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Journal of Applied Biomechanics

© 2012 Human Kinetics, Inc.

Note. This article will be published in a forthcoming issue of

the Journal of Applied Biomechanics. The article appears here

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Section: Original Research

Article Title: A Biomechanical Study of Side Steps at Different Distances

Authors: Yuki Inaba1, Shinsuke Yoshioka

2, Yoshiaki Iida

1, Dean C. Hay

3, Senshi Fukashiro

1

Affiliations: 1Department of Life Sciences, University of Tokyo, Meguro, Tokyo, Japan.

2Faculty of Sport and Health Science, Ritsumeikan University, Kusatsu, Shiga, Japan.

3School of Physical and Health Education, Nipissing University, North Bay, Ontario, Canada.

Journal: Journal of Applied Biomechanics

©2012 Human Kinetics, Inc.




Article Type: Original Research

A Biomechanical Study of Side Steps at Different Distances

Full names and email address of all authors:

- Yuki Inaba, M.S.1;email: [email protected]

- Shinsuke Yoshioka, Ph.D.2; email: [email protected]

- Yoshiaki Iida, M.S.1; email: [email protected]

- Dean C. Hay, Ph.D.3; email: [email protected]

- Senshi Fukashiro, Ph.D.1; email: [email protected]

Institutional Affiliation:

1 Department of Life Sciences, University of Tokyo, Meguro, Tokyo, Japan

2 Faculty of Sport and Health Science, Ritsumeikan University, Kusatsu, Shiga, Japan

3 School of Physical and Health Education, Nipissing University, North Bay, ON, Canada

Funding: No funding was received.

Conflict of Interests Disclosure: None of the authors have commercial or other interests that

may conflict with the integrity of this scientific work

Correspondence address:

-Name: Yuki Inaba

-Address: 3-8-1 Komaba, Meguro, Tokyo, 153-8902, Japan

-Email: [email protected]

Running Title: A Biomechanical Study of Side Steps

mailto:[email protected]






Abstract

Lateral quickness is a crucial component of many sports. However, biomechanical

factors which contribute to quickness in lateral movements have not been understood well.

Thus, the purpose of this study was to quantify three dimensional kinetics of hip, knee, and

ankle joints in side steps to understand the function of lower extremity muscle groups. Side

steps at nine different distances were performed by nine male subjects. Kinematic and ground

reaction force data were recorded, and net joint torque and work were calculated by a

standard inverse-dynamics method. Extension torques and work done at hip, knee, and ankle

joints contributed substantially to the changes in side step distances. On the other hand, hip

abduction work was not as sensitive to the changes in the side step distances. The main roles

of hip abduction torque and work were to accelerate the center of mass laterally in the earlier

phase of the movement and to keep the trunk upright, but not to generate large power for

propulsion.

Keywords: joint torque/ work, hip abduction, quickness, inverse dynamics, three

dimensional motion analysis

Word Count: 4533 words [introduction-discussion]




Introduction

Quickness in lateral movements is crucial for skill performance in various sports

such as basketball, tennis, and football. In those sports, athletes are required to have an ability

to move quickly in the lateral direction to catch a ball, to defend an opponent, or to evade a

defender. Therefore, considerable attention has been given to investigating the abilities that

are required to perform lateral movements quickly, like change-of-direction movements. For

example, Little and Williams1 examined relations between a 10 m test, a flying 20 m test, and

a zigzag test and found relatively low coefficients of determination between them. Based on

the results, they concluded that the ability to accelerate, sprint, and change direction are

relatively independent. Elsewhere, it was reported that the correlations between agility tests

and leg extensor strength or power are generally low.2-3

The conclusion was that though leg

extensor strength and power could predict a portion of the ability of the change of direction

movement, technical factors and muscle groups other than extensors should be also taken into

consideration to predict the ability better.

In order to reveal which muscle groups or what kind of technique contribute to

enhance quickness in lateral movements, we believe it is essential to investigate the

mechanisms of the movements biomechanically. Over the past few decades, a lot of

researchers have provided biomechanical and electromyographic data of the change of

direction movements. However, most of those studies were aimed to reveal the causes or

prevention methods of knee ligament injuries4-13

that could occur during lateral movements.

For instance, Besier et al.14

have reported that large knee joint valgus/varus and

external/internal moments are applied at the supporting limb during cutting movements, and

suggested that those could be the causes of the knee ligament injuries. These studies have

quantified the joint moments at the supporting limb and identified the knee injury

mechanisms. However, it is not understood how the joint moments at lower limb contribute




to propel the body laterally, or to the intended direction. Neptune et al.15

investigated

side-shuffle and v-cut movements to provide a database of kinematic and electromyographic

data to describe muscle functions during those movements as well as to understand the causes

of ankle sprain injury. They reported coordination of lower limb muscles, and identified

possible muscle functional roles during those movements by combining the kinematic and

eletromyographic data. Yet, which muscle groups most contribute to generate lateral velocity

is not understood, and that is the important question that needs to be solved in order to find

the determinant factor of quickness in lateral movements.

The inverse dynamics method which calculate net joint torques, power, and work has

been used to understand mechanical function of groups of lower limb muscles16

in forward

and upward movements, such as running,17-20

and vertical and horizontal jumps.21-25

Conducting joint kinetic analysis on these movements were effective because the objective of

the movement was clearer, such as maximizing velocity toward upward or forward direction.

On the other hand the change of direction movement literally involves movements in at least

two directions. In fact, the contribution of joint kinetics to generate lateral propulsion in

movements only toward the lateral direction is poorly understood. Thus, the purpose of this

study was to investigate the contribution of hip, knee and ankle joint three-dimensional

kinetics in generating lateral propulsion in side steps from quiet stance.

We expected that the kinetic variables that contribute to lateral velocity at take-off

would increase when the step distances increased. Side steps consist mainly of hip, knee, and

ankle extension/flexion, and hip abduction/adduction motions. Among those, a greater degree

of hip abduction motion is a unique feature to side steps when compared to upward or

forward movements. Thus, hip abduction motion is sometimes considered as a key movement

for lateral movements and training of hip abductors are recommended for lateral

movements.26-27

Kea et al.28

reported that hip abduction and adduction strength were not the




determinant of side hopping distances. Some researchers who insist that hip abduction is a

key movement for lateral movements also insist that hip abductors act as a stabilizer, too. It is

obvious that hip abduction motion is involved more in side steps than in sagittal plane

movements; however, the role of hip abduction motion in generating lateral velocity or to

provide stability is not understood. As for extension, a low but significant relation was found

between leg extensor strength and power and agility performance.2-3

Based on these findings,

it was predicted that even though hip abduction motion is important for lateral movements,

increased side stepping distances depend on extension kinetics but do not depend on hip

abduction kinetics. Thus, we hypothesized that kinetic variables related to lower limb joint

extension would increase with the step distances while those of hip abduction would not.

Methods

Experimental design

Nine healthy male subjects (age: 24.3 ± 1.1 y; height: 1.74 ± 0.06 m; body mass:

75.6 ± 9.9 kg; mean ± standard deviation) participated in the experiment. The present study

was approved by the ethics committee of the University of Tokyo. The subjects read the

description of the purpose, risks, and basic procedures of the experiment and gave their

written informed consents for participation.

Subjects performed leftward side steps at nine different distances that were set to

20-100 % (D20-D100: at intervals of 10 %) of their height. The target distances were marked

with pieces of color tape on the floor. Subjects pushed off the floor with their right foot, and

landed with their left foot. They kept their arms akimbo and faced forward throughout the

trial. The subjects were asked to make the movement time as short as possible. After adequate

practice, they performed side steps at nine different distances in a random order to account

for fatigue, until three successful trials for each distance were obtained.




Data collection

Thirty one reflective markers were attached to the anatomical landmarks of the

subjects to record three-dimensional body movements by 3D-Motion Analysis System with

seven cameras at 200 Hz (HAWK Digital System, Motion Analysis Corporation, Santa Rosa,

CA, USA). Twelve segments were defined by the anatomical landmarks: head, trunk, right

and left upper arms, right and left forearms, right and left thighs, right and left shanks, right

and left feet. The right foot was placed on a force plate (9281B, Kistler, Switzerland), and

ground reaction forces (GRF) were recorded at 1000 Hz.

Data Analysis

Kinematic and kinetic data were calculated for the push-off leg. Step phase was

defined as the range from the moment of start of the step movements to the moment of

take-off of push-off foot, which was determined by GRF data. All data were normalized to

step phase. In the analysis, the mean of the data of three steps were used as the representative

value of each step distance of each subject.

Kinematic Data

The origin of the global axes was set to the corner of the force plate. The X, Y, and Z

axes were set to medial-lateral, anterior-posterior, and vertical directions, respectively. The

subjects stepped along the direction of global X-axis. The obtained positional data of markers

attached to the body were smoothed using a fourth-order zero phase shift Butterworth

low-pass filter at the cut off frequency of 6 Hz. The cut-off frequency was determined by

conducting a residual analysis.29

The center of mass of the whole body (COM) was calculated

based on the body segment parameters reported by de Leva et al.30

The position of COM was

expressed with respect to the position of center of pressure (COP) measured by the forceplate

under the right foot. The distance between COP and COM also represents the foot placement

with respect to COM. The lateral and the vertical components of velocity of COM at take-off




(vlateral and vvertical) with respect to the global axes were calculated by numerical

differentiation.

The local coordinate systems were set to each of pelvis, thigh, shank, and foot

segments. They were all right-handed orthogonal systems which were defined using the cross

products of unit vectors defined by anatomical landmarks on each segment. The x-axis of the

pelvis segment was the unit vector from the left anterior iliac spine (ASIS) to the right ASIS;

the z-axis was the cross product of the x-axis and the unit vector from the midpoint of right

and left posterior iliac spine to the midpoint of right and left ASIS; and the y-axis was the

cross product of the z- and x-axes. The z-axis of the thigh segment was the unit vector from

knee joint center to hip joint center; the x-axis was the cross product of the unit vector

defined by the cross product of the vector from the hip joint center to the lateral epicondyle of

the femur and the vector from the hip joint center to the medial epicondyle of femur and

z-axis; and the y-axis was the cross product of the z-axis and x-axis. The z-axis of the leg

segment was the unit vector from ankle joint center to knee joint center; the x-axis was the

cross product of the unit vector defined by the cross product of the vector from knee joint

center to lateral malleolus and the vector from knee joint center to medial malleolus and

z-axis; and the y-axis was the cross product of z-axis and x-axis. The y-axis of the foot

segment was the unit vector from calcaneal tuberosity to head of the second metatarsal; the z

axis was the cross products of the unit vector from head of the first metatarsal to head of the

fifth metatarsal and y-axis; and the x-axis was the cross products of the y-axis and z-axis. The

local coordinate system of a proximal segment of a joint was used as a rotation axis. Joint

angles were calculated as the relative position of distal segment with respect to proximal

segment by using Cardan angle definition (x-y’-z’’ sequence).29

The rotation angle around the

x-axis was defined as hip and knee extension/flexion and ankle plantarflexion/dorsiflexion,

around the y’-axis as hip abduction/adduction, knee valgus/varus, and ankle




eversion/inversion, and around the z’’-axis as hip and knee external/internal rotation and

ankle abduction/adduction. The range of joint angle change was calculated for each axis of

rotation as the difference between minimum and maximum values of joint angles around each

axis in the step phase.

The orientation of the push-off leg was analyzed by computing the angle of the line

connecting right ankle joint center and right hip joint center, with respect to the global

medial-lateral axis. Also, the orientation of trunk was analyzed by computing the angle of the

line connecting right ASIS and left ASIS, with respect to the global medial-lateral axis.

Kinetic Data

The orientation of the ground reaction forces (GRF) in the frontal plane was

calculated as follows:

The orientation of the GRF in the frontal plane = arctan (GRFvertical/GRFlateral)

Three dimensional net joint torques were calculated by the standard inverse-dynamics

calculation, using GRF data and kinematic data.29

The net joint torque calculated in this study

is expressed as the internal (muscle) moments with respect to proximal segment local

coordinate system. Joint power was calculated as the product of joint torque and joint angular

velocity. Total joint work was calculated by integrating the joint power from the start of the

movement to toe-off. Positive and negative joint work was computed as the numerical

integration of only positive or negative values, respectively. The sum of positive or negative

work was the sum of positive or negative portion of the work at all joints around each axis

during the whole step phase. Joint power, joint angular velocity, and joint work were

calculated with respect to the proximal segment local coordinate system.

Statistics

One-way repeated-measures analysis of variance (ANOVA) tests were conducted for

vlateral and vvertical, frontal angle of GRF, the lateral position of lowest point of the COM, range




of motion, peak value of the joint torque, joint angular velocity, and joint power and joint

work at hip, knee, and ankle joints. When the effect of distance was found, post-hoc multiple

comparison Tukey tests were conducted for establishing differences between step distances

(significance level p<0.05).

Results

Even though hip and knee extension torques and ankle plantarflexion torque

increased as the step distances increased, hip abduction torque did not increase with the step

distances (Fig.1) as opposed to the change in hip abduction angles (Fig.4). Significant effects

of step distances (p<0.05) were found in peak torques at all axes of the hip, knee, and ankle

joints except knee rotation. The peak amplitude of hip extension torque increased from 0.42 ±

0.18 N m•kg-1

at D20 to 2.01 ± 0.29 N m•kg-1

at D100 (D20<D40<D60<D80<D90-D100;

p<0.05, partial η2=0.91, (a) in Fig.1). Also, increases in peak torques were observed in knee

extension torque from 0.27 ± 0.39 N m•kg-1

at D20 to 2.11 ± 0.66 N m•kg-1

at D100

(D20-D30<D40< D50<D80-D90; p<0.05, partial η2=0.85, (b) in Fig.1). However, there

were no significant increases from D60 to D100 in knee extension peak torques. The peak

ankle plantarflexion torque also increased from 1.11 ± 0.37 N m•kg-1

at D20 to 1.78 ± 0.27 N

m•kg-1

at D50 but did not increase in distances longer than D50 (D20<D40< D70-D100;

p<0.05, partial η2=0.72, (c) in Fig.1). On the other hand, even though the effect of the

distance was found (p<0.05), the peak amplitude of hip abduction torque did not increase

with the step distances (D20>D80-D100; p<0.05, partial η2=0.62, (d) in Fig.1). There were no

significant changes from 1.29±0.25 N m•kg-1

at D20 to 1.16 ± 0.26 N m•kg-1

at D70 in hip

abduction torque. It even decreased from D20 to 1.09 ± 0.29 N m•kg-1

at D100 ((d) in Fig.1).

The peak ankle eversion torque increased from 0.25 ± 0.14 N m•kg-1

at D20 to 0.99 ± 0.19 N

m•kg-1

at D100 with the step distances ((e) in Fig.1). However, since the direction of angular




velocity and the joint torque was opposite, the resultant joint power was negative. When the

peaks of joint torques were plotted against vlateral, joint torques that were most sensitive to the

changes in vlateral were those of hip and knee extension, and ankle plantarflexion (Fig.3 left).

The sum of the positive joint work of hip, knee, and ankle joints increased from 0.42

± 0.17 J•kg-1

at D20 to 3.2 ± 0.42 J•kg-1

at D100 as the step distances increased

(D20-D30<D40<D50<D60-D70<D80<D90<D100; p<0.05) (Fig.2). The rate of increase in

positive joint work from D20 to D100 (D100/D20) was greatest in hip extension, and positive

joint work at D100 was 31.3 times greater than that at D20. The rates of increase were

relatively large in work at knee extension (D100/D20=15.4), ankle plantarflexion

(D100/D20=11.7), and ankle eversion (D100/D20=11.3). However, the rate of increase in hip

abduction positive work was as small as 2.09 because it did not increase with the step

distances longer than D60 (D20<D40<D60; p<0.05). The other rates of increase were smaller

(D100/D20 = 2.88~4.54) than those in hip, knee, and ankle joint extension work.

The sum of the negative joint work decreased with the step distances from -0.23 ±

0.14 J•kg-1

at D20 to -2.01 ± 0.66 J•kg-1

at D100 (D20-D30>D50>D70>D90, D100; p<0.05)

(Fig.2). Large negative work was done at hip and knee extension. Negative hip abduction

work decreased from (-3.15 ± 5.21)×10-3

J•kg-1

at D20 to -0.27 ± 0.20 J•kg-1

at D100 with

the step distances (D20-D40>D80~D100; p<0.05). The sum of the total joint work at hip,

knee, and ankle joints increased with the step distances from -0.20 ± 0.07 J/kg at D20 to 1.22

± 0.73 J•kg-1

at D100 (D20<D50<D100; p<0.05). The proportions of hip abduction work to

the sum of the total work were relatively high at the shorter distances compared to those at

longer distances. Total hip abduction work decreased from D60 to D100. When total joint

works were plotted against vlateral, joint works which were most sensitive to the changes in

vlateral were those done at the knee extension/flexion axis, and the ankle




plantarflexion/dorsiflexion axis (Fig.3 right).

The most notable changes found in the range of angle changes when the step

distances increased occurred at hip, knee, and ankle extension/flexion motions and hip

abduction/adduction motions (significant effect of step distance, p<0.05, Fig.4). The range of

hip extension/flexion angle changes increased from 16.4 ± 3.5 degrees at D20 to 47.8 ± 6.7

degrees at D100 ((f) in Fig.4). The range of knee extension/flexion angles increased from

27.5 ± 6.1 degrees at D20 to 58.2 ± 5.7 degrees at D100 ((g) in Fig.4). Also, ankle

plantarflexion/dorsiflexion angles increased from 8.1 ± 3.2 degrees at D20 to 55.6 ± 4.0

degrees at D100 ((h) in Fig.4). Hip abduction/adduction angles increased from 8.4 ± 2.6

degrees at D20 to 41.4 ± 3.9 degrees at D100 ((i) in Fig.4).

The lateral component of velocity of COM at take-off (vlateral) significantly increased

from D20 to D100 (Table 1) as expected. Even though the vertical component of velocity of

COM at take-off (vvertical) also increased with the step distances, it did not show as much

increases as vlateral did. Also, vvertical at any distance was much smaller than vlateral.

COM moved down lower and more laterally at the end of the contermovement phase.

Even though the lateral position of COM with respect to center of pressure (COP) at the

lowest point of COM located significantly more laterally from 0.16 ± 0.06 m at D20 to 0.32 ±

0.04 m at D60, there were no significant changes at the distances longer than D60. The

orientation of the push-off leg decreased with the step distances from D20 to D80. Significant

changes did not exist in the orientation of the trunk (Table 1).

A significant effect of step distance (p<0.05) was also found in the orientation of

GRF in the frontal plane (Table 1). It significantly decreased from D20 to D60, which

indicates that the GRF was tilted more laterally as the step distances increased (Table 1).

However, in the steps at distances longer than D60, the changes in the orientation of GRF in

frontal plane were smaller than the changes from D20 to D60.




Discussion

The purpose of this study was to investigate the contribution of hip, knee, and ankle

joint kinetics in performing side steps by evaluating the three dimensional kinematics and

kinetics of the push-off limb in side steps of increasing distances. We hypothesized that the

kinetic variables related to joint extension would increase with the step distances while those

of hip abduction would not. The main findings of our study that joint kinetic variables around

extension/flexion axis at hip and knee, and around the plantarflexion/dorsiflexion axis at the

ankle increased more than those at the hip abduction/adduction axis supported the hypothesis.

Therefore, it was indicated that lateral propulsion in side steps was mainly increased by

producing joint work primarily around hip and knee extension/flexion axes and ankle

plantarflexion/dorsiflexion axis, but not around the abduction/adduction or external/internal

rotation axes.

The changes in vlateral over the step distances were greater than the changes in vvertical

(Table1). Also, vlateral was always much greater than vvertical (Table1). These results confirm

that the lateral propulsion was mainly affected by controlling the lateral step distances.

Therefore, peaks of joint torques and total joint work was plotted against vlateral to observe the

changes in kinetics when the lateral propulsions are increased (Fig.3). Since joint torques

which were most sensitive to the changes in vlateral were those of hip and knee extension, and

ankle plantarflexion (Fig.3 left), it was indicated that lateral propulsion was mainly affected

by those joint torques. Also, the results that total joint work at the knee extension/flexion axis

and at the ankle plantarflexion/dorsiflexion axis increased with vlateral (Fig.3 right) indicate

that the increases in the lateral step distances, or increases in the lateral propulsion were

accomplished by increasing those total joint works. Even though the total work at hip

extension/flexion axis did not increase as much as at the knee and ankle extension/flexion

axes, the positive work at hip extension/flexion axis increased with the step distances. Thus,




positive joint work at hip extension/flexion axis also contributed to lateral propulsion. These

results are also consistent with the muscle functions predicted by electromyographic and

kinematic data reported by Neptune et al.15

that concentric activity of hip and knee extensors

and ankle plantar flexors provide acceleration in lateral movements. In addition, it is

reasonable to increase the lateral step distances by increasing joint extension motions, taking

the anatomical configuration of the human lower extremity into account. Calculating the

physiological cross sectional area of extensors and other muscles of the lower extremity

based on the parameters reported by Friederich and Brand,31

about seventy percent of the

total physiological cross sectional area is involved in extension or flexion. Also, the range of

motion was greater in flexion and extension than motions in the frontal and transverse

planes.32

Subjects took the strategy to activate the powerful extensors to perform greater joint

work.

Though the amplitude of hip and knee extension torques and ankle plantarflexion

torque increased with the step distances, the distances that the joint torques reached the

plateau were different between joints (Fig.2). In fact, it has been reported that when

increasing jump height from submaximal height to maximal height, joint torques increase in

the order from distal to proximal joints.33

They suggested that their subjects took the strategy

to increase the contribution of distal segments and minimize the movements of proximal

segments that have larger moment of inertia in submaximal jumps for enhancing movement

effectiveness. Hence, it is argued since the subjects of our study were instructed to minimize

the movement time, they took the strategy to increase the joint torques in the order from

distal to proximal joints. That is why ankle plantarflexion and knee extension torque reached

plateau earlier while hip extension torque kept increasing with step distance. This strategy

that minimized the movement of the proximal, or hip joint motion explains why joint total




work at the hip extension/flexion axis did not change as much as work at the knee and ankle

extension/flexion axes in the shorter distances.

Hip abduction torque and total work were not as sensitive to increases in vlateral as

hip, knee, and ankle extension torques and total work (Fig.3). These results suggest that it is

unlikely that hip abduction work directly contributed to increase the lateral step distances or

lateral propulsions. Nevertheless, the range of hip abduction angle changes increased with the

step distances. Also, the proportion of the hip abduction work to the sum of the total work

was relatively large at the shorter distances. Thus, the role of hip abduction movement in side

steps should not be neglected.

The positive work done at the hip abduction/adduction axis is suggested to

contribute to the acceleration of COM in the earlier phase of the side steps. The positive work

done at the hip abduction/adduction axis increased from D20 to D60, but it did not increase in

the distances longer than that (Fig.2). At the same time, the lowest position of COM moved

more laterally (Table 1), and the orientation of GRF tilted more laterally from D20 to D60

(Table1). It has been previously reported that in walking hip abductors actively control the

medial-lateral balance of COM by accelerating COM medially.34

Therefore, it is suggested

that the positive work done at hip abduction/adduction axis played a role in accelerating

COM in the earlier phase of the movement, and controlled the COM position by the time that

COM reached its lowest position. In the steps at shorter distances, not as much hip, knee, and

ankle extension work were required because the lateral propulsion to be acquired was smaller.

Therefore, the step movements were mostly accomplished just by accelerating COM by

positive hip abduction work. In the steps at longer distances, greater extension work was

necessary for acquiring greater total work to produce faster initial lateral velocity at take-off.

Still, a certain amount of positive hip abduction work must have been done to tilt the body

laterally and adjust the direction of COM acceleration to be acquired by extension work.




Fukashiro et al.23

suggested that the trunk orientation is tuned by the start of the push-off

phase so that the body is accelerated to the desired direction during the push-off phase.

Therefore, it is suggested that hip abduction work contributed to adjust the lateral position of

the COM before the start of the push-off phase so that the acceleration of COM that results

from joint extension work at hip, knee, and ankle joints during the push-off phase is directed

to the right direction in the steps of longer distances.

In addition, it is considered that hip abduction/adduction torque and joint work

contributed to keep the trunk upright. Even though the orientation of the push-off limb

decreased with the step distances, trunk orientation did not change. The push-off leg is

thought to tilt laterally because of gravity. Lyon and Day35

has reported that since COM

“falls” passively by gravity in stepping, the motion of COM is determined at the time when

the leading limb leaves the ground. If no joint torque was exerted, both push-off leg and trunk

would fall laterally by gravity. However, in this study, the orientation of the trunk was kept

upright at all phase of the steps. In walking, it has been said that the hip abductors act to keep

the pelvis level36

or to control the frontal plane body balance.37

Therefore, it is indicated

that hip abduction torque played role in keeping the trunk upright while the push-off leg was

tilted more and more laterally by gravity. This suggestion is in agreement with a previous

study that suggested that hip abductors and adductors did not act as the power generators but

as the hip stabilizer.15

In addition, the results of our study support the results of a previous

study that relatively low correlations exist between hip abductor and adductor strengths and

single-leg lateral and medial hop tests,28

since strengthening of stabilizer would not directly

contribute to increase the stepping distances. Therefore, we conclude that hip abduction work

mainly contributed to accelerating COM laterally in the earlier phase of the movement, and to

keep the trunk upright, but did not directly act to increase the side step distances.




The practical implications of this work are to strengthen the extensors and plantar

flexors of the lower extremity to increase the lateral step distances or lateral propulsion in

side steps since hip and knee extension, and ankle plantarflexion torques and work increased

with the lateral step distances. In addition, COM needs to be positioned laterally with respect

to COP or foot before the positive hip and knee extension works and ankle plantarflexion

works are done. Hence, it is recommended to facilitate quick lateral foot placement in order

to accelerate COM in a shorter duration. A certain level of strength in the hip abductors is

required to perform lateral movements quickly since they control the direction of COM

acceleration and the posture of the trunk. However, since hip abductors did not play a main

role in increasing side step distances, strengthening of extensor muscle groups will yield

greater performance gains than training hip abductors.




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Figure 1: Joint torques of hip, knee, and ankle joints of push-off limb during side steps at

D20, D40, D60, D80, and D100. Mean variables of nine subjects are plotted. The peak

amplitude of (a) hip extension torque, (b) knee extension torque, (c) ankle plantarflexion

torque, and (e) ankle eversion torque increased with the step distances. The peak amplitude of

(d) hip abduction torque did not increase with the step distances.




Figure 2: (top) positive joint work and (bottom) negative joint work. Black bar: hip and knee

extension/flexion and ankle plantarflexion/dorsiflexion work. Gray bar: hip

abduction/adduction, knee valgus/varus, and ankle eversion/inversion work. White bar: hip

and knee exteral/internal rotation and ankle abduction/ adduction work.




Figure 3: (left) The relationship between lateral velocity of COM at take-off and peak torque

of hip (top), knee (middle), and ankle joint (bottom). (right) The relationship between lateral

velocity of COM at take-off and total work of hip (top), knee (middle), and ankle joint

(bottom). Mean variables of nine subjects are plotted, and vertical line represents SD.




Figure 4: Joint angles of hip, knee, and ankle joints of push-off limb during side steps at D20,

D40, D60, D80, and D100. Mean variables of nine subjects are plotted. The range of joint

angle changes in (f) hip extension/flexion, (g) knee extension/flexion, (h) ankle

plantarflexion/dorsiflexion, and (i) hip abduction/adduction increased with the step distances.




Table 1: Mean and SD of vlateral, vvertical, the orientation of GRF, the push-off limb, and the

trunk in frontal plane, and COP-COM distance when COM is at the lowest position. *: partial

η2

Step Distance D20 D30 D40 D50 D60 D70 D80 D90 D100 Statistics (p<0.05) Effect

Size*

Vlateral

(m•s-1)

mean 0.47 0.76 1.01 1.33 1.64 1.92 2.19 2.40 2.56 D20<D30<D40

<D50<D60<D70

<D80<D90<D100

0.99

SD 0.06 0.12 0.11 0.10 0.08 0.08 0.12 0.13 0.12

Vvertical

(m•s-1)

mean -0.04 -0.01 0.11 0.17 0.17 0.17 0.24 0.32 0.33

D20<D50-D100 0.55 SD 0.07 0.05 0.11 0.12 0.14 0.17 0.17 0.19 0.26

Orientation

of the GRF

(deg.)

mean 79.9 78.0 75.8 73.4 71.1 69.4 68.2 68.1 66.8 D20-D30>D40>D50

>D60-D70>D100 0.93

SD 1.3 1.6 1.9 2.6 2.6 2.8 3.5 3.8 3.6

Orientation

of the

Push-off

Limb (deg.)

mean 86.8 82.5 79.2 76.5 73.8 70.7 69.1 69.2 69.8

D20>D40>D60>

D80-D100 0.85

SD 3.2 3.9 4.0 4.8 5.9 6.4 6.4 7.6 8.7

Orientation

of the Trunk

(deg.)

mean -0.1 0.8 0.8 1.0 1.4 1.0 1.2 1.1 0.2 n.s.

SD 3.1 2.8 2.7 2.8 2.6 3.2 3.2 2.7 2.9

COP-COM

Distance

(m)

mean 0.16 0.23 0.27 0.29 0.32 0.36 0.38 0.37 0.36

D20<D30<D60-D100 0.78 SD 0.08 0.10 0.09 0.04 0.04 0.04 0.04 0.05 0.07

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