voluntary activation and decreased force production of the qs after total knee arthroplasty

Voluntary Activation and DecreasedForce Production of the QuadricepsFemoris Muscle After Total KneeArthroplasty

APTA is a sponsor of theDecade, an international,multidisciplinary initiativeto improve health-relatedquality of life for people withmusculoskeletal disorders.

Background and Purpose. Quadriceps femoris muscle weakness as mani-fested by a decrease in force-generating capability is a persistent problemafter total knee arthroplasty (TKA). The authors hypothesized that(1) patients with a TKA would have decreased quadriceps femoris muscleperformance (weakness) and impaired volitional activation when com-pared with a group of older adults without knee pathology, (2) pain andage would account for a large portion of the variability in volitionalactivation after surgery, and (3) volitional activation in the TKA groupwould account for a large portion of the variability in force production.Subjects. Comparison subjects were 52 volunteers (mean age�72.2 years,SD�5.34, range�64–85). The TKA group comprised 52 patients (meanage�64.9 years, SD�7.72, range�49–78) with a diagnosis of osteoarthritiswho had undergone a tricompartmental, cemented TKA. Methods. Kneeextension force was measured using a burst superimposition technique,where a supramaximal burst of electrical stimulation was superimposed ona maximal voluntary isometric contraction (MVIC). The amount of failureof volitional activation is determined by the amount of electrical augmen-tation of force beyond a person’s MVIC at the instant of the application ofthe electrical burst. Results. The average normalized knee extension forceof the TKA group was 64% lower than that of the comparison group. Theaverage volitional activation deficit in the TKA group (26%) was 4 times asgreat as the comparison group’s deficit (6%). Age did not correlate withquadriceps femoris muscle activation, and knee pain explained only asmall portion of the variance in knee extension force (r2�.17). Volitionalactivation was highly correlated with knee extension force production(r2�.65). Discussion and Conclusion. Considerable quadriceps femorismuscle inhibition after surgery has several implications for recovery.Rehabilitation programs that focus on volitional exercise alone areunlikely to overcome this pronounced failure of activation. Early interven-tions focused at improving quadriceps femoris muscle voluntary activationmay improve efforts to restore muscle force. [Mizner RL, Stevens JE,Snyder-Mackler L. Voluntary activation and decreased force production ofthe quadriceps femoris muscle after total knee arthroplasty. Phys Ther.2003;83:359–365.]

Key Words: Knee replacement, Muscle inhibition, Volitional activation.

Ryan L Mizner, Jennifer E Stevens, Lynn Snyder-Mackler

Physical Therapy . Volume 83 . Number 4 . April 2003 359

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Total knee arthroplasty (TKA) predictablyreduces knee pain, but it has had limited successin restoring quadriceps femoris muscle force-generating capacity and function to that of

age-matched people without osteoarthritis.1–6 Decreasedquadriceps femoris muscle production is a major impair-ment following TKA.1,6,7 Knee extension force deficits of30% to 40% compared with knee extension force inage-matched subjects without knee disease have beenreported to exist a year or more after surgery.2 Impair-ment of quadriceps femoris muscle performance hasbeen correlated with fall risk,8 ambulation speed,9–11

speed and quality of sit-to-stand transfers,11 and perfor-mance during stair climbing in individuals greater than60 years of age.6

Despite the relationship between knee extension forceand functional ability, decreased quadriceps femorismuscle performance after TKA has gone relatively unex-amined. Investigators1–6 have measured knee extensionforce as an outcome variable months to years after surgery.Although these studies provide valuable information forunderstanding the long-term condition of the knee exten-sors following TKA, they do not provide information con-cerning the cause of this persistent decrease in force. Theearly period after surgery has received little scrutiny, yet thisperiod is when patients typically begin outpatient rehabil-itation to address, among other things, decreased quadri-ceps femoris muscle performance.

Both atrophy and failure of volitional activation of thequadriceps femoris muscle have been suggested ascauses of deceased muscle force in people with kneeosteoarthritis as well as in older adults.12–18 Failure ofvoluntary activation can be operationally defined as theinability to produce all available force of a muscledespite maximal conscious effort.19–21 A failure of volun-

tary activation can result from pain,22 effusion,23,24 andjoint damage,13 all of which are potentially present inpatients after TKA.

Diminished activation has been implicated as a contrib-uting factor in preventing rapid and full recovery ofquadriceps femoris muscle force following anterior cru-ciate ligament reconstruction and in patients with pain-ful patellofemoral disorders.16,19,25 Typically, twitch-interpolation or burst superimposition of electricalstimulation has been used to quantify the extent ofvoluntary activation failure of a muscle.16,21 Neithertechnique has been used to examine activation deficitsin patients after TKA. Determining the extent of volun-tary activation of patients may prove critical to designingand implementing effective rehabilitation programs.Hurley et al14 reported that strength training, whichincluded 4 weeks of intensive isokinetic training toaddress decreased quadriceps femoris muscle perfor-mance, had limited success in resolving voluntary activa-tion failure and improving force production in patientswith a substantial activation failure. The purposes of ourinvestigation were: (1) to quantify the extent of quadri-ceps femoris muscle force deficits and voluntary activa-tion deficits in patients who had undergone TKA com-pared with older people without known knee pathologyand (2) to determine the effect of knee pain and age onthe voluntary activation of the knee extensors of thelower extremity that underwent the TKA. We hypothe-sized that (1) patients after TKA would have lowernormalized quadriceps femoris muscle force anddecreased voluntary activation when compared with agroup of older adults without knee pathology, (2) painand age would account for a large portion of thevariability in voluntary activation after surgery, and(3) voluntary activation in the TKA group would accountfor a large portion of the variability in force production.

RL Mizner, PT, MPT, is a doctoral student, Biomechanics and Movement Science Program, Department of Physical Therapy, University ofDelaware, Newark, Del.

JE Stevens, PT, MPT, PhD, was a doctoral student, Biomechanics and Movement Science Program, University of Delaware, at the time of the study.Dr Stevens is currently Post-doctoral Associate, Department of Physical Therapy, University of Florida.

L Snyder-Mackler, PT, ScD, SCS, ATC, is Professor, Department of Physical Therapy, University of Delaware, 301 McKinly Laboratory, Newark, DE19716 (USA) ([email protected]). Address all correspondence to Dr Snyder-Mackler.

All authors provided concept/research design, writing, and data analysis. Mr Mizner and Dr Stevens provided data collection. Dr Snyder-Macklerprovided project management and fund procurement. Mr Mizner and Dr Snyder-Mackler provided consultation (including review of themanuscript before submission).

This study was approved by the Human Subjects Review Board of the University of Delaware.

This work was supported by the National Institutes of Health (#1R01HD041055-01A1) and the Foundation for PhysicalTherapy (Mary McMillan Scholarship, PODS I and II Scholarships). The authors will receive no financial benefit from thepublication of these findings.

This article was submitted May 22, 2002, and was accepted October 28, 2002.

360 . Mizner et al Physical Therapy . Volume 83 . Number 4 . April 2003

Method

SubjectsTwo groups of subjects were studied: older adults with-out knee pathology (comparison group) and patientswho had undergone a primary TKA 3 to 4 weeks prior tothe measurement session (Table). The comparisongroup comprised 52 volunteers (mean age�72.2 years,SD�5.34, range�64–85) recruited from local seniorcenters and exercise facilities in the Wilmington, Del,area. All subjects in the comparison group participatedin a regular exercise program that included at least 30minutes of regular cardiovascular exercise (such aswalking, cycling, swimming, or tennis) 3 times per week.The TKA group comprised 52 patients (mean age�64.9years, SD�7.72, range�49–78) with a diagnosis of osteo-arthritis who had undergone a tricompartmental,cemented TKA.

Patients were recruited from a consortium of orthopedicsurgeons from the Wilmington, Del, area who a per-formed tricompartmental, cemented TKA with a medialparapatellar surgical approach. Potential subjects for theTKA group were excluded if they had a body mass index(BMI�weight [in kilograms]/[height (in meters)]2)greater than 40 (morbidly obese) or if they had evidenceof: (1) musculoskeletal impairments, other than theTKA, that limited function in the lower extremity to betested; (2) uncontrolled blood pressure; (3) diabetesmellitus, because even subtle peripheral neuropathyaffects conduction of the electrical stimulation; (4) neo-plasms; or (5) neurological disorders. All subjects gavewritten informed consent.

Muscle Force and Voluntary Activation MeasurementAll subjects participated in a measurement session of amaximal voluntary isometric contraction (MVIC) of thequadriceps femoris muscle with a burst superimpositiontechnique. They were seated in an electromechanicaldynamometer (Kin-Com 500 H).* The TKA group satwith the hip flexed to 90 degrees and the knee flexed to75 degrees, and the comparison group sat with the hipand knee flexed to 90 degrees. The arthroplasty groupwas tested at 75 degrees instead of 90 degrees because weanticipated that a relatively large number of subjectseither would be unable to achieve 90 degrees of flexionat 3 to 4 weeks after surgery or would be unable toachieve that range without pain.

The axis of the dynamometer was positioned at the axisof rotation of the knee joint, and the distal edge of theshin attachment was placed 2 in (5.08 cm) proximal tothe lateral malleolus of the test leg. A waist and a trunkstrap were used for stabilization. Two self-adhesive elec-trodes (7.6 cm � 12.7 cm)† were placed over thequadriceps femoris muscle at the motor point of thevastus medialis and proximal rectus femoris muscles(Fig. 1). Subjects performed 2 submaximal contractionsand 1 MVIC lasting 2 to 3 seconds each in order to warmup the muscle and to familiarize the patient with thetesting procedure.

After 5 minutes of rest, subjects were instructed tomaximally contract the quadriceps femoris muscle forapproximately 4 seconds. Verbal encouragement andvisual output of their force were used to motivate the

* Chattecx Corp, 6431 Pythian Rd, Harrison, TN 37341-3902.† CONMED Corp, 310 Broad St, Utica, NY 13501.

Table.Group Descriptionsa

TKA Group Comparison Group

n X SD Range n X SD Range

SexMen 32 28Women 20 24

Age (y) 64.9b 7.72 49–78 72.2 5.34 64–85

BMI (kg/m2) 29.2b 3.62 21.9–37.1 25.2 3.80 19.9–35.9

Involved CAR 0.742b,c 0.168 0.342–1.00 NA

Uninvolved CAR 0.927 0.065 0.76–1.00 0.943 0.053 0.73–1.00

Involved normalized MVIC (N/BMI) 8.80b,c 4.85 2.97–24.3 NA

Uninvolved normalized MVIC (N/BMI) 24.14 8.02 11.9–42.9 24.15 6.01 9.04–41.1

a TKA�total knee arthroplasty, BMI�body mass index, CAR�central activation ratio (maximal volitional force/maximal force during burst of stimulation),MVIC�maximal voluntary isometric contraction, N�newtons, NA�not applicable.b Significant difference between the TKA group and the comparison group as analyzed with an independent t test (P�.05).c Significant difference between the involved and uninvolved lower extremities in the TKA group as analyzed with a paired t test (P�.05).

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subjects to produce an MVIC. Approximately 3 secondsinto the contraction, the stimulator (Grass S8800 stimu-lator with a Grass model SIU8T stimulus isolation unit‡)delivered a supramaximal electrical stimulus ofmonophasic rectangular waves at a rate of 100 pulses persecond for 100 milliseconds at 135 V. The knee exten-sion force was measured and recorded using custom-written software (Labview 4.0.1 and 5.0)§ with a 200-Hzsampling rate.

If maximal voluntary force output was achieved and noaugmentation of force was observed due to the stimula-tion (ie, there was already optimal recruitment), thenthe testing session was concluded for that limb. Ifaugmentation was present during the application of theelectrical stimulus, the test was repeated. Five minutes ofrest was provided between contractions in an effort tominimize muscular or neuromuscular fatigue. A maxi-mum of 3 trials was recorded. The highest volitionalforce achieved during the 3 attempts was used foranalysis. A weight correction was performed automati-cally by the computer program by adding the baselineforce while the patient was relaxed to the force measure-ment. Burst superimposition testing was performed onthe uninvolved limbs of the TKA group and then on theoperated limb. Only the right lower extremity was testedin the comparison group. The burst superimpositiontechnique has been shown to be highly reliable insubjects without pathology (mean age�24.2 years,range�17–32), with repeated testing that demonstratedan intraclass correlation coefficient of .98.26

Pain MeasurementA numeric rating scale was used to quantify knee painduring burst superimposition testing. Subjects with TKAwere asked to verbally rate the pain in and around theknee during the burst superimposition test on a scalefrom 0 to 10, where 0 represented no pain and 10represented the worst pain imaginable. Subjects wereasked to rate only knee pain and not the discomfort inthe thigh associated with the level of electrical stimula-tion during test. The knee pain rating given during theattempt that produced the greatest force was used foranalysis. Numeric rating scales are easy to administer andhave exhibited a Pearson product moment correlationof greater than .94 in within day test-retest collections inpeople with arthritis.27

Data Management and AnalysisTwo measures of knee extension force production wereused for analysis: peak volitional force normalized toBMI and a quadriceps index (QI). Peak volitional forcewas normalized to allow for comparison with the unin-jured group. The QI was determined by dividing theMVIC of the involved quadriceps femoris muscle by theMVIC of the contralateral, uninvolved quadriceps femo-ris muscle.

The extent of failure of volitional activity of the quadri-ceps femoris muscle during the testing was quantifiedusing the central activity ratio (CAR) described byKent-Braun and Le Blanc.28 The CAR was calculated bydividing the maximal volitional force by the maximalforce produced by the combination of volitional effortand a superimposed burst (Fig. 2). A CAR of 1 indicatescomplete activation of the muscle with no augmentation‡ Grass Instruments, 570 Liberty St, Braintree, MA 02184.

§ National Instruments, 6504 Bridge Point Pkwy, Austin, TX 78730.

Figure 1.Electrode placement for burst superimposition testing.


of the maximal volitional force observed during theburst of electrical stimulation.

Differences in force production, volitional activation,age, and BMI between groups were analyzed usingindependent t tests. Differences in force production andvolitional activation between involved and uninvolvedlower extremities in the TKA group were analyzed withpaired t tests. The effects of age, QI, and knee painduring burst superimposition testing of TKA group wereanalyzed using regression analysis. A probability level ofless than .05 was considered significant for all tests.

ResultsThe TKA group was younger and had a greater BMI thanthe comparison group (Table). Quadriceps femorisforce production and volitional activation in theinvolved lower extremity were lower in the TKA groupthan in the comparison group (Table). The TKA groupdisplayed a deficit in the average, normalized voluntaryforce of 64% compared with the comparison group’saverage, normalized voluntary force (Table). There wasno difference between the normalized voluntary force orthe CAR of the uninvolved quadriceps femoris muscle ofthe TKA group and the quadriceps femoris muscle of thecomparison group. The average CAR for the TKA groupwas 0.742 (26% volitional activation deficit) as comparedwith the comparison group’s 0.943 (6% volitional acti-vation deficit).

Linear regression analysis indicated that age of the TKAgroup did not explain the variance in the CAR variable(Fig. 3). The knee pain of the TKA group during burstsuperimposition testing showed a small relationship toCAR (r2�.17) (Fig. 4). Only half (26 of 52) of thesubjects with TKA reported knee pain during burstsuperimposition testing. The subjects in the TKA group

who had knee pain during testing had greater failure ofvolitional activation than those without knee pain(Fig. 5). Volitional activation of the TKA groupexplained a large portion of the variance in their QI witha curvilinear model of regression (r2�.65) (Fig. 6).

DiscussionThe hypotheses that patients after TKA would produceless force and exhibit greater failure of volitional activa-tion of the quadriceps femoris compared with a compar-ison group were supported by the data. Although theTKA group had more men, had a greater average BMI,was younger, and was tested at a knee angle closer to theangle of greatest mechanical advantage for the quadri-ceps femoris muscle than the comparison group, there

Figure 2.Example of a force trace recorded during a burst superimposition test ofthe quadriceps femoris muscle. The central activation ratio (CAR) for thistest is 0.76 (maximal volitional force [135 N]/maximal force duringburst of stimulation [178 N]). TKA�total knee arthroplasty,MVIC�maximal voluntary isometric contraction.

Figure 3.Graphic representation of the linear relationship between the age ofsubjects with total knee arthroplasty and the amount of volitionalactivation of their involved quadriceps femoris muscle 3 to 4 weeks aftersurgery.

Figure 4.Relationship between volitional activation and knee pain during burstsuperimposition testing. NRS�numeric rating scale.


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were profound deficits in force production and a largeaverage failure of volitional activation. The best predic-tor of quadriceps femoris muscle force production wasthe CAR. This relationship emphasizes that subjects whomanifested the greatest decrease in muscle force follow-ing surgery also displayed the greatest inhibition.

Knee pain appears to contribute a small amount to thefailure of voluntary activation, and we believe this is arelevant clinical finding to consider in developing reha-bilitation protocols. We believe that efforts to increasemuscle force production in patients with painful quad-

riceps femoris muscle contraction should take into con-sideration that these patients are more likely to havemuscle inhibition. Simply eliminating pain will not pro-vide the panacea for eliminating knee extensioninhibition.

The subjects’ age did not provide additional informationfor identifying those subjects with volitional activitydeficits. Researchers12,21 have identified small age-related deficits in volitional activation of the quadricepsfemoris muscle in older adults. In our study, any age-related deficits in volitional activation were likely negli-gible in the presence of the large activation failure weobserved.

Younger patients will likely undergo TKA as the durabil-ity of prostheses continues to improve. Current pros-thetic devices have a revision rate of less than 10% up to20 years following surgery. Knee replacement in youngerpatients is also supported by previous studies thatshowed that patients with greater function, as measuredby self-assessment questionnaire, prior to surgeryachieved the greatest functional status following sur-gery.3 The results of our study show that even a relativelyyoung patient (ie, 50–55 years of age) who has had aTKA is not immune from exhibiting extensive failure ofvolitional activation with a related decrease in quadri-ceps femoris muscle force following surgery. Chronic,weak knee extensor muscles may make longer functionallife of a total knee prosthesis impossible.

Failure of volitional activation may play an importantrole in the cause of the persistent decreased quadricepsfemoris muscle production in patients following TKA.Volitional activation deficits of the quadriceps femorismuscle found in studies of patellofemoral dysfunctionand knee osteoarthritis have been shown to relate todecreased quadriceps femoris muscle produc-tion.15,17,19,22 Manal and Snyder-Mackler19 showed thatpatients with volitional activity deficits with patellarcontusions had more than twice the percentage ofdecreased quadriceps femoris muscle force than thosewithout reflex inhibition. The average failure of activa-tion of the patients with reflex inhibition and patellarcontusion was 14%. The average failure of activation ofthe TKA group in our study (26%) was considerablylarger.

Our data illustrate that decreased quadriceps femorismuscle performance is present 1 month after TKA.Muscle force measurements are not often a part of theassessment of outcomes, whereas reduction in painfollowing surgery is often enough to lead to claims ofexcellent surgical success.5 We believe the strong rela-tionship between quadriceps femoris muscle force pro-duction and performance during stair climbing, gait,

Figure 5.Comparison of volitional activation of knees of subjects with a total kneearthroplasty grouped by those with or without pain during burst super-imposition testing. MVIC�maximal voluntary isometric contraction.

Figure 6.Exponential regression analysis showing the model of quadriceps index(side-to-side muscle force comparison) accounting for the variance incentral activation ratio.


and transfers6,10,11 should not be ignored. Simply achiev-ing pain relief and restoring a functional range ofmotion in the postoperative knee does not precludestriving for resolution of decreased quadriceps femorismuscle production. Inadequate quadriceps femoris mus-cle rehabilitation could have long-term negative conse-quences in patient outcomes and may lead to increasedfall risk with advancing age.

ConclusionThe results of our study suggest that postoperativerehabilitation should include tactics to reduce factorsthat may propagate poor volitional activation of thequadriceps femoris muscle. Attempting to provide ade-quate stimulus to promote gains in muscle force produc-tion with traditional rehabilitation exercises, in ouropinion, will be unlikely to succeed if the patient has apronounced failure of volitional activation. More aggres-sive strategies to control pain and pain-provoking inflam-mation, coupled with the use of electrically elicitedcontractions for muscle force training or musclere-education, may be more successful in overcomingdeficits in volitional activation. Tools such as biofeed-back also may be useful in prompting the patient tomaximize muscle contractions and to develop strategiesto improve activation during resistive exercises designedto increase muscle force production.

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voluntary activation and decreased force production of the qs after total knee arthroplasty

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