Abstract
Background Because people with total knee arthroplasty (TKA) have persistent functional limitations and disability, identifying modifiable risk factors for persistent disability is warranted. Before surgery, people have pervasive lower extremity muscle weakness. The fact that hip abductor muscle strength is often not targeted in postoperative rehabilitation may contribute to functional limitations.
Objective Study objectives were: (1) to examine the reliability of handheld dynamometry (HHD) for measuring hip abductor strength and (2) to determine whether hip abductor strength contributes to physical function beyond the contribution of quadriceps muscle strength.
Design This was a cross-sectional study.
Methods Two-hundred ten participants underwent quadriceps and hip abductor muscle strength testing and measurement of physical function (performance-based and self-reported outcomes). Correlation and regression equations were built to determine the relationships of strength, pain, and functional ability. A subset of 16 participants underwent hip abductor strength testing at 2 sessions to determine the reliability of the measure.
Results Measuring hip abductor strength with HHD yielded excellent relative reliability, with an intraclass correlation coefficient (ICC [2,3]) of .95 and a 95% confidence interval of .86 to .98, but moderate absolute reliability, with a minimal detectable change (with 95% confidence) of 47.6 N and a 95% confidence interval of 35.5 to 76.5. Hip abductor strength made a significant additional contribution to performance-based measures of physical function after anthropometric covariates and quadriceps muscle strength were accounted for. Hip abductor strength did not show bivariate correlations with patient-reported measures of physical function and did not contribute to patient-reported physical function after covariates and quadriceps muscle strength were accounted for.
Limitations A cause-and-effect relationship between hip abductor strength and physical function could not be established.
Conclusions In people with unilateral TKA, HHD testing of hip abductor strength is reliable. Hip abductor strength contributes to performance-based but not patient-reported measures of physical function in people with unilateral TKA.
Total knee arthroplasty (TKA) results in long-term pain relief and patient satisfaction.1 Despite this prognosis, many people continue to have impairments and functional limitations compared with age-matched control groups.2,3 These impairments and functional limitations persist beyond 6 months after surgery, the time at which the recovery of physical function seems to plateau.4 People after TKA take 30% to 115% more time to complete the timed Stair Climbing Test (SCT), require 20% to 62% more time to complete the Timed “Up & Go” Test (TUG), and cover 17% to 27% less distance in the Six-Minute Walk Test (6MWT) than people who have not had TKA.2,3,5–7 After TKA, people also report limitations in kneeling, squatting, moving laterally, turning and cutting, carrying loads, stretching, leg strength, playing tennis, dancing, gardening, and sexual activity.8
Although quadriceps muscle strength is an important predictor of physical function after TKA,2,9,10 it does not fully explain physical function in people who have had TKA,2,10 and other factors should be considered. The hip abductor muscles have the potential to influence the performance of functional tasks, such as walking. The hip abductor muscles function to stabilize the pelvis in the frontal plane during walking and especially during single-limb stance.11,12 In addition to their role in frontal-plane stability, the hip abductor muscles on the supported side assist in rotating the pelvis on the nonsupported side forward, enhancing the advancement of the nonsupported limb during walking.11,12 Before surgery, patients with knee osteoarthritis (OA) have hip abductor muscles on the involved side that are 24% weaker when tested isometrically13 and 23% weaker when tested concentrically14 than people who do not have knee OA. A recent randomized controlled trial revealed that strengthening of the hip abductor muscles reduced knee pain and improved self-reported and performance-based physical function in patients with knee OA,15 suggesting that these muscles play a role in functional performance. This finding was substantiated by another study in which hip muscle strength predicted performance in the SCT, Figure 8 Walk Test, and 5-Chair-Rise Test early after TKA.16 Collectively, these findings suggest that assessing hip abductor muscle strength in patients with TKA is important and that targeting deficits in this muscle group may maximize functional recovery after surgery.
In earlier work, electromechanical dynamometry was used to evaluate hip muscle strength in patients after TKA.16 Although this method is the gold standard for strength testing, not all physical therapy clinics are equipped with the device. Handheld dynamometers are a good alternative for electromechanical dynamometry given their ease of use, portability, and reasonable cost. Handheld dynamometers were previously shown to be valid and reliable in people who were healthy17 and people with hip OA.18 Establishing the reliability of a measure in a targeted population is necessary, and, to our knowledge, no study has examined the reliability of hip abductor muscle strength testing with a handheld dynamometer in patients with TKA.
Therefore, the purpose of the study was 2-fold. First, we planned to examine the test-retest reliability of handheld dynamometry for the measurement of hip abductor strength in people after TKA. Second, we intended to determine whether hip abductor strength contributes to performance-based and self-reported physical function beyond the contribution of quadriceps muscle strength. We expected good to strong test-retest reliability with the use of a handheld dynamometer to assess hip abductor strength on the side of surgery in people with TKA as well as a small amount of change between tests. We hypothesized that hip abductor strength contributes to physical function beyond the contribution of quadriceps muscle strength in people with unilateral TKA.
Method
Study Design and Setting
This study was designed as a cross-sectional study. Participants recruited for this study attended 1 testing session at the University of Delaware Physical Therapy Clinic. A subgroup of participants recruited for reliability testing completed an additional testing session within about 1 week of the first testing session.
Participants
Participants who were 50 to 85 years of age and who had undergone unilateral primary TKA for knee OA 6 to 48 months earlier were recruited for this study. Participants were recruited from the practice of a local group of orthopedic surgeons who perform tricompartmental cemented TKA with a medial parapatellar approach and who use either a posterior stabilized prosthesis or a posterior cruciate ligament–retaining prosthesis. Potential participants were excluded if they had undergone surgery in the lower extremities other than the unilateral TKA or had musculoskeletal involvement in the lower extremities or the spine that limited their physical function. Potential participants also were excluded if they had neurological impairments, uncontrolled blood pressure, altered sensation in the feet, or a body mass index of greater than 50 kg/m2. All participants signed informed consent forms approved by the Human Subjects Review Board at the University of Delaware before participation.
Hip Abductor Muscle Strength
The maximum isometric strength of the hip abductor muscles was tested with a handheld dynamometer (Lafayette Manual Muscle Test System, model 01163, Lafayette Instruments, Lafayette, Indiana) and a stabilization strap (Figure). Participants were positioned in a side-lying position with the limb to be tested facing up. The tested hip was in neutral flexion/extension and neutral rotation. The knee of the tested limb was in full extension. The hip and knee of the contralateral limb were in slight flexion for comfort and stability. The handheld dynamometer was placed on the lateral aspect of the tested limb just proximal to the lateral femoral epicondyle. The nonelastic stabilization strap was used to secure the dynamometer against the leg and to provide resistance to hip abduction. The strap was wrapped around the leg and underneath the table. Participants were asked to slowly abduct their hips until further movement was not possible because of the strap. The tightness of the strap was adjusted so that the hip was in 0 degrees of abduction. Once the strap was tightened appropriately, participants were asked to push as hard as they could against the dynamometer for 5 seconds. Three trials were performed per side, and 1 tester conducted all of the test and retest measurements. This method of strength testing has been shown to be valid and reliable in people who are healthy.17
Setup used to measure hip abductor strength by handheld dynamometry.
Quadriceps Muscle Strength
Quadriceps muscle strength was quantified by maximum voluntary isometric contraction. Participants were seated on an isokinetic dynamometer (Kin-Com, Chattecx Corp, Harrison, Tennessee) with the hip at 90 degrees of flexion and the knee at 75 degrees of flexion. Two straps were used to stabilize the pelvis and the trunk. Participants were instructed to produce their maximal isometric contraction. Three trials were performed per side, and the maximum volitional force generated was used to quantify quadriceps muscle strength. Isometric quadriceps muscle strength testing has been shown to be a reliable measure in people with knee OA19 and people after TKA.20 Quadriceps muscle strength and hip abductor strength are presented as nonnormalized values (in newtons) and as values normalized with the recommended allometric scaling method (N/kg0.67).21,22
Knee Outcome Survey Activities of Daily Living Scale (KOS-ADLS) and Knee Pain
The KOS-ADLS is a knee-specific self-reported outcome measure for assessing functional limitations imposed by the knee condition.23 The KOS-ADLS has 14 items; each item is scored from 0 to 5 (Likert scale). Answers are summed and reported as a percentage score (0–100), with higher scores reflecting greater self-perceived functional ability. The KOS-ADLS has been shown to be a valid, reliable, and responsive measure of functional limitations in people after TKA.23,24 Knee pain was assessed on a scale from 0 to 5 with a question from the KOS-ADLS; a score of 0 indicated no pain, and a score of 5 indicated pain that prevented the participant from engaging in all daily activities.
Global Rating Scale (GRS)
Participants were asked to rate their overall ability on a scale from 0% to 100%; 0% represented a complete loss of function, and 100% represented the ability to perform all activities without limitations.23
Performance-Based Measures of Physical Function
Incorporating performance-based measures into the assessment of physical function after TKA is imperative because patients with TKA are known to overestimate their functional ability on self-reported outcome measures.25,26 Functional performance was assessed with the TUG, SCT, and 6MWT. The TUG measures the time taken by a participant to stand from an armed chair (seat height of 46 cm), walk for 3 m, and return to sit on the same chair. The TUG has been shown to be a reliable and responsive test in people with TKA.27 The SCT measures the time taken by a participant to ascend and descend a flight of 12 steps. The SCT has been shown to be a valid,28 reliable,28 and responsive27 measure of functional performance in people with TKA. The 6MWT measures the distance covered by a participant walking on a flat surface for 6 minutes. The 6MWT has been shown to have excellent reliability and responsiveness in people with TKA.27,29 Participants were asked to perform all of the functional tests as quickly as they could while still feeling safe and comfortable. Participants performed 2 trials of the TUG and the SCT; the average of the 2 trials is presented in seconds. For the 6MWT, participants performed only 1 trial; the results are presented in meters.
Data Analysis
The method proposed by Bonett30 was used to estimate the required sample size for reliability testing. The parameters used for sample size estimation were an intraclass correlation coefficient (ICC) of .90 and a 95% confidence interval (CI) width of .2. This method suggested that 15 participants represented an adequate sample size; therefore, we recruited 16 participants for reliability testing. Intraclass correlation coefficient model 2,3 was used to quantify the test-retest reliability of hip abductor strength measurement with a handheld dynamometer. The correlation coefficient obtained from the ICC was used to compute the standard error of measurement and the minimal detectable change with 95% confidence (MDC95). Bland-Altman plots were used to determine the spread of the error or the difference between the test and the retest to examine for systematic bias.
Hierarchical linear regression models were used to examine the contribution of hip abductor strength to physical function after covariates, including quadriceps muscle strength, were accounted for. Body height and weight were controlled for in all regression models to account for the influence of body size on functional performance.31 The selection of other potential covariates to account for in the regression models was based on previous literature, and only the covariates that had significant bivariate correlations with the functional measures were controlled for in the regression models. Pearson correlation coefficients and Spearman rho values were used to examine bivariate correlations between variables. Body size, age, sex, and time from surgery also are known to influence functional performance.4,32 We also considered knee pain as a potential covariate to control for because it is known that knee pain influences functional performance in people with TKA.2
We ran a separate regression model for each of the physical function outcome measures. Body height and weight and other covariates that correlated significantly with functional performance were added in the first step as independent variables. In the second step of the regression model, we added quadriceps muscle strength in the limb that had undergone surgery as an independent variable to account for its known contribution to physical function.2,9,10 In the third step, we added hip abductor muscle strength in the limb that had undergone surgery as an independent variable. The independent contribution of hip abductor strength to physical function was tested by the magnitude of the change in the R2 value. Quadriceps and hip abductor muscle strength values used in the bivariate correlation analysis and regression models were nonnormalized values (in newtons). The regression models were tested for linearity, normality, homoscedasticity, and multicollinearity. All analyses were performed with IBM SPSS Statistics 20 (IBM Corp, Armonk, New York). The significance level was set at .05.
Role of the Funding Source
This study was supported by National Institutes of Health grants P20RR016458 and K12HD055931. The authors extend their appreciation to the College of Applied Medical Sciences Research Center and the Deanship of Scientific Research at King Saud University for funding Dr Alnahdi's participation in this research.
Results
Sixteen participants were recruited for the test-retest reliability portion of the present study, but the data from 1 participant were outliers, more than 2.5 times above the group median; because these data had the potential to artificially inflate the reliability estimate, they were not included in the analysis. Therefore, data from 15 participants are presented (Tab. 1). Measuring the strength of the hip abductor muscle group with a handheld dynamometer as described here had excellent test-retest reliability (ICC [2,3]=.95) (Tab. 2). On the basis of this reliability coefficient, the MDC95 was computed for hip abductor strength both as absolute values (newtons) and normalized using allometric scaling (Tab. 2). There was no systematic bias when the handheld dynamometer was used to test the strength of the hip abductor muscles (eFigure). The 95% CI of the mean difference between the second testing session and the first testing session contained the line of equality (0), indicating the absence of systematic error between testing sessions.
Characteristics of Participants Recruited for Reliability Testing (n=15)a
Test-Retest Results for Hip Abductor Strengtha
To examine the contribution of hip abductor strength to physical function, we tested 210 participants with unilateral TKA (Tab. 3). Bivariate correlation analysis was done as an interim analysis to provide support for the regression analysis and to aid in understanding the relationship between variables before they were entered into the regression analysis. Quadriceps muscle strength in the limb that had undergone surgery showed a significant inverse correlation with the TUG (r=−.38) and SCT (r=−.49) and a significant direct correlation with the 6MWT (r=.41) and KOS-ADLS (r=.18) (Tab. 4). Hip abductor strength in the limb that had undergone surgery showed a significant inverse correlation with the TUG (r=−.39) and SCT (r=−.48) and a significant direct correlation with the 6MWT (r=.37) (Tab. 4).
Characteristics of Participants Recruited for Testing Contribution of Hip Abductor Strength to Physical Function (N=210)a
Bivariate Correlations Among Variables (N=210)a
The covariates height, weight, age, sex, and knee pain explained between 27% and 57% of the variance in physical function measures (Tab. 5). Quadriceps muscle strength was a significant predictor of the TUG, SCT, 6MWT, and KOS-ADLS, explaining an additional 5.8%, 9.6%, 7.6%, and 2.4% of the variance in these measures beyond what was already explained by the covariates (Tab. 5). Regression analysis revealed no additional contribution of quadriceps muscle strength in explaining the GRS results (Tab. 5). Hip abductor strength was a significant predictor of performance in the TUG and SCT, explaining an additional 2.1% and 1.9% of the variance in these measures beyond what was already explained by the covariates and quadriceps muscle strength (Tab. 5). Regression analysis revealed no additional contribution of hip abductor strength in explaining the 6MWT, KOS-ADLS, and GRS results (Tab. 5).
Hierarchical Linear Regression Predicting Physical Function (N=210)a
Discussion
The present study was conducted to examine the test-retest reliability of using a handheld dynamometer to measure the strength of the hip abductor muscles and to determine whether hip abductor strength contributes to performance-based and self-reported physical function beyond the contribution of quadriceps muscle strength after TKA. The first hypothesis was supported; using a handheld dynamometer to measure hip abductor strength had excellent relative reliability (ICC [2,3]=.95) but moderate absolute reliability (MDC95=47.6 N; 95% CI=35.5, 76.5).33 When expressed as a percentage of the average hip abductor strength, the MDC95 was 26.8%, with a 95% CI ranging from 20% to 43% (Tab. 2). The wide CI of the MDC95 found in the present study may reduce its clinical applicability and suggests that another study with a larger sample size may be needed to estimate the MDC95 with a narrower CI for hip abductor strength. Piva et al16 reported an ICC of .92 (95% CI=.8, .97) with a commercial dynamometer; that value is close to the reliability coefficient reported in the present study. Pua et al18 showed that measuring hip abductor strength with a handheld dynamometer was reliable (ICC=.84; 95% CI=.55, .94) for patients with hip OA. The lower ICC and wide CI reported by Pua et al18 may reflect different populations or testing techniques. Pua et al18 tested their participants in the supine position and relied on the strength of the examiner to keep the test isometric; in contrast, we used a nonelastic strap to provide resistance, thereby removing the potentially biasing influence of the examiner's strength. On the basis of the results of the present study, we recommend using a side-lying position with a nonelastic strap during testing of the hip abductor muscles.
The second hypothesis was that hip abductor strength contributes to physical function beyond the contribution of quadriceps muscle strength in people with unilateral TKA. The results of the present study support this hypothesis. The correlation values for hip abductor strength and performance-based functional tests were similar to those for quadriceps muscle strength and performance-based functional tests (r =.37–.48). In addition, quadriceps and hip abductor muscle strength were moderately correlated (r=.58). Therefore, although the regression models revealed only a small additional contribution of hip abductor strength, the lack of a larger change might be explained by the moderate association between quadriceps and hip abductor muscle strength. Because quadriceps muscle strength was entered into the regression models before hip abductor strength, the variance in function that was shared by quadriceps muscle strength and hip abductor strength was assigned to the quadriceps muscle. This covariance made the contribution of the hip abductor muscle look small, although some of the variance assigned to the quadriceps muscle was shared with the hip abductor muscle. The converse also could be seen when hip abductor strength was entered into the model before quadriceps muscle strength; the shared variance was assigned to hip abductor strength rather than quadriceps muscle strength (eTable). The conclusion is that quadriceps muscle strength and hip abductor strength primarily explain similar components of function but that each strength metric does supply additional, complementary information.
Participants with stronger hip abductor muscles completed the TUG and SCT faster than those with weak hip abductor muscles. The design of the present study did not allow us to establish a definite causal relationship between hip abductor strength and physical function in participants with TKA, but it provided evidence that hip abductor strength was related to physical function; this relationship could be a causal indication that improving hip abductor strength will improve physical function. Given this relationship, clinicians are encouraged to consider targeting deficits in hip abductor strength in people with TKA in their rehabilitation protocols to maximize the recovery of physical function. This intervention is supported by the work of Piva et al,16 who reported that hip abductor strength was a significant predictor of performance during stair climbing, the 5-Chair-Rise Test, and the Figure 8 Walk Test after covariates and quadriceps muscle strength were controlled for.
A direct comparison of the present study and the study of Piva et al16 is not feasible. We used different covariates in the regression models, and given the small sample size in the study of Piva et al,16 there is the potential for inflated R2 values due to model overfitting.34,35 We also included pain in the knee that did not undergo surgery as a covariate in the regression models because this limb also predicts physical function after TKA2; we found a significant bivariate correlation with physical function. However, it is interesting that Piva et al16 found that hip abductor strength significantly predicted performance in the Figure 8 Walk Test.16 The Figure 8 Walk Test requires more lateral stability of the body than the straight walking in the performance-based tests used in the present study. The need for lateral movement and stability in the Figure 8 Walk Test may increase the importance of the hip abductor muscles in this test.
Hip abductor strength did not contribute to the prediction of self-reported measures of physical function. This result agrees with the report of Piva et al,16 who found no relationship between hip abductor strength and the physical function subscale of the Western Ontario and McMaster Universities Osteoarthritis Index.16 Pain in both the knee that had undergone surgery and the knee that had not undergone surgery and participants' anthropometry contributed significantly to self-reported measures of physical function, predicting 57% and 27% of the variance in the KOS-ADLS and GRS, respectively (Tab. 5). Participants' weight correlated negatively with the KOS-ADLS, indicating that participants who weighed more had lower self-reported functional ability. The pain level in both knees, especially the knee that had undergone surgery, correlated negatively with the KOS-ADLS and GRS (Tab. 4). Therefore, participants with higher pain levels in both knees had lower self-reported functional ability. Performance-based tests and patient-reported questionnaires measure different domains of physical function and disability.25,26,36 On the basis of our results and previous findings, the factors that contribute to performance-based measures of physical function may be different from those that contribute to self-perceived physical ability, and both should be used to evaluate disability and recovery after TKA.
Quadriceps muscle strength was a significant predictor of both performance-based measures of physical function (TUG, SCT, and 6MWT) and self-reported measures of physical function (KOS-ADLS) (Tab. 5). After height, weight, sex, and pain in the knee that had not undergone surgery were accounted for, quadriceps muscle strength still significantly predicted measures of physical function, highlighting the importance of quadriceps muscle strength for physical functioning after TKA. This finding is consistent with previous reports for people with TKA.2,9,10,16,37–39 Several prospective clinical trials have found that postoperative rehabilitation protocols focusing on early, progressive, and intensive quadriceps strengthening produce functional outcomes surpassing those produced by more traditional treatment paradigms. On the basis of the results of the present study, quadriceps muscle strengthening should continue to be the focus of outpatient rehabilitation. Although we anticipated asymmetrical hip abductor and quadriceps muscle weakness, only the quadriceps muscle showed substantial asymmetry between limbs, with the limb that had undergone surgery being 13% weaker than the limb that had not undergone surgery (Tab. 3). There was no difference in hip abductor strength between the limbs.
The present study is not without limitations. The cross-sectional design prevented the establishment of causality between hip abductor strength and physical function after TKA. However, the cross-sectional design allowed us to provide evidence of an association between hip abductor strength and physical function. The participants in the present study were tested 6 to 48 months after TKA surgery. The observed pattern of association between hip abductor strength and physical function might not be the same for participants earlier than 6 months or later than 48 months after surgery. On the other hand, the large sample recruited for the present study enhanced the external validity and generalizability of the findings to people with TKA.
In conclusion, both the strength of the hip and the strength of the knee are related to functional performance. It appears that hip strength and knee strength explain similar portions of functional ability, although each joint provides unique information about functional performance. Future trials evaluating rehabilitation protocols after TKA should include components of hip abductor strengthening. Measuring hip abductor strength with a handheld dynamometer in people with TKA is reliable.
Footnotes
Dr Alnahdi and Dr Snyder-Mackler provided concept/idea/research design. All authors provided writing and data analysis. Dr Alnahdi provided data collection. Dr Zeni and Dr Snyder-Mackler provided facilities/equipment. Dr Snyder-Mackler provided project management, fund procurement, institutional liaisons, and consultation (including review of manuscript before submission).
This study was approved by the Human Subjects Review Board at the University of Delaware.
This study was supported by National Institutes of Health grants P20RR016458 and K12HD055931. The authors extend their appreciation to the College of Applied Medical Sciences Research Center and the Deanship of Scientific Research at King Saud University for funding Dr Alnahdi's participation in this research.
- Received July 26, 2013.
- Accepted March 14, 2014.
- © 2014 American Physical Therapy Association
References
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