Voluntary activation and decreased force production of the quadriceps femoris muscle after total knee arthroplasty. (Research Report).

Total knee arthroplasty (TKA) predictably reduces knee pain, but it has had limited success in restoring quadriceps femoris muscle force-generating capacity and function to that of age-matched people without osteoarthritis. (1-6) Decreased quadriceps femoris muscle production is a major impairment following TKA. (1,6,7) Knee extension force deficits of 30% to 40% compared with knee extension force in age-matched subjects without knee disease have been reported to exist a year or more after surgery. (2) Impairment of quadriceps femoris muscle performance has been correlated with fall risk, (8) ambulation speed, (9-11) speed and quality of sit-to-stand transfers, (11) and performance during stair climbing in individuals greater than 60 years of age. (6)

Despite the relationship between knee extension force and functional ability, decreased quadriceps femoris muscle performance after TKA has gone relatively unexamined. Investigators (1-6) have measured knee extension force as an outcome variable months to years after surgery. Although these studies provide valuable information for understanding the long-term condition of the knee extensors following TKA, they do not provide information concerning the cause of this persistent decrease in force. The early period after surgery has received little scrutiny, yet this period is when patients typically begin outpatient rehabilitation to address, among other things, decreased quadriceps femoris muscle performance.

Both atrophy and failure of volitional activation of the quadriceps femoris muscle have been suggested as causes of deceased muscle force in people with knee osteoarthritis as well as in older adults. (12-18) Failure of voluntary activation can be operationally defined as the inability to produce all available force of a muscle despite maximal conscious effort. (19-21) A failure of voluntary activation can result from pain, (22) effusion (23,24) and joint damage (13) all of which are potentially present in patients after TKA.

Diminished activation has been implicated as a contributing factor in preventing rapid and full recovery of quadriceps femoris muscle force following anterior cruciate ligament reconstruction and in patients with painful patellofemoral disorders. (16,19,25) Typically, twitch-interpolation or burst superimposition of electrical stimulation has been used to quantify the extent of voluntary activation failure of a muscle. (16,21) Neither technique has been used to examine activation deficits in patients after TKA. Determining the extent of voluntary activation of patients may prove critical to designing and implementing effective rehabilitation programs. Hurley et al (14) reported that strength training, which included 4 weeks of intensive isokinetic training to address decreased quadriceps femoris muscle performance, had limited success in resolving voluntary activation failure and improving force production in patients with a substantial activation failure. The purposes of our investigation were: (1) to quantify the extent of quadriceps femoris muscle force deficits and voluntary activation deficits in patients who had undergone TKA compared with older people without known knee pathology and (2) to determine the effect of knee pain and age on the voluntary activation of the knee extensors of the lower extremity that underwent the TKA. We hypothesized that (1) patients after TKA would have lower normalized quadriceps femoris muscle force and decreased voluntary activation when compared with a group of older adults without knee pathology, (2) pain and age would account for a large portion of the variability in voluntary activation after surgery, and (3) voluntary activation in the TKA group would account 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) ([e-mail]). 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.

Method

Subjects

Two groups of subjects were studied: older adults without knee pathology (comparison group) and patients who had undergone a primary TKA 3 to 4 weeks prior to the measurement session (Table). The comparison group comprised 52 volunteers (mean age = 72.2 years, SD = 5.34, range = 64-85) recruited from local senior centers and exercise facilities in the Wilmington, Del, area. All subjects in the comparison group participated in a regular exercise program that included at least 30 minutes of regular cardiovascular exercise (such as walking, cycling, swimming, or tennis) 3 times per week. 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.

Patients were recruited from a consortium of orthopedic surgeons from the Wilmington, Del, area who a performed tricompartmental, cemented TKA with a medial parapatellar surgical approach. Potential subjects for the TKA group were excluded if they had a body mass index (BMI = weight [in kilograms]/[[height (in meters)].sup.2]) greater than 40 (morbidly obese) or if they had evidence of: (1) musculoskeletal impairments, other than the TKA, that limited function in the lower extremity to be tested; (2) uncontrolled blood pressure; (3) diabetes mellitus, because even subtle peripheral neuropathy affects conduction of the electrical stimulation; (4) neoplasms; or (5) neurological disorders. All subjects gave written informed consent.

Muscle Force and Voluntary Activation Measurement

All subjects participated in a measurement session of a maximal voluntary isometric contraction (MVIC) of the quadriceps femoris muscle with a burst superimposition technique. They were seated in an electromechanical dynamometer (Kin-Com 500 H). * The TKA group sat with the hip flexed to 90 degrees and the knee flexed to 75 degrees, and the comparison group sat with the hip and knee flexed to 90 degrees. The arthroplasty group was tested at 75 degrees instead of 90 degrees because we anticipated that a relatively large number of subjects either would be unable to achieve 90 degrees of flexion at 3 to 4 weeks after surgery or would be unable to achieve that range without pain.

The axis of the dynamometer was positioned at the axis of rotation of the knee joint, and the distal edge of the shin attachment was placed 2 in (5.08 cm) proximal to the lateral malleolus of the test leg. A waist and a trunk strap were used for stabilization. Two self-adhesive electrodes (7.6 cm x 12.7 cm) ([dagger]) were placed over the quadriceps femoris muscle at the motor point of the vastus medialis and proximal rectus femoris muscles (Fig. 1). Subjects performed 2 submaximal contractions and 1 MVIC lasting 2 to 3 seconds each in order to warm up the muscle and to familiarize the patient with the testing procedure.

[FIGURE 1 OMITTED]

After 5 minutes of rest, subjects were instructed to maximally contract the quadriceps femoris muscle for approximately 4 seconds. Verbal encouragement and visual output of their force were used to motivate the subjects to produce an MVIC. Approximately 3 seconds into the contraction, the stimulator (Grass S8800 stimulator with a Grass model SIU8T stimulus isolation unit ([double dagger])) delivered a supramaximal electrical stimulus of monophasic rectangular waves at a rate of 100 pulses per second for 100 milliseconds at 135 V. The knee extension force was measured and recorded using custom-written software (Labview 4.0.1 and 5.0) ([section]) with a 200-Hz sampling rate.

If maximal voluntary force output was achieved and no augmentation of force was observed due to the stimulation (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 maximum of 3 trials was recorded. The highest volitional force achieved during the 3 attempts was used for analysis. A weight correction was performed automatically by the computer program by adding the baseline force while the patient was relaxed to the force measurement. 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 stimulation 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

Two measures of knee extension force production were 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 uninjured group. The QI was determined by dividing the MVIC of the involved quadriceps femoris muscle by the MVIC of the contralateral, uninvolved quadriceps femoris muscle.

The extent of failure of volitional activity of the quadriceps femoris muscle during the testing was quantified using the central activity ratio (CAR) described by 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 complete activation of the muscle with no augmentation of the maximal volitional force observed during the burst of electrical stimulation.

[FIGURE 2 OMITTED]

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 than in the comparison group (Table). The TKA group displayed a deficit in the average, normalized voluntary force of 64% compared with the comparison group's average, normalized voluntary force (Table). There was no difference between the normalized voluntary force or the CAR of the uninvolved quadriceps femoris muscle of the TKA group and the quadriceps femoris muscle of the comparison group. The average CAR for the TKA group was 0.742 (26% volitional activation deficit) as compared with the comparison group's 0.943 (6% volitional activation deficit).

Linear regression analysis indicated that age of the TKA group did not explain the variance in the CAR variable (Fig. 3). The knee pain of the TKA group during burst superimposition testing showed a small relationship to CAR ([r.sup.2] = .17) (Fig. 4). Only half (26 of 52) of the subjects with TKA reported knee pain during burst superimposition testing. The subjects in the TKA group who had knee pain during testing had greater failure of volitional activation than those without knee pain (Fig. 5). Volitional activation of the TKA group explained a large portion of the variance in their QI with a curvilinear model of regression ([r.sup.2] = .65) (Fig. 6).

[FIGURES 3-6 OMITTED]

Discussion

The hypotheses that patients after TKA would produce less force and exhibit greater failure of volitional activation of the quadriceps femoris compared with a comparison group were supported by the data. Although the TKA group had more men, had a greater average BMI, was younger, and was tested at a knee angle closer to the angle of greatest mechanical advantage for the quadriceps femoris muscle than the comparison group, there were profound deficits in force production and a large average failure of volitional activation. The best predictor of quadriceps femoris muscle force production was the CAR. This relationship emphasizes that subjects who manifested the greatest decrease in muscle force following surgery also displayed the greatest inhibition.

Knee pain appears to contribute a small amount to the failure of voluntary activation, and we believe this is a relevant clinical finding to consider in developing rehabilitation protocols. We believe that efforts to increase muscle force production in patients with painful quad riceps femoris muscle contraction should take into consideration that these patients are more likely to have muscle inhibition. Simply eliminating pain will not provide the panacea for eliminating knee extension inhibition.

The subjects' age did not provide additional information for identifying those subjects with volitional activity deficits. Researchers (12,21) 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 negligible in the presence of the large activation failure we observed.

Younger patients will likely undergo TKA as the durability of prostheses continues to improve. Current prosthetic devices have a revision rate of less than 10% up to 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 surgery. (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 quadriceps 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 production. (15,17,19,22) Manal and Snyder-Mackler (19) showed that patients with volitional activity deficits with patellar contusions had more than twice the percentage of decreased quadriceps femoris muscle force than those without reflex inhibition. The average failure of activation of the patients with reflex inhibition and patellar contusion was 14%. The average failure of activation of the TKA group in our study (26%) was considerably larger.

Our data illustrate that decreased quadriceps femoris muscle performance is present 1 month after TKA. Muscle force measurements are not often a part of the assessment of outcomes, whereas reduction in pain following surgery is often enough to lead to claims of excellent surgical success. (5) We believe the strong relationship between quadriceps femoris muscle force production and performance during stair climbing, gait, and transfers (6,10,11) should not be ignored. Simply achieving pain relief and restoring a functional range of motion in the postoperative knee does not preclude striving for resolution of decreased quadriceps femoris muscle production. Inadequate quadriceps femoris muscle rehabilitation could have long-term negative consequences in patient outcomes and may lead to increased fall risk with advancing age.

Conclusion

The results of our study suggest that postoperative rehabilitation should include tactics to reduce factors that may propagate poor volitional activation of the quadriceps femoris muscle. Attempting to provide adequate stimulus to promote gains in muscle force production with traditional rehabilitation exercises, in our opinion, will be unlikely to succeed if the patient has a pronounced failure of volitional activation. More aggressive strategies to control pain and pain-provoking inflammation, coupled with the use of electrically elicited contractions for muscle force training or muscle re-education, may be more successful in overcoming deficits in volitional activation. Tools such as biofeedback also may be useful in prompting the patient to maximize muscle contractions and to develop strategies to improve activation during resistive exercises designed to increase muscle force production.

Table.

Group Descriptions (a)

                              TKA Group

                              n     [bar]X          SD       Range

Sex
  Men                         32
  Women                       20
Age (y)                             64.9 (b)        7.72        49-78
BMI (kg/[m.sup.2])                  29.2 (b)        3.62      21.9-37.1
Involved CAR                         0.742 (b,c)    0.168    0.342-1.00
Uninvolved CAR                       0.927          0.065     0.76-1.00
Involved normalized MVIC
  (N/BMI)                            8.80 (b,c)     4.85      2.97-24.3
Uninvolved normalized MVIC
  (N/BMI)                           24.14           8.02      11.9-42.9

                              Comparison Group

                              n     [bar]X          SD       Range

Sex
  Men                         28
  Women                       24
Age (y)                             72.2            5.34       64-85
BMI (kg/[m.sup.2])                  25.2            3.80     19.9-35.9
Involved CAR                        NA
Uninvolved CAR                       0.943          0.053    0.73-1.00
Involved normalized MVIC
  (N/BMI)                           NA
Uninvolved normalized MVIC
  (N/BMI)                           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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* Chattecx Corp, 6431 Pythian Rd, Harrison, TN 37341-3902.

([dagger]) CONMED Corp, 310 Broad St, Utica, NY 13501.

([double dagger]) Grass Instruments, 570 Liberty St, Braintree, MA 02184.

([section]) National Instruments, 6504 Bridge Point Pkwy, Austin, TX 78730.