier jump front to back could cause an increase in

56


anterior tibial translation and greater ACL loading, thus predisposing the ACL to injury

or re-injury. This information should be carefully considered when deciding whether to

incorporate the barrier jump front to back into an ACL rehab program due to the

increased quadriceps activation that it promotes.

Peak EMG Flexion Angles for Each Muscle During All Jumps

Based on our results, there were no significant differences in the peak EMG

flexion angles of the vastus medialis when comparing all five exercises. However,

significant differences in peak EMG flexion angles of the remaining muscles were

observed. We found that the peak EMG flexion angles of the vastus lateralis and biceps

femoris during the lateral bounding exercise differed significantly from all other

exercises performed in that a smaller flexion angle was observed during this exercise.

Conversely, the medial hamstrings behaved in the opposite manner during the lateral

bounding exercise by having the largest peak EMG flexion angle. These results lead the

researchers to conclude that the medial hamstrings are the last muscle to activate during

the lateral bounding exercise. As a result, the medial hamstrings are unable to provide

stabilization to the posterior aspect of the knee to offset the earlier activation of the

combined quadriceps.

Previous research has found that the hamstrings provide posterior stabilization to

the knee by counteracting the anterior shear force produced by the quadriceps, and thus

reducing the amount of force placed on the ACL (Noyes & Barber-Westin, 2012;

Renstrom et al., 1986). Reduced activation of the hamstrings relative to the quadriceps

causes an increase in restraint force to be placed on the ACL (Beynnon & Fleming, 1998;

Croisier et al., 2008). In addition, weakness of the hamstrings may increase the risk of an

57


ACL rupture by contributing to a greater ground reaction force being transmitted to the

knee joint upon landing (Hewett, Myer, & Ford, 2006). Therefore, decreased activation

and strength of the hamstrings relative to the quadriceps may increase one’s susceptibility

to ACL injury (Boden, Griffin, & Garret, 2000, Myer et al., 2009, and Chappel,

Creighton, Giuliani, Yu, & Garret, 2007).

This information is imperative to consider when deciding which exercises are the

safest and most appropriate for ACL rehabilitation. Although our initial results

demonstrated that lateral bounding had the most balanced Q:H coactivation ratio, further

data analysis was required to determine the flexion angles that the maximal muscle

contractions for each muscle were occurring. In order to safely prescribe these exercises,

the clinician must have knowledge of the timing of the maximum quadriceps activation in

relation to the maximum hamstrings activation. This information can be used to

determine exercises that promote earlier hamstring activation to provide posterior

stabilization to the knee and thus, protect the ACL.

Peak EMG Flexion Angles for Each Muscle Within Each Exercise

Based on our results, there were statistically significant differences between the

peak EMG flexion angles during the barrier jump front to back, barrier jump side to side,

and the scissor jump. The peak EMG flexion angles of the hamstrings were smaller than

the peak EMG flexion angles of the quadriceps, meaning that the MH and BF activate at

a smaller flexion angle to provide posterior stabilization to the knee before the VL and

VM activate. Numerous investigators have reported that quadriceps contraction increases

ACL strain at smaller angles (0°-30°) of knee flexion (Arms et al. 1984; Beynnon et al.

1995; & Li et al. 1999). In addition, most non-contact ACL injuries occur when the knee

58


is moving towards terminal extension making it imperative that the hamstrings activate at

a smaller flexion angle in order to provide posterior stabilization to the knee joint and

decrease the amount of restraint force placed on the ACL (Renstrom et al., 1986; Arms et

al., 1984; Boden et al., 2000). This information can be used to guide the clinician in

choosing exercises that promote earlier hamstring activation and thus, more protection of

the ACL. Our results suggest that the barrier jump front to back, barrier jump side to

side, and the scissor jump could be a safe exercises to use when beginning to incorporate

dynamic activities into an ACL rehabilitation program as they promote earlier activation

of the hamstrings and thus, increased stabilization of the knee joint.

In contrast to the barrier jump side to side, barrier jump front to back, and scissor

jump where the peak EMG flexion angle of the medial hamstrings was smaller than the

other muscles, during the lateral bounding exercise, the peak EMG flexion angle of the

medial hamstrings was significantly larger than the other three muscles. This means that

the combined quadriceps activate at a smaller flexion angle than the medial hamstrings

activate during this exercise. As previously discussed, several studies have shown that

ACL loading increases as the knee flexion angle decreases (Arms et al. 1984; Beynnon et

al. 1995; Li et al. 1999; Renstrom et al., 1986; & Boden et al., 2000). Furthermore,

Colby et al. (2000) compared quadriceps to hamstrings activation at various knee flexion

angles during dynamic activities including side step cutting, cross-cutting, stopping, and

landing and found that the peak quadriceps activity occurred at 22˚ of knee flexion

suggesting that increased strain is placed on the ACL at this angle. Since the quadriceps

are ACL antagonists, over-activation and earlier activation of the quadriceps in relation to

the hamstrings can result in increased strain and greater likelihood of ACL injury.

59


Careful consideration should be used when determining when and/or whether to

incorporate bounding into an ACL rehabilitation program as it facilitates earlier

activation of the quadriceps and thus, increased strain and risk of injury to the ACL.

Finally, there were no statistically significant differences in peak EMG flexion

angles during the squat jump. The peak EMG flexion angles during this exercise are

similar between the quadriceps and hamstrings with peak muscle activation for both

muscle groups occurring at approximately 62˚ of knee flexion. As previous research has

found, most non-contact ACL injuries occur while an athlete is accelerating, decelerating,

cutting side to side, or pivoting (Noyes & Barber-Westin; Begalle, DiStefano, Blackburn,

& Padua 2012). Since the squat jump does not encourage any lateral motion during the

exercise, it appears that this could be the safest exercise to utilize earlier on in ACL

rehabilitation as it could facilitate a more balanced coactivation pattern between the

quadriceps and hamstrings throughout the exercise.

Based on our findings, it appears that the barrier jump front to back, barrier jump

side to side, and scissor jump may be safe exercises to utilize early on in an ACL

rehabilitation program as they facilitate earlier activation of the hamstrings, which is

necessary to provide posterior stabilization to the knee. In addition, the squat jump

facilitates peak muscle activity at similar knee flexion angles between the quadriceps and

hamstrings, suggesting that this exercise could also be safe to use in ACL rehabilitation.

Overall, it is important for athletes to master performing front to back motions, such as

the barrier jump front to back before attempting to perform lateral motions that require

more dynamic stability to protect the knee. Exercises such as lateral bounding should be

60


used with caution, especially during the early stages of rehabilitation due to the earlier

activation of the quadriceps in relation to the hamstrings.

Recommendations and Limitations

Based on our initial findings, the most balanced (smallest) coactivation ratios

were observed during the squat jump and lateral bounding exercises. The largest (most

quadriceps-dominant) Q:H ratios were observed during the barrier jump side to side and

barrier jump front to back exercises. However, examining these ratios in isolation does

not provide the clinician with enough information to determine which exercises are the

most appropriate at various stages of the rehabilitation program. Although examining the

coactivation ratios was the primary purpose of this study, upon further examination of the

data, we determined that the timing of the peak muscle activity is a fundamental factor to

consider when prescribing these exercises.

Based on the coactivation ratio data, lateral bounding appeared to be the safest

exercise due to the smallest (most balanced) ratio. However, when examining the timing

of the peak quadriceps and hamstrings contractions, we determined that the hamstrings

are unable to stabilize the posterior aspect of the knee prior to the quadriceps contraction.

Therefore, it is our recommendation that this exercise should be used with caution due to

the potential for increased strain placed on the ACL. In contrast, the barrier jump front to

back had the highest (least balanced) ratio, but had earlier hamstring peak contraction

which provided stability to the knee. Overall, our data presents one of the challenges in

prescribing safe and effective exercise for ACL prevention and rehabilitation. Although

coactivation ratios are an important component to exercise prescription, without

61


examining the timing of peak muscle contractions, the clinician cannot choose the

optimal exercise to retrain the muscles while also protecting the ligaments of the knee.

We examined muscle activation in a healthy, recreationally active population with

a small sample size of 18 subjects. Thus, a limitation of this study is that the results

cannot be generalized to the overall active population. In addition, because our

population was recreationally active, defined as 60 minutes of exercise at least 3 days a

week, the results cannot be generalized to other specific athletic populations, such as

collegiate athletes. Future studies should include a greater number of subjects with the

potential to study specific cohorts of athletes, such as soccer players, basketball players,

or volleyball players. In addition, there was some variability between the subject’s ratios.

A small number of subjects had ratios that were significantly larger (Q:H ratio=31.85:1)

than the mean rations, which could indicate that these subjects are more quadriceps

dominant, and thus more prone to ACL injury. Another limitation of this study was that

it only included 5 exercises. Although we tried to pick exercises that moved in all planes

of motion, playing sports such as soccer and basketball require an athlete to move in a

multitude of different directions quickly and our exercises may not cover every motion

associated with playing these sports. Finally, a limitation of our study was the method of

MVIC collection. The MVIC collection was conducted at the beginning of the study and

it was limited by the amount of manual force provided by the researcher. Future studies

should utilize the BioDex system for MVIC collection to ensure that the amount of force

provided is equivalent between subjects.

62


Conclusions

We evaluated the Q:H coactivation ratios among five high velocity, closed chain

plyometric exercises, as well the knee flexion angles that coincide with peak muscle

activity. Results of our study identified that the barrier jump front to back, barrier jump

side to side, and scissor jump facilitated earlier activation of the hamstrings in relation to

the quadriceps suggesting that these exercises provide the most stability to the posterior

aspect of the knee, thus protecting the ACL. In contrast, lateral bounding facilitates

earlier quadriceps activation and therefore should be used with caution in the early stages

of ACL rehabilitation due to the anterior shear force placed on the ACL from the

quadriceps. Having knowledge of both the overall Q:H ratios as well as the timing of

peak muscle contraction allows for better exercise prescription and progression and could

also be used in injury prevention programs to decrease the likelihood of ACL injury or

re-injury.

63


References

Aagaard, P., Simonsen, E., Andersen, J., Magnusson, S., Bojsen-Moller, F., & Dyhre-

Poulsen, P. (2000). Antagonist muscle coactivation during isokinetic knee

extension. Scandinavian journal of medicine & science in sports, 10(2), 58-67.

Retrieved from http://onlinelibrary.wiley.com/doi/10.1034/j.1600-

0838.2000.010002058.x/pdf

Aagaard, P., Simonsen, E., Magnusson, P., Larsson, B., & Dyhre-Poulson, P. (1998). A

new concept for isokinetic hamstring : Quadriceps muscle strength ratio. The

American Journal of Sports Medicine, 26(2), 231-237. Retrieved from

http://faculty.fullerton.edu/gnoffal/Courses/561 Course/iso3.pdf

Alentorn-Geli, E., Myer, G.D., Silvers, H.J., Samitier, G., Romero, D., Lazaro-Haro, C.,

& Cugat, R. (2009). Prevention of non-contact anterior cruciate ligament injuries

in soccer players. Part 1: Mechanism of injury and underlying risk factors. Knee

Surgery, Sports Traumatology, Arthroscopy, 17(8), 705-729. Retrieved from

http://www.scribd.com/doc/229492642/Alentorn-Geli-Et-Al-2009-Prevention-of-

Non-contact-Anterior-Cruciate-Ligament-Injuries-in-Soccer-Players-Part-1

Arendt, E., Agel, J., & Dick, R. (1999). Anterior cruciate ligament injury patterns among

collegiate men and women. Journal of Athletic Training, 34(2), 86-92. Retrieved

from http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1322895/pdf/jathtrain00006-

0014.pdf

Arms, S.W., Pope, M.H., Johnson, R. J., Fischer, R.A., Arvidsson, I., Eriksson, E. (1984).

The biomechanics of anterior cruciate ligament rehabilitation and reconstruction.

The American Journal of Sports Medicine, 12(1), 8-18. Retrieved from

http://www.ncbi.nlm.nih.gov/pubmed/6703185

Baltzopoulos, V. and Brodie, D. (1989) Isokinetic dynamometry: applications and

limitations. SportsMedicine, 8, 101-116.

Baratta, R., Solomonow, M., Zhou, B., Letson, D., Chuinard, R., & D'Ambrosia, R.

(1988). Muscular coactivation. The role of the antagonist musculature in

maintaining knee stability. American journal of sports medicine, 16(2), 113-22.

Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/3377094/

Begalle, R. L., DiStefano, L. J., Blackburn, T., & Padua, D. A. (2012). Quadriceps and

hamstrings coactivation during common therapeutic exercises. Journal of Athletic

Training, 47(4), 396-405.

Beynnon, B.D. & Fleming, B.C. (1998). Anterior cruciate ligament strain in-vivo: a

review of previous work. Journal of Biomechanics, 31(6), 519-525. Retrieved

from http://www.ncbi.nlm.nih.gov/pubmed/9755036

64


Beynnon, B.D., Fleming, B.C., Johnson, R.J., Nichols, C.E., Pope, M.H., Renstrom, P.A.

(1995). Anterior cruciate ligament strain behavior during rehabilitation exercises

in vivo. The American Journal of Sports Medicine, 23(1), 24-34. Retrieved from


Boden, B. P., Griffin, L. Y., Garret, W. E. (2000). Etiology and prevention of noncontal

acl injury. The Physician and Sports Medicine, 28(4), 1-12. Retrieved from

http://www.frvbc.com/newsletter/pdf/Etiology%20and%20Prevention%20of%20

Noncontact%20ACL%20Injury%20word.pdf

Chappell, J. D., Creighton, R. A., Giuliani, C., Yu, B., & Garrett, W. E. (2007).

Kinematics and electromyography of landing preparation in vertical stop-jump:

Risks for noncontact anterior cruciate ligament injury. The American Journal of

Sports Medicine, 35, 235-241.

Cheung, R.H., Smith, A.W., & Wong, D.P. (2012). H:Q ratios and bilateral leg strength

in college field and court sport players. Journal of Human Kinetics, 33, 63-71.

Retrieved from http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3588678/

Colby, S., Francisco, A., Yu, B., Kirkendall, D., Finch, M., & Garrett, W. (2000).

Electromyographic and kinematic analysis of cutting maneuvers: implications for

anterior cruciate ligament injury. The American Journal of Sports Medicine,

28(2), 234-240. Retrieved from http://ajs.sagepub.com/content/28/2/234.full.pdf

Coombs, R., & Garbutt, G. (2002). Developments in the use of the hamstring/quadriceps

ratio for the assessment of muscle balance. Journal of sports science and

medicine, (1), 56-62. Retrieved from

http://www.jssm.orwww.jssm.org/vol1/n3/1/n3-1text.php

Croisier, J.L., Ganteaume, S., Binet, J., Genty, M., Ferret, J.M. (2008). Strength

imbalances and prevention of hamstring injury in professional soccer players: a

prospective study. American Journal of Sports Medicine, 36,1469–1475.

DeMorat, G., Weinhold, P., Blackburn, T., Chudik, S., & Garrett, W. E. (2004).

Aggressive quadriceps loading can induce noncontact anterior cruciate ligament

injury. The American Journal of Sports Medicine, 32, 477-483.

De Villarreal, S.S., Requena, E., Newton, R.U. (2010). Does plyometric training

improve strength performance? A meta-analysis. Journal of Science and

Medicine in Sport, 13, 513-522.

DiStefano, L.J., Blackburn, J.T., Marshall, S.W., & Padua, D.A. (2009). Gluteal muscle

activation during common therapeutic exercises. Journal of Orthopaedic &

65


Sports Physical Therapy, 39(7), 532-540. Retrieved from


Draganich, L.F., Jaeger, R.J., & Kralj, A.R. (1989). Coactivation of the hamstrings and

quadriceps during extension of the knee. Journal of bone and joint surgery.

American volume, 71(7), 1075-1081. Retrieved from:


Gottlob, C.A., Baker, C.L. (2000). Anterior cruciate ligament reconstruction:

socioeconomic issues and cost effectiveness. American Journal of Orthopedics,

29, 472-476.

Heidt, R.S., Sweeterman, L.M., Richelle, M.S., Carlonas, L., Traub, J.A., & Tekulve,

F.X. (2000). Avoidance of soccer injuries with preseason conditioning. The


http://www.portalsaudebrasil.com/artigospsb/treinofunc9.pdf

Hewett, T. E., Myer, G. D., & Ford, K. R. (2006). Anterior cruciate ligament injuries in

female athletes, part 1: Mechanics and risk factors. The American Journal of

Sports Medicine, 34, 299-311.

Hewett, T.E., Stroupe, A.L., Nance, T.A., & Noyes, F.R. (1996). Plyometric training in

female athletes: decreased impact forces and increased hamstring torques. The


http://ajs.sagepub.com/content/24/6/765.full.pdf+html

Hewett, T.E. (2007) ACL injury prevention programs. In: Hewett TE, Shultz SJ, Griffin

L.Y. (eds) Understanding and preventing non-contact ACL injuries, 1st edition.

Human Kinetics, Champaign, pp 57–60.

Holcomb, W.R., Rubley, M.D., Lee, H.J., and Guadagnoli, M.A. (2007) Effect of

hamstring-emphasized resistance training on hamstring: quadriceps strength

ratios. Journal of Strength & Conditioning Research, 21, 41–47.

Kannus, P. (1994) Isokinetic evaluation of muscular performance: implications for

muscle testing and rehabilitation. International Journal of Sports Medicine

15(Suppl.1), 11-15.

Kisner, C., & Colby, L. A. (2012). Therapeutic exercise: Foundations and techniques.

Philadelphia: F.A. Davis.

Li, G., Rudy, T.W., Sakane, M., Kanamori, A., Ma, C.B., Woo, S.L. (1999). The

importance of quadriceps and hamstring muscle loading on knee kinematics and

in-situ forces in the ACL. Journal of Biomechanics, 32(4), 395-400.

66


Markolf, K.L., Burchfield, D.M., Shapiro, M.M., Shepard, M.F., Finerman, G.A.,

Slauterbeck, J.L. (1995). Combined knee loading states that generate high

anterior cruciate ligament forces. Journal of Orthopedic Research, 13 (6), 930-

935. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/8544031

Meunier, A., Odensten, M., & Good, L. (2007). Long-term results after primary repair or

non-surgical treatment of anterior cruciate ligament rupture: a randomized study

with a 15-year follow up. Scandinavian Journal of Medical Science in Sports, 17,

230-237.

Myer, G. D., Ford, K. R., Barber Foss, K., Liu, C., Nick, T. G., & Hewett, T. E. (2009).

The relationship of hamstrings and quadriceps strength to anterior cruciate

ligament injury in female athletes. Clinical Journal of Sports Medicine, 19, 3-8.

Myer, G.D., Ford, K.R., Palumbo, J.P., & Hewett, T.E. (2005). Neuromuscular training

improves performance and lower extremity biomechanics in female athletes.

Journal of Strength and Conditioning Association, 19(1), 51-60. Retrieved from

http://www.t.motionanalysis.com/pdf/2005_myer.pdf

Noyes, F. R., & Barber-Westin, S. (2012). Acl injuries in the female athlete.

Nyland, J.A., Caborn, D.N., Shapiro, R., & Johnson, D.L. (1999). Crossover cutting

during hamstring fatigue produces transverse plane knee control deficits. Journal

of Athletic Training, 34, 137–143.

Podraza, J.T. & White, S.C. (2010). Effect of knee flexion angle on ground reaction

forces, knee moments, and muscle co-contraction during an impact-like

deceleration landing: implications for the non-contact mechanism of ACL injury.

The Knee, 17, 291-295.

Potteiger, J.A., Lockwood, R.H., Haub, M.D., Dolezal, B.A., Almuzaini, K.S., Schroeder,

J.M., et al., (2005). Muscle power and fiber characteristics following 8 weeks of

plyometric training. Journal of Strength and Conditioning Research, 13, 275-

279.

Rahimi, R., Arshadi, P., Behpur, N., Boroujerdi, S.S., & Rahimi, M. (2006). Evaluation

of plyometrics, weight training, and their combination on angular velocity.

Journal of Strength and Conditioning Research, 4(1), 1-6. Retrieved from

http://facta.junis.ni.ac.rs/pe/pe2006/pe2006-01.pdf

Renstrom, P., Arms, S.W., Stanwyck, T.S., Johnson, R.J., Pope, M.H. (1986). Strain

within the anterior cruciate ligament during hamstring and quadriceps activity.

American Journal of Sports Medicine, 14 (1), 83-87. Retrieved from

http://ajs.sagepub.com/content/14/1/83.full.pdf+html

67


Rosene, J. M., Fogarty, T. D., & Mahaffey, B. L. (2001). Isokinetic

hamstrings:quadriceps ratios in intercollegiate athletes. Journal of Athletic

Training, 36(4), 378-383.

Salmon, L., Russel, V., Musgrove, T., Pinezewski, L., Refshauge, K. (2005). Incidence

and risk factors for graft rupture and contralateral rupture after anterior cruciate

ligament reconstruction. Arthroscopy, 21(8), 948-957. Retrieved from

http://www.nsosmc.com.au/resources/Rerupture.pdf

Withrow, T. J., Huston, L. J., Wojtys, E. M., & Ashton-Miller, J. A. (2008). Effect of

varying hamstring tension on anterior cruciate ligament strain during in vitro

impulsive knee flexion and compression loading. The Journal of Bone & Joint

Surgery, 90, 815-823.

Yu, B., & Garret, W. (2007). Mechanisms of non‐contact acl injuries. British Journal of

Sports Medicine, 41(1), 47-51. Retrieved from

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2465243/

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