1) Patients who had undergone total knee arthroplasty (TKA) 3-4 weeks prior had significantly lower quadriceps force (64% lower) and greater voluntary activation deficits (26% vs 6%) compared to older adults without knee pathology. 2) Age did not correlate with quadriceps activation for TKA patients, and knee pain only explained a small amount of variability in force production. 3) Voluntary activation deficits in TKA patients accounted for a large portion (r^2=0.65) of the variability in their knee extension force production.

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. 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 of
age-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 of
ment 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 electrical
ment of quadriceps femoris muscle performance has stimulation has been used to quantify the extent of
been correlated with fall risk,8 ambulation speed,9 –11 voluntary activation failure of a muscle.16,21 Neither
speed and quality of sit-to-stand transfers,11 and perfor- technique has been used to examine activation deficits
mance 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, which
and functional ability, decreased quadriceps femoris included 4 weeks of intensive isokinetic training to
muscle 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 patients
Although these studies provide valuable information for with a substantial activation failure. The purposes of our
understanding 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 pathology
period is when patients typically begin outpatient rehabil- and (2) to determine the effect of knee pain and age on
itation to address, among other things, decreased quadri- the voluntary activation of the knee extensors of the
ceps femoris muscle performance. lower extremity that underwent the TKA. We hypothe-
sized that (1) patients after TKA would have lower
Both atrophy and failure of volitional activation of the normalized quadriceps femoris muscle force and
quadriceps femoris muscle have been suggested as decreased voluntary activation when compared with a
causes of deceased muscle force in people with knee group of older adults without knee pathology, (2) pain
osteoarthritis as well as in older adults.12–18 Failure of and age would account for a large portion of the
voluntary activation can be operationally defined as the variability in voluntary activation after surgery, and
inability to produce all available force of a muscle (3) voluntary activation in the TKA group would account
despite 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 of
Delaware, 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, DE
19716 (USA) (smack@udel.edu). 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-Mackler
provided project management and fund procurement. Mr Mizner and Dr Snyder-Mackler provided consultation (including review of the
manuscript 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 Physical
Therapy (Mary McMillan Scholarship, PODS I and II Scholarships). The authors will receive no financial benefit from the
publication 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. 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 9.04–41.1
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).
Method Muscle Force and Voluntary Activation Measurement
All subjects participated in a measurement session of a
Subjects maximal voluntary isometric contraction (MVIC) of the
Two groups of subjects were studied: older adults with- quadriceps femoris muscle with a burst superimposition
out knee pathology (comparison group) and patients technique. They were seated in an electromechanical
who had undergone a primary TKA 3 to 4 weeks prior to dynamometer (Kin-Com 500 H).* The TKA group sat
the measurement session (Table). The comparison with the hip flexed to 90 degrees and the knee flexed to
group comprised 52 volunteers (mean age 72.2 years, 75 degrees, and the comparison group sat with the hip
SD 5.34, range 64 – 85) recruited from local senior and knee flexed to 90 degrees. The arthroplasty group
centers and exercise facilities in the Wilmington, Del, was tested at 75 degrees instead of 90 degrees because we
area. All subjects in the comparison group participated anticipated that a relatively large number of subjects
in a regular exercise program that included at least 30 either would be unable to achieve 90 degrees of flexion
minutes of regular cardiovascular exercise (such as at 3 to 4 weeks after surgery or would be unable to
walking, cycling, swimming, or tennis) 3 times per week. achieve that range without pain.
The TKA group comprised 52 patients (mean age 64.9
years, SD 7.72, range 49 –78) with a diagnosis of osteo- The axis of the dynamometer was positioned at the axis
arthritis who had undergone a tricompartmental, of rotation of the knee joint, and the distal edge of the
cemented TKA. shin attachment was placed 2 in (5.08 cm) proximal to
the lateral malleolus of the test leg. A waist and a trunk
Patients 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 the
formed tricompartmental, cemented TKA with a medial quadriceps femoris muscle at the motor point of the
parapatellar surgical approach. Potential subjects for the vastus medialis and proximal rectus femoris muscles
TKA 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 warm
greater than 40 (morbidly obese) or if they had evidence up the muscle and to familiarize the patient with the
of: (1) musculoskeletal impairments, other than the testing procedure.
TKA, that limited function in the lower extremity to be
tested; (2) uncontrolled blood pressure; (3) diabetes After 5 minutes of rest, subjects were instructed to
mellitus, because even subtle peripheral neuropathy maximally contract the quadriceps femoris muscle for
affects conduction of the electrical stimulation; (4) neo- approximately 4 seconds. Verbal encouragement and
plasms; or (5) neurological disorders. All subjects gave visual output of their force were used to motivate the
written 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. 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 Analysis
Figure 1. Two measures of knee extension force production were
Electrode 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 the
into the contraction, the stimulator (Grass S8800 stimu- MVIC of the involved quadriceps femoris muscle by the
lator 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 per
second 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 quantified
written software (Labview 4.0.1 and 5.0)§ with a 200-Hz using the central activity ratio (CAR) described by
sampling 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. Figure 2.
Example of a force trace recorded during a burst superimposition test of
the quadriceps femoris muscle. The central activation ratio (CAR) for this
test is 0.76 (maximal volitional force [135 N]/maximal force during
burst 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 the
burst of electrical stimulation.
Differences in force production, volitional activation,
age, and BMI between groups were analyzed using
independent t tests. Differences in force production and
volitional activation between involved and uninvolved
lower extremities in the TKA group were analyzed with
paired t tests. The effects of age, QI, and knee pain
during burst superimposition testing of TKA group were
analyzed using regression analysis. A probability level of
less than .05 was considered significant for all tests.
Results
The TKA group was younger and had a greater BMI than
the comparison group (Table). Quadriceps femoris
force production and volitional activation in the
involved 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 burst
displayed a deficit in the average, normalized voluntary superimposition testing. NRS numeric rating scale.
force of 64% compared with the comparison group’s
average, normalized voluntary force (Table). There was
no difference between the normalized voluntary force or who had knee pain during testing had greater failure of
the CAR of the uninvolved quadriceps femoris muscle of volitional activation than those without knee pain
the TKA group and the quadriceps femoris muscle of the (Fig. 5). Volitional activation of the TKA group
comparison group. The average CAR for the TKA group explained a large portion of the variance in their QI with
was 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 produce
Linear 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 the
superimposition 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 the
subjects 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, there
Physical Therapy . Volume 83 . Number 4 . April 2003 Mizner et al . 363

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 knee
arthroplasty grouped by those with or without pain during burst super-
thetic devices have a revision rate of less than 10% up to
imposition 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 patellar
Figure 6. contusions had more than twice the percentage of
Exponential 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 in
central 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 of
were profound deficits in force production and a large the TKA group in our study (26%) was considerably
average failure of volitional activation. The best predic- larger.
tor of quadriceps femoris muscle force production was
the CAR. This relationship emphasizes that subjects who Our data illustrate that decreased quadriceps femoris
manifested the greatest decrease in muscle force follow- muscle performance is present 1 month after TKA.
ing surgery also displayed the greatest inhibition. Muscle force measurements are not often a part of the
assessment of outcomes, whereas reduction in pain
Knee pain appears to contribute a small amount to the following surgery is often enough to lead to claims of
failure 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. 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 preclude
striving 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-
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quences 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 postoperative
rehabilitation 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 the
quadriceps femoris muscle. Attempting to provide ade- 14 Hurley MV, Jones DW, Newham DJ. Arthrogenic quadriceps inhi-
bition and rehabilitation of patients with extensive traumatic knee
quate stimulus to promote gains in muscle force produc-
injuries. Clin Sci (Lond). 1994;86:305–310.
tion with traditional rehabilitation exercises, in our
opinion, 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.
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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 muscle
re-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.
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