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Research Reports |
SJ Farquhar, PT, MPT, is a doctoral student in the Graduate Program in Biomechanics and Movement Science and the Department of Physical Therapy, University of Delaware.
DS Reisman, PT, PhD, is Assistant Professor, Department of Physical Therapy, University of Delaware.
L Snyder-Mackler, PT, ScD, SCS, ATC, FAPTA, is Professor, Department of Physical Therapy, 301 McKinly Laboratory, University of Delaware, Newark, DE 19716 (USA).
Address all correspondence to Dr Snyder-Mackler at: smack{at}udel.edu
Submitted February 5, 2007;
Accepted December 28, 2007
 | Abstract |
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Subjects and Methods: Twenty-four subjects (12 subjects with unilateral TKA and 12 control subjects) were recruited; those with TKA were tested 3 months and 1 year following surgery. Motion analysis of an STS task was synchronized with 2 force platforms and electromyography. Outcome measures included joint angles and moments, electromyography, vertical ground reaction forces, muscle strength, and functional performance tests.
Results: Subjects with TKA showed improvements in symmetry of motion, strength, and functional performance from 3 months to 1 year following TKA. Compared with control subjects, subjects with TKA relied on increased hip flexion and a larger hip extensor moment to perform the STS task.
Discussion and Conclusion: The increased hip extensor moment demonstrated that subjects adopted a strategy to avoid the use of the quadriceps femoris muscle, yet this strategy persisted as quadriceps femoris muscle strength improved. This pattern may be a learned movement pattern that may not resolve without retraining.
| Introduction |
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 TopAbstract Introduction MethodResultsDiscussionConclusionReferences |
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Quadriceps femoris muscle weakness in the surgically treated (involved) limb has been reported to persist for 1 year4 and for up to 3 years10 following TKA. At 3 months following TKA, quadriceps femoris muscle strength correlates with both functional performance9,11 and the symmetry of walking and the sit-to-stand (STS) movement.11 Cross-sectional motion analysis of the STS task demonstrated differences between involved and uninvolved limbs (differences between the sides in subjects with TKA) as well as between control subjects and subjects 3 months,11 1 year,12 and up to 6 years13 following TKA. Specifically, the hip and knee contralateral to the TKA were shown to bear more load, demonstrating higher extensor moments11,13 and ground reaction forces.11,13 Compensation patterns adopted in an effort to avoid pain may lead to altered loading patterns that may place additional stresses on the uninvolved limb11; these patterns may have the long-term consequence of advancing OA in the hip or knee contralateral to the TKA.14–16
Investigations that test subjects longitudinally often report the results of questionnaires17 or functional tests or strength.18,19 Investigations of kinematics have included subjects over a range of years following TKA,13 have reported on a mixture of subjects with unilateral and bilateral TKA,4,13 or have reported differences only at the knee.12 Although these reports are valuable, they do not provide information about longitudinal changes in movement patterns over discrete time intervals. Thus, little is known about changes in STS task performance over time in subjects following TKA.
Changes in patterns may play a role in the noncognate progression of OA in other joints of the lower extremities.16 Assessment of subjects at discrete intervals may provide insight into how these movement patterns change over time. Previously, differences between the sides during an STS task were reported in subjects 3 months following TKA11 when the subjects’ knee angle was constrained at 90 degrees of flexion during sitting. Differences in kinematics and kinetics when subjects with TKA are allowed to self-select their starting position are unknown.
Therefore, the purpose of this study was to investigate changes in STS task performance in subjects 3 months and 1 year following TKA and differences between these subjects and control subjects who were healthy. We hypothesized that if subjects 3 months after TKA self-selected their starting position, there would be asymmetries in the motions and moments of the hips and knees relative to the motions and moments in matched control subjects. We hypothesized that quadriceps femoris muscle strength and function would show improvements 1 year after TKA and that strength, function, and movement patterns would be more similar to those of control subjects.
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TopAbstractIntroductionMethodResultsDiscussionConclusionReferences |
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Subjects participated in functional and strength testing and motion analysis 3 months following surgery and again 1 year following surgery. Twelve of the 14 subjects returned for testing 1 year following surgery; both subjects who did not return were scheduled for TKA on the contralateral knee. Only data for the 12 subjects (6 women and 6 men) who returned for motion analysis and functional and strength testing at 1 year were used in the analyses. All results for subjects 3 months and 1 year following TKA were compared with the results for control subjects. Twelve subjects without injury (5 women and 7 men) were matched by age (±5 years), height (±5%), and body mass index (±5%) to subjects 1 year following TKA (Tab. 1). Subjects with TKA also were matched to control subjects by limb; for example, a subject with a right TKA had a matched control subject with the right limb designated as the so-called involved limb.
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Motion Analysis
Subjects sat on an armless, backless chair, with the chair height set to the knee joint line. No restrictions were placed on the position of the lower extremities, and subjects rose from the chair at a self-selected pace. Subjects were asked to hold the arms across the chest or in the lap and were instructed not to use the arms to assist with rising from the chair.
Motion analysis of the STS task was performed by use of a 3-dimensional, 6-camera motion analysis system (VICON Peak).* Two forceplates
positioned in the floor captured the ground reaction forces under each leg during the STS task. Analog data (forceplate data and electromyography [EMG]) were sampled at 1,080 Hz, and video data were sampled at 120 Hz. Retroreflective markers were placed bilaterally on the heads of the fifth metatarsals, lateral malleoli, lateral femoral condyles, greater trochanters, and iliac crests; 2 markers were placed on the heel counter of each shoe. Rigid thermoplastic shells, each with 4 retroreflective markers glued on, were affixed bilaterally to the lower leg and thigh and over the sacrum by use of elastic wraps (SuperWrap)
or double-sided tape to minimize movement artifacts.
Lower-extremity joint kinetics and kinematics were time normalized to 100% of the STS task, as defined from the start to the end of standing. The start of standing and the end of standing are both determined by the angular velocity of the pelvic marker. When the angular velocity moves above zero, the start of standing has begun, and when it returns to zero, the end of standing has been reached. This method was chosen to eliminate reliance on a variable associated with one of the limbs because of the evidence of asymmetry in subjects with TKA. "Seat-off," the instant at which the buttocks leave the seat, was operationally defined as the time of the peak vertical ground reaction force21 (Fig. 1).
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Customized software (Labview 8)|| identified flexion and extension, peak internal extensor and flexor moments, and vertical ground reaction forces. Peak data points used for analyses were chosen to coincide with seat-off.
EMG
Electromyographic activity was recorded with a 16-channel system# interfaced with the VICON system for simultaneous recording. Active surface electrodes (1.2 cm in diameter, interelectrode distance of 1.8 cm) were taped over the mid-muscle belly of the vastus lateralis and the biceps femoris muscles. Elastic bands were wrapped over the electrodes to minimize movement. The subject was positioned on a padded plinth in order to isometrically test each muscle for the verification of electrode placement and to record resting baseline and maximum signals.
The raw EMG data were filtered by use of customized software (Labview 8) that filtered the signals with a 350-Hz low-pass Butterworth filter. Following full-wave rectification, a linear envelope of the signal was created by use of a phase-corrected 20-Hz low-pass Butterworth filter. This linear envelope was normalized to the maximum signal obtained during either the maximal voluntary isometric contraction (MVIC) trial or the dynamic trial. The maximum EMG signal used for normalization was the average of the peak value plus the values at 25 frames before and after the peak value (total of 51 frames) to allow for an accurate representation of the signal. All subsequent EMG data were normalized against this maximum value. Electromyographic data collected during the STS task were analyzed to identify the peak level of muscle activation.
Quadriceps Muscle Strength Measurement
The MVIC of the quadriceps femoris muscle was assessed isometrically.24–27 In brief, subjects were seated in an electromechanical dynamometer (Kin-Com 500H)** with the knee flexed to 75 degrees. Subjects performed 2 submaximal contractions and 1 MVIC lasting 2 to 3 seconds each to become familiar with the testing procedure and to warm up the muscle.
After 5 minutes of rest, subjects were instructed to contract the quadriceps femoris muscle maximally for approximately 3 seconds. Verbal encouragement and visual output of their force were used to motivate the subjects to produce an MVIC. The MVIC force was measured and recorded by use of customized software (Labview 4.0.1 and 5.0).|| A maximum of 3 trials were performed on each leg. The highest volitional force achieved was used for analysis. In subjects with TKA, the uninvolved limb was tested before the involved limb.
Functional Testing
Measures of functional performance included the TUG Test, the SCT, and the 6MWT. The TUG Test measures the time it takes a subject to rise from an armchair (seat height of 46 cm), walk 3 m, turn around, and return to sitting in the same chair.28 The SCT measures the time it takes a subject to ascend and descend a flight of twelve 7-in-high steps; use of one handrail is permitted. The stair-climbing task was found to be sensitive to change with physical activity interventions in people with knee OA.29 One practice test was performed, and the average of 2 tests was used for analysis with both the TUG Test and the SCT.
The 6MWT is a self-paced functional test used to provide an assessment of the extent to which impairments affect mobility. The 6MWT measures the distance a person can walk in 6 minutes. It is a highly reliable measure in healthy, older adult populations (r=.95)30 and is considered to be one of the most responsive measures of function following TKA.3,31 For all tests, subjects were instructed to walk as quickly and as safely as they were able.
Data Analysis
Joint angles, joint moments, vertical ground reaction forces, and peak magnitudes of muscle activity were analyzed by use of an analysis of variance (ANOVA) with 2 repeated measures comparing subjects with TKA at 3 months and 1 year following surgery (time) and involved versus uninvolved limbs (limb). If an interaction effect was present, then main effects were not investigated. Paired t tests were performed as post hoc tests. Comparisons of control subjects with those with TKA were performed by use of an ANOVA (limb x group) at both 3 months and 1 year following TKA. Independent t tests were used for post hoc comparisons if significant main effects or an interaction was present. The results of the ANOVAs are reported in Tables 2, 3, and 4. The results of the post hoc tests are reported in Figures 2, 3, 4, 5, 6, and 7.
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| Results |
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TopAbstractIntroductionMethodResultsDiscussionConclusionReferences |
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Differences at the Knee
The peak knee flexion angle did not change over time, nor was it different in subjects with TKA and control subjects (Tab. 2 and Fig. 3). For the knee extensor moment there was an effect of limb (Tab. 2), and at 3 months after TKA, values from post hoc tests were significantly higher for the uninvolved limb than for the involved limb (Fig. 3). At 1 year following TKA, there were no differences between the limbs (Fig. 3). Compared with control subjects, subjects 3 months after TKA showed an interaction effect for both knee angle (Tab. 2) and knee extensor moment (Tab. 2); post hoc tests revealed that only the knee extensor moment of the involved limb was lower than that in control subjects (Fig. 3). Compared with control subjects, subjects 1 year after TKA showed an effect of group for both knee flexion angle (Tab. 2) and knee extensor moment (Tab. 2); however, post hoc tests did not reveal any differences.
EMG Differences
The normalized peak magnitude of the vastus lateralis muscle in subjects 3 months and 1 year following TKA showed a limb x time interaction (Tab. 3). At 3 months following TKA, post hoc tests showed that the normalized peak magnitude was lower on the involved side than on the uninvolved side; by 1 year following TKA, there were no differences between the sides (Fig. 4). Compared with control subjects, subjects 3 months after TKA showed an effect of limb, an effect of group, and an interaction effect (Tab. 3). Post hoc tests revealed that the normalized peak magnitude was higher for the uninvolved limb 3 months following TKA than for control subjects (Fig. 4). Compared with control subjects, subjects 1 year after TKA showed an effect of limb (Tab. 3). There were no differences in the normalized peak magnitude between subjects 1 year following TKA and control subjects (Fig. 4).
The normalized peak magnitude of the biceps femoris muscle in subjects 3 months and 1 year following TKA showed a limb x time interaction (Tab. 3). Values from post hoc tests were higher for the uninvolved side than for the involved side 3 months following TKA (Fig. 5). The peak magnitude for the uninvolved side decreased by 1 year following TKA (Fig. 5), resulting in no differences between the sides. Compared with control subjects, subjects 3 months following TKA showed an effect of limb and an interaction effect (Tab. 3). Values from post hoc tests were higher for the uninvolved limb 3 months following TKA than for control subjects (Fig. 5). At 1 year following TKA, there continued to be an effect of limb (Tab. 3). However, there were no post hoc differences in peak magnitude between subjects 1 year following TKA and control subjects (Fig. 5).
Vertical Ground Reaction Forces
In subjects with TKA, there was a limb x time interaction (Tab. 2). Post hoc tests at 3 months following TKA revealed that vertical ground reaction forces for the involved limb were significantly lower than those for the uninvolved limb (Fig. 6). By 1 year, the vertical ground reaction forces for the involved limb increased significantly, resulting in no differences between the sides (Fig. 6). Compared with control subjects, subjects with TKA showed no effect of limb or group at both times (Tab. 2 and Fig. 6).
Quadriceps Femoris Muscle Strength
Subjects 3 months and 1 year after TKA showed an effect of limb and an effect of time (Tab. 4). At 3 months following TKA, post hoc tests revealed that the involved quadriceps femoris muscle was significantly weaker than the uninvolved quadriceps muscle (Fig. 7). By 1 year, the strength of the involved limb improved, but this limb continued to be weaker than the uninvolved limb (Fig. 7). Compared with control subjects, subjects 3 months following TKA showed an effect of limb (Tab. 4). Post hoc tests revealed that subjects 3 months after TKA were weaker in the involved limb but not in the uninvolved limb than control subjects (Fig. 7). There were no differences in strength between subjects 1 year following TKA and control subjects (Fig. 7).
Functional Testing
Subjects performed significantly better on the SCT but not on the TUG Test or the 6MWT at 1 year following TKA than at 3 months after surgery (Fig. 8). Compared with control subjects, subjects 3 months following TKA performed significantly more slowly on the SCT but not on the TUG Test or the 6MWT. There were no differences between subjects 1 year following TKA and control subjects on the TUG Test, the SCT, or the 6MWT (Fig. 8).
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| Discussion |
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TopAbstractIntroductionMethodResultsDiscussionConclusionReferences |
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Relationship of Movement Patterns and Strength
At 3 months following TKA, the altered strategy used to complete the STS consisted of unloading of the involved limb with the use of greater hip flexion, which resulted in higher hip extensor moments. At 3 months following TKA, the quadriceps femoris muscles in the involved limb were still weak. Increased hip flexion reduced the demand on the knee extensor muscles, transferring it to the hip extensor muscles35 during the STS task; therefore, an altered movement strategy that places less demand on the knee extensor muscles is logical at 3 months following surgery. However, at 1 year, resolution of the strength asymmetries and asymmetries in STS task performance resulted in a movement pattern that was atypical relative to that in control subjects, suggesting that this pattern is a learned movement pattern that may not resolve without retraining.
At 3 months after TKA, subjects may use hip flexion because they cannot generate the necessary knee extensor muscle torque. In the early phases following surgery, this seems to be a reasonable compensation pattern. Yu et al36 demonstrated that angular motions of the hip contribute to horizontal and vertical velocity during the STS task, in which the angular motion of the knee contributes to horizontal velocity and prevents collapse. This difference in the contributions of the joints may play a role in reliance on the hip extensor muscles in subjects with TKA. The knee extensor muscles continue to act to prevent collapse, but the contribution to horizontal velocity may be reduced. This strategy was likely adopted because of pain or weakness or in an effort to avoid using the knee extensor muscles and is often present in people with OA prior to TKA.
What is the potential impact of the persistence of this abnormal movement pattern after the resolution of the impairments that likely lead to its development? Our subjects following TKA used their hip extensor muscles more than their counterparts who were not injured. Long-term stresses on the hip joints may be a contributing factor in the progression of OA, particularly in the uninvolved hip.16,37 A large hip extensor moment has been shown to be a contributor to increased wear on the anterior portion of the femur38 and has been implicated in the development of hip OA.37,38 Therefore, the high stresses present in functional tasks coupled with altered joint loading may be contributing factors in the evolution of OA, particularly in the contralateral hip.
Functional Outcomes
Despite the altered movement strategy for the STS task that persisted at 1 year, subjects 1 year following TKA were not different from control subjects in terms of functional test scores. However, it is important to note that although there was a lack of statistical significance, subjects with TKA had lower scores on all of the functional tests than control subjects. These results indicate that improvements in timed functional test scores, although useful, may not reveal important differences in movement performance that could have future consequences for the development of OA in other joints of the lower extremities.
Physical Therapy Intervention
Why would subjects show improvements in strength, symmetry, and functional performance yet continue to demonstrate an altered STS movement strategy 1 year following TKA? All of our subjects participated in physical therapy at a clinic that focuses its efforts on the symmetry of quadriceps femoris muscle strength, weight bearing, and gait retraining. Our physical therapy protocol20 for subjects following TKA includes measurements of quadriceps muscle strength bilaterally, and efforts are focused on strengthening the involved limb to make it as strong as the uninvolved limb. Weights, neuromuscular electrical stimulation, and functional training are all used to improve strength and functional control of the quadriceps femoris muscles. The criterion for the progression of the exercises is that the subject can complete the exercises correctly but still is maximally fatigued at the end of each set. Standardization of this protocol in our clinic has produced excellent outcomes.18–20
During physical therapy, it is common for gait to be assessed regularly, even as a subject moves around the clinic to go to different exercise stations. Other functional tasks, such as the STS task and negotiation of stairs, are not assessed or trained with the same regularity. However, the results of the present study indicate the potential importance of including evaluation and training for tasks such as the STS task and negotiation of stairs, particularly because these tasks subject the knee joint to higher stresses and forces than walking.39
The STS task could easily be incorporated as a functional retraining exercise into physical therapy following TKA. Through movement reeducation strategies, a subject could be taught to complete the STS task using a more typical movement pattern. Furthermore, this task could be incorporated into physical therapy rehabilitation as an exercise to improve the use of the involved limb. Given the potential impact of a poor movement strategy during the STS task on the progression of OA in the contralateral limb, functionally reeducating subjects to perform the STS task may be one component of physical therapy rehabilitation that could affect long-term outcomes following TKA.
Potential Limitations
There are limitations to the present investigation. Allowing subjects to self-select their starting position allows insight into how the task is naturally performed. Because we allowed subjects to self-select their starting position, we expected that asymmetries would persist. We were surprised to discover that the self-selected starting position resulted in a lack of differences 1 year after TKA. However, the self-selected starting position contributed to the variability of the results which, in a small sample size, can lead to the underestimation of an effect, or a type II error. In addition, our decision to use an alpha value of .05 for our small sample also increased the risk of a type I error. The risk of a type I error also existed because of the large number of comparisons made in the present study. Thus, the small sample size could have been a limiting factor in the lack of differences between control subjects and subjects 1 year following TKA.
The self-selected starting position likely played a role in the interaction effect when subjects 3 months after TKA were compared with control subjects. All subjects were allowed to select their starting position; the inherent variability in the healthy control subjects,40 confounded by the small sample size, played a role in generating this interaction. Chair height also plays a role in STS task performance13,37; we chose 100% of tibial height in order to standardize that aspect of the task. However, it has been found that chair height primarily affects moments at the knee but minimally affects moments at the hip37; therefore, chair height likely did not account for our results.
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TopAbstractIntroductionMethodResultsDiscussionConclusionReferences |
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| Footnotes |
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The Human Subjects Review Board at the University of Delaware approved this study.
These data were presented, in part, at the 2007 Annual Meeting of the Orthopaedic Research Society; March 2–5, 2008; San Francisco, Calif; and at the 2008 Combined Sections Meeting of the American Physical Therapy Association; February 6–9, 2008; Nashville, Tenn.
* VICON, 14 Minns Business Park, West Park, Oxford, United Kingdom OX2 0JB. 
Bertec Corp, 6171 Huntley Rd, Suite J, Columbus, OH 43229.
Fabrifoam Inc, 900 Springdale Dr, Exton, PA 19341.
C-Motion Inc, 15821-A Crabbs Branch Way, Rockville, MD 20855.
|| National Instruments Corp, 11500 N Mopac Expressway, Austin, TX 78759.
# Motion Lab Systems Inc, 15045 Old Hammond Hwy, Baton Rouge, LA 70816.
** Isokinetic International, 6426 Morning Glory Dr, Harrison, TN 37341.
SPSS Inc, 233 S Wacker Dr, Chicago, IL 60606.
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