Research Report                                  Voluntary Activation and Decreased                                  Force...
T        otal knee arthroplasty (TKA) predictably                              tary activation can result from pain,22 eff...
Table.Group Descriptionsa                                                 TKA Group                                       ...
If maximal voluntary force output was achieved and no                                                                     ...
Figure 2.Example of a force trace recorded during a burst superimposition test ofthe quadriceps femoris muscle. The centra...
riceps femoris muscle contraction should take into con-                                                                   ...
and transfers6,10,11 should not be ignored. Simply achiev-                 9 Connelly DM, Vandervoort AA. Effects of detra...
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Voluntary activation and decreased force production of the qs after total knee arthroplasty


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

  1. 1. Research Report Voluntary Activation and Decreased Force Production of the Quadriceps Femoris Muscle After Total Knee Arthroplasty Background and Purpose. Quadriceps femoris muscle weakness as mani- fested by a decrease in force-generating capability is a persistent problem after total knee arthroplasty (TKA). The authors hypothesized that (1) patients with a TKA would have decreased quadriceps femoris muscle performance (weakness) and impaired volitional activation when com- pared with a group of older adults without knee pathology, (2) pain and age would account for a large portion of the variability in volitional activation after surgery, and (3) volitional activation in the TKA group would 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 (mean age 64.9 years, SD 7.72, range 49 –78) with a diagnosis of osteoarthritis who had undergone a tricompartmental, cemented TKA. Methods. Knee extension force was measured using a burst superimposition technique, where a supramaximal burst of electrical stimulation was superimposed on a maximal voluntary isometric contraction (MVIC). The amount of failure of volitional activation is determined by the amount of electrical augmen- tation of force beyond a person’s MVIC at the instant of the application of the electrical burst. Results. The average normalized knee extension force of the TKA group was 64% lower than that of the comparison group. The average volitional activation deficit in the TKA group (26%) was 4 times as APTA is a sponsor of the great as the comparison group’s deficit (6%). Age did not correlate with Decade, an international, quadriceps femoris muscle activation, and knee pain explained only a multidisciplinary initiative small portion of the variance in knee extension force (r 2 .17). Volitional to improve health-related quality of life for people with activation was highly correlated with knee extension force production musculoskeletal disorders. (r 2 .65). Discussion and Conclusion. Considerable quadriceps femoris muscle inhibition after surgery has several implications for recovery. Rehabilitation programs that focus on volitional exercise alone are unlikely to overcome this pronounced failure of activation. Early interven- tions focused at improving quadriceps femoris muscle voluntary activation may improve efforts to restore muscle force. [Mizner RL, Stevens JE, Snyder-Mackler L. Voluntary activation and decreased force production of the 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
  2. 2. T otal knee arthroplasty (TKA) predictably tary activation can result from pain,22 effusion,23,24 and reduces knee pain, but it has had limited success joint damage,13 all of which are potentially present in in restoring quadriceps femoris muscle force- patients after TKA. generating capacity and function to that ofage-matched people without osteoarthritis.1– 6 Decreased Diminished activation has been implicated as a contrib-quadriceps femoris muscle production is a major impair- uting factor in preventing rapid and full recovery ofment following TKA.1,6,7 Knee extension force deficits of quadriceps femoris muscle force following anterior cru-30% to 40% compared with knee extension force in ciate ligament reconstruction and in patients with pain-age-matched subjects without knee disease have been ful patellofemoral disorders.16,19,25 Typically, twitch-reported to exist a year or more after surgery.2 Impair- interpolation or burst superimposition of electricalment of quadriceps femoris muscle performance has stimulation has been used to quantify the extent ofbeen correlated with fall risk,8 ambulation speed,9 –11 voluntary activation failure of a muscle.16,21 Neitherspeed and quality of sit-to-stand transfers,11 and perfor- technique has been used to examine activation deficitsmance during stair climbing in individuals greater than in patients after TKA. Determining the extent of volun-60 years of age.6 tary activation of patients may prove critical to designing and implementing effective rehabilitation programs.Despite the relationship between knee extension force Hurley et al14 reported that strength training, whichand functional ability, decreased quadriceps femoris included 4 weeks of intensive isokinetic training tomuscle performance after TKA has gone relatively unex- address decreased quadriceps femoris muscle perfor-amined. Investigators1– 6 have measured knee extension mance, had limited success in resolving voluntary activa-force as an outcome variable months to years after surgery. tion failure and improving force production in patientsAlthough these studies provide valuable information for with a substantial activation failure. The purposes of ourunderstanding the long-term condition of the knee exten- investigation were: (1) to quantify the extent of quadri-sors following TKA, they do not provide information con- ceps femoris muscle force deficits and voluntary activa-cerning the cause of this persistent decrease in force. The tion deficits in patients who had undergone TKA com-early period after surgery has received little scrutiny, yet this pared with older people without known knee pathologyperiod is when patients typically begin outpatient rehabil- and (2) to determine the effect of knee pain and age onitation to address, among other things, decreased quadri- the voluntary activation of the knee extensors of theceps femoris muscle performance. lower extremity that underwent the TKA. We hypothe- sized that (1) patients after TKA would have lowerBoth atrophy and failure of volitional activation of the normalized quadriceps femoris muscle force andquadriceps femoris muscle have been suggested as decreased voluntary activation when compared with acauses of deceased muscle force in people with knee group of older adults without knee pathology, (2) painosteoarthritis as well as in older adults.12–18 Failure of and age would account for a large portion of thevoluntary activation can be operationally defined as the variability in voluntary activation after surgery, andinability to produce all available force of a muscle (3) voluntary activation in the TKA group would accountdespite maximal conscious effort.19 –21 A failure of volun- for 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) ( 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
  3. 3. Table.Group Descriptionsa TKA Group Comparison Group n X SD Range n X SD Range Sex Men 32 28 Women 20 24 Age (y) 64.9b 7.72 49–78 72.2 5.34 64–85 2 b BMI (kg/m ) 29.2 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 9.04–41.1a 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).Method Muscle Force and Voluntary Activation Measurement All subjects participated in a measurement session of aSubjects maximal voluntary isometric contraction (MVIC) of theTwo groups of subjects were studied: older adults with- quadriceps femoris muscle with a burst superimpositionout knee pathology (comparison group) and patients technique. They were seated in an electromechanicalwho had undergone a primary TKA 3 to 4 weeks prior to dynamometer (Kin-Com 500 H).* The TKA group satthe measurement session (Table). The comparison with the hip flexed to 90 degrees and the knee flexed togroup comprised 52 volunteers (mean age 72.2 years, 75 degrees, and the comparison group sat with the hipSD 5.34, range 64 – 85) recruited from local senior and knee flexed to 90 degrees. The arthroplasty groupcenters and exercise facilities in the Wilmington, Del, was tested at 75 degrees instead of 90 degrees because wearea. All subjects in the comparison group participated anticipated that a relatively large number of subjectsin a regular exercise program that included at least 30 either would be unable to achieve 90 degrees of flexionminutes of regular cardiovascular exercise (such as at 3 to 4 weeks after surgery or would be unable towalking, cycling, swimming, or tennis) 3 times per week. achieve that range without pain.The TKA group comprised 52 patients (mean age 64.9years, SD 7.72, range 49 –78) with a diagnosis of osteo- The axis of the dynamometer was positioned at the axisarthritis who had undergone a tricompartmental, of rotation of the knee joint, and the distal edge of thecemented TKA. shin attachment was placed 2 in (5.08 cm) proximal to the lateral malleolus of the test leg. A waist and a trunkPatients were recruited from a consortium of orthopedic strap were used for stabilization. Two self-adhesive elec-surgeons from the Wilmington, Del, area who a per- trodes (7.6 cm 12.7 cm)† were placed over theformed tricompartmental, cemented TKA with a medial quadriceps femoris muscle at the motor point of theparapatellar surgical approach. Potential subjects for the vastus medialis and proximal rectus femoris musclesTKA group were excluded if they had a body mass index (Fig. 1). Subjects performed 2 submaximal contractions(BMI weight [in kilograms]/[height (in meters)]2) and 1 MVIC lasting 2 to 3 seconds each in order to warmgreater than 40 (morbidly obese) or if they had evidence up the muscle and to familiarize the patient with theof: (1) musculoskeletal impairments, other than the testing procedure.TKA, that limited function in the lower extremity to betested; (2) uncontrolled blood pressure; (3) diabetes After 5 minutes of rest, subjects were instructed tomellitus, because even subtle peripheral neuropathy maximally contract the quadriceps femoris muscle foraffects conduction of the electrical stimulation; (4) neo- approximately 4 seconds. Verbal encouragement andplasms; or (5) neurological disorders. All subjects gave visual output of their force were used to motivate thewritten informed consent. * Chattecx Corp, 6431 Pythian Rd, Harrison, TN 37341-3902. † CONMED Corp, 310 Broad St, Utica, NY 13501.Physical Therapy . Volume 83 . Number 4 . April 2003 Mizner et al . 361
  4. 4. If maximal voluntary force output was achieved and no augmentation of force was observed due to the stimula- tion (ie, there was already optimal recruitment), then the testing session was concluded for that limb. If augmentation was present during the application of the electrical stimulus, the test was repeated. Five minutes of rest was provided between contractions in an effort to minimize muscular or neuromuscular fatigue. A maxi- mum of 3 trials was recorded. The highest volitional force achieved during the 3 attempts was used for analysis. A weight correction was performed automati- cally by the computer program by adding the baseline force while the patient was relaxed to the force measure- ment. Burst superimposition testing was performed on the uninvolved limbs of the TKA group and then on the operated limb. Only the right lower extremity was tested in the comparison group. The burst superimposition technique has been shown to be highly reliable in subjects without pathology (mean age 24.2 years, range 17–32), with repeated testing that demonstrated an intraclass correlation coefficient of .98.26 Pain Measurement A numeric rating scale was used to quantify knee pain during burst superimposition testing. Subjects with TKA were asked to verbally rate the pain in and around the knee during the burst superimposition test on a scale from 0 to 10, where 0 represented no pain and 10 represented the worst pain imaginable. Subjects were asked to rate only knee pain and not the discomfort in the thigh associated with the level of electrical stimula- tion during test. The knee pain rating given during the attempt that produced the greatest force was used for analysis. Numeric rating scales are easy to administer and have exhibited a Pearson product moment correlation of greater than .94 in within day test-retest collections in people with arthritis.27 Data Management and AnalysisFigure 1. Two measures of knee extension force production wereElectrode placement for burst superimposition testing. used for analysis: peak volitional force normalized to BMI and a quadriceps index (QI). Peak volitional force was normalized to allow for comparison with the unin-subjects to produce an MVIC. Approximately 3 seconds jured group. The QI was determined by dividing theinto the contraction, the stimulator (Grass S8800 stimu- MVIC of the involved quadriceps femoris muscle by thelator with a Grass model SIU8T stimulus isolation unit‡) MVIC of the contralateral, uninvolved quadriceps femo-delivered a supramaximal electrical stimulus of ris muscle.monophasic rectangular waves at a rate of 100 pulses persecond for 100 milliseconds at 135 V. The knee exten- The extent of failure of volitional activity of the quadri-sion force was measured and recorded using custom- ceps femoris muscle during the testing was quantifiedwritten software (Labview 4.0.1 and 5.0)§ with a 200-Hz using the central activity ratio (CAR) described bysampling rate. Kent-Braun and Le Blanc.28 The CAR was calculated by dividing the maximal volitional force by the maximal force produced by the combination of volitional effort and a superimposed burst (Fig. 2). A CAR of 1 indicates‡ Grass Instruments, 570 Liberty St, Braintree, MA 02184. complete activation of the muscle with no augmentation§ National Instruments, 6504 Bridge Point Pkwy, Austin, TX 78730.362 . Mizner et al Physical Therapy . Volume 83 . Number 4 . April 2003
  5. 5. 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, Figure 3.MVIC maximal voluntary isometric contraction. Graphic representation of the linear relationship between the age of subjects with total knee arthroplasty and the amount of volitional activation of their involved quadriceps femoris muscle 3 to 4 weeks after surgery.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 group Figure 4.than in the comparison group (Table). The TKA group Relationship between volitional activation and knee pain during burstdisplayed a deficit in the average, normalized voluntary superimposition testing. NRS numeric rating scale.force of 64% compared with the comparison group’saverage, normalized voluntary force (Table). There wasno difference between the normalized voluntary force or who had knee pain during testing had greater failure ofthe CAR of the uninvolved quadriceps femoris muscle of volitional activation than those without knee painthe TKA group and the quadriceps femoris muscle of the (Fig. 5). Volitional activation of the TKA groupcomparison group. The average CAR for the TKA group explained a large portion of the variance in their QI withwas 0.742 (26% volitional activation deficit) as compared a curvilinear model of regression (r 2 .65) (Fig. 6).with the comparison group’s 0.943 (6% volitional acti-vation deficit). Discussion The hypotheses that patients after TKA would produceLinear regression analysis indicated that age of the TKA less force and exhibit greater failure of volitional activa-group did not explain the variance in the CAR variable tion of the quadriceps femoris compared with a compar-(Fig. 3). The knee pain of the TKA group during burst ison group were supported by the data. Although thesuperimposition testing showed a small relationship to TKA group had more men, had a greater average BMI,CAR (r 2 .17) (Fig. 4). Only half (26 of 52) of the was younger, and was tested at a knee angle closer to thesubjects with TKA reported knee pain during burst angle of greatest mechanical advantage for the quadri-superimposition testing. The subjects in the TKA group ceps femoris muscle than the comparison group, therePhysical Therapy . Volume 83 . Number 4 . April 2003 Mizner et al . 363
  6. 6. riceps femoris muscle contraction should take into con- sideration that these patients are more likely to have muscle inhibition. Simply eliminating pain will not pro- vide the panacea for eliminating knee extension inhibition. The subjects’ age did not provide additional information for identifying those subjects with volitional activity deficits. Researchers12,21 have identified small age- related deficits in volitional activation of the quadriceps femoris 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 we observed. Younger patients will likely undergo TKA as the durabil-Figure 5. ity of prostheses continues to improve. Current pros-Comparison of volitional activation of knees of subjects with a total kneearthroplasty grouped by those with or without pain during burst super- thetic devices have a revision rate of less than 10% up toimposition testing. MVIC maximal voluntary isometric contraction. 20 years following surgery. Knee replacement in younger patients is also supported by previous studies that showed that patients with greater function, as measured by self-assessment questionnaire, prior to surgery achieved the greatest functional status following sur- gery.3 The results of our study show that even a relatively young patient (ie, 50 –55 years of age) who has had a TKA is not immune from exhibiting extensive failure of volitional activation with a related decrease in quadri- ceps femoris muscle force following surgery. Chronic, weak knee extensor muscles may make longer functional life of a total knee prosthesis impossible. Failure of volitional activation may play an important role in the cause of the persistent decreased quadriceps femoris muscle production in patients following TKA. Volitional activation deficits of the quadriceps femoris muscle found in studies of patellofemoral dysfunction and knee osteoarthritis have been shown to relate to decreased quadriceps femoris muscle produc- tion.15,17,19,22 Manal and Snyder-Mackler19 showed that patients with volitional activity deficits with patellarFigure 6. contusions had more than twice the percentage ofExponential regression analysis showing the model of quadriceps index decreased quadriceps femoris muscle force than those(side-to-side muscle force comparison) accounting for the variance incentral activation ratio. without reflex inhibition. The average failure of activa- tion of the patients with reflex inhibition and patellar contusion was 14%. The average failure of activation ofwere profound deficits in force production and a large the TKA group in our study (26%) was considerablyaverage failure of volitional activation. The best predic- larger.tor of quadriceps femoris muscle force production wasthe CAR. This relationship emphasizes that subjects who Our data illustrate that decreased quadriceps femorismanifested the greatest decrease in muscle force follow- muscle performance is present 1 month after surgery also displayed the greatest inhibition. Muscle force measurements are not often a part of the assessment of outcomes, whereas reduction in painKnee pain appears to contribute a small amount to the following surgery is often enough to lead to claims offailure of voluntary activation, and we believe this is a excellent surgical success.5 We believe the strong rela-relevant clinical finding to consider in developing reha- tionship between quadriceps femoris muscle force pro-bilitation protocols. We believe that efforts to increase duction and performance during stair climbing, gait,muscle force production in patients with painful quad-364 . Mizner et al Physical Therapy . Volume 83 . Number 4 . April 2003
  7. 7. and transfers6,10,11 should not be ignored. Simply achiev- 9 Connelly DM, Vandervoort AA. Effects of detraining on knee exten-ing pain relief and restoring a functional range of sor strength and functional mobility in a group of elderly women. J Orthop Sports Phys Ther. 1997;26:340 –346.motion in the postoperative knee does not precludestriving for resolution of decreased quadriceps femoris 10 Gibbs J, Hughes S, Dunlop D, et al. Predictors of change in walking velocity in older adults. J Am Geriatr Soc. 1996;44:126 –132.muscle production. Inadequate quadriceps femoris mus-cle rehabilitation could have long-term negative conse- 11 Moxley Scarborough D, Krebs DE, Harris BA. Quadriceps musclequences in patient outcomes and may lead to increased strength and dynamic stability in elderly persons. Gait Posture. 1999;10: 10 –20.fall risk with advancing age. 12 Harridge SD, Kryger A, Stensgaard A. Knee extensor strength, activation, and size in very elderly people following strength training.Conclusion Muscle Nerve. 1999;22:831– 839.The results of our study suggest that postoperativerehabilitation should include tactics to reduce factors 13 Hurley MV. The effects of joint damage on muscle function, proprioception and rehabilitation. Man Ther. 1999;2:11–17.that may propagate poor volitional activation of thequadriceps femoris muscle. Attempting to provide ade- 14 Hurley MV, Jones DW, Newham DJ. Arthrogenic quadriceps inhi- bition and rehabilitation of patients with extensive traumatic kneequate stimulus to promote gains in muscle force produc- injuries. Clin Sci (Lond). 1994;86:305–310.tion with traditional rehabilitation exercises, in ouropinion, will be unlikely to succeed if the patient has a 15 Hurley MV, Newham DJ. The influence of arthrogenous muscle inhibition on quadriceps rehabilitation of patients with early, unilat-pronounced failure of volitional activation. More aggres- eral osteoarthritic knees. Br J Rheumatol. 1993;32:127–131.sive strategies to control pain and pain-provoking inflam-mation, coupled with the use of electrically elicited 16 Morrissey MC. Reflex inhibition of thigh muscles in knee injury: causes and treatment. Sports Med. 1989;7:263–276.contractions for muscle force training or musclere-education, may be more successful in overcoming 17 O’Reilly S, Jones A, Doherty M. Muscle weakness in osteoarthritis. Curr Opin Rheumatol. 1997;9:259 –262.deficits in volitional activation. Tools such as biofeed-back also may be useful in prompting the patient to 18 Slemenda C, Brandt KD, Heilman DK, et al. Quadriceps weaknessmaximize muscle contractions and to develop strategies and osteoarthritis of the knee. Ann Intern Med. 1997;127:97– improve activation during resistive exercises designed 19 Manal TJ, Snyder-Mackler L. Failure of voluntary activation of theto increase muscle force production. quadriceps femoris muscle after patellar contusion. J Orthop Sports Phys Ther. 2000;30:655– 663.References 20 Snyder-Mackler L, De Luca PF, Williams PR, et al. Reflex inhibition1 Bolanos AA, Colizza WA, McCann PD, et al. A comparison of of the quadriceps femoris muscle after injury or reconstruction of theisokinetic strength testing and gait analysis in patients with posterior anterior cruciate ligament. J Bone Joint Surg Am. 1994;76:555–560.cruciate-retaining and substituting knee arthroplasties. J Arthroplasty. 21 Stackhouse SK, Stevens JE, Lee SC, et al. Maximum voluntary1998;13:906 –915. activation in nonfatigued and fatigued muscle of young and elderly2 Finch E, Walsh M, Thomas SG, Woodhouse LJ. Functional ability individuals. Phys Ther. 2001;81:1102–1109.perceived by individuals following total knee arthroplasty compared to 22 O’Reilly SC, Jones A, Muir KR, Doherty M. 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Rehabilitation complications following knee surgery. Clinthritis: a source for patient information. J Arthroplasty. 2000;15: Sports Med. 1999;18:905–925.289 –294. 26 Snyder-Mackler L, Binder-Macleod SA, Williams PR. Fatigability of6 Walsh M, Woodhouse LJ, Thomas SG, Finch E. Physical impairments human quadriceps femoris muscle following anterior cruciate liga-and functional limitations: a comparison of individuals 1 year after ment reconstruction. Med Sci Sports Exerc. 1993;25:783– knee arthroplasty with control subjects. Phys Ther. 1998;78:248 –258. 27 Ferraz MB, Quaresma MR, Aquino LR, et al. Reliability of pain scales in the assessment of literate and illiterate patients with rheuma-7 Berman AT, Bosacco SJ, Israelite C. Evaluation of total knee arthro- toid arthritis. J Rheumatol. 1990;17:1022–1024.plasty using isokinetic testing. Clin Orthop. 1991;(271):106 –113. 28 Kent-Braun JA, Le Blanc R. Quantitation of central activation8 Lord SR, Rogers MW, Howland A, Fitzpatrick R. Lateral stability, failure during maximal voluntary contractions in humans. Muscle Nerve.sensorimotor function and falls in older people. J Am Geriatr Soc. 1996;19:861– 869.1999;47:1077–1081.Physical Therapy . Volume 83 . Number 4 . April 2003 Mizner et al . 365