Abstract
Background Quadriceps femoris muscle strengthening is a common rehabilitation exercise for knee osteoarthritis (OA). More information is needed to determine whether targeting muscle power is a useful adjunct to strengthening for people with knee OA.
Objective The purpose of this study was to identify the predictive ability of knee extensor strength and knee extensor power in the performance of physical tasks in adults with knee OA.
Design This study used a cross-sectional design.
Methods Fifty-five participants with clinical knee OA were included (43 women; mean [SD] age=60.9 [6.9] years). Dependent variables were: timed stair ascent, timed stair descent, and the Six-Minute Walk Test (6MWT). Independent variables were: peak knee extensor strength and mean peak knee extensor power. Covariates were: age, body mass index, and self-efficacy. Multiple regression analyses were run for each dependent variable with just covariates, then a second model including strength, and then a third model including power. The R2 values were compared between models.
Results Power explained greater variance than strength in all models. Over and above the covariates, power explained an additional 6% of the variance in the 6MWT, increasing the R2 value from .33 to .39; 8% in the stair ascent test, increasing the R2 value from .52 to .60; and 3% in the stair descent test, increasing the R2 value from .44 to .47.
Limitations The sample demonstrated very good mobility and muscle function scores and may not be indicative of those with severe knee OA.
Conclusions In adults with knee OA, knee extensor power was a stronger determinant of walking and stair performance when compared with knee extensor strength. Clinicians should consider these results when advising patients on exercise to maintain or improve mobility.
Osteoarthritis (OA) is a chronic musculoskeletal disease that affects the articular cartilage of lower limb joints. Approximately 13.0% of Canadians live with this disease,1 with incidence rates that increase with age.2 The costs associated with the direct and indirect management of knee OA in Canada reached $27.5 billion in 2010.1 This number is expected to increase exponentially over the next 30 years due to the aging population.1 Even more concerning than the economic burden are the intangible costs associated with OA, such as disability, pain, and psychological effects. The knee is one of the most commonly affected joints.3 As there is no cure for the disease, researchers are concerned with identifying modifiable factors that could improve physical and psychological functioning for people affected by knee OA.
Quadriceps femoris muscle strength is a modifiable factor that may protect against decline of physical function. Deficits in quadriceps femoris muscle strength are associated with incident knee OA.4,5 However, strength, defined as the ability of a muscle to produce force,6 may not be the ideal measure of muscle function in knee OA. Muscle power refers to the ability to generate force rapidly (ie, product of force and velocity of muscle contraction).6 With age, decline in muscle power is approximately twice as pronounced as decline in muscle strength.7 Relatively little work has evaluated a role for muscle power in knee OA. Furthermore, exercise programs targeting rehabilitation of older adults with mobility issues tend to focus on strength and balance components,8 with few utilizing power training. In a trial of 33 women with knee OA, participants were randomly assigned to complete high-speed muscle training (power training, n=12), slow-speed training (conventional strength training, n=10), or a control intervention (warm-up and stretching exercises, n=11).9 The high-speed training group had greater improvements in overall muscle performance measures compared with the slow-speed training group but performed similarly to the slow training group and controls during functional tasks such as the 400-m walk, Berg Balance Scale, and timed chair rise.9 The small sample size may have limited the ability to identify statistical significance among groups. We need to better understand the relationships of muscle power and clinical outcomes to determine whether muscle power is a useful target for rehabilitation in knee OA.
The purpose of this study was to estimate the relative contributions of knee extensor power versus knee extensor strength to performance on the Six-Minute Walk Test (6MWT), a timed stair ascent task, and a timed stair descent task, after accounting for age, body mass index (BMI), and self-efficacy, among community-dwelling older adults with knee OA.
Method
A cross-sectional study design was used. The study took place between October 2012 and December 2013.
Participants
A convenience sample of community-dwelling older adults between the ages of 40 and 70 years with clinical knee OA participated. Participants were recruited at 1 rheumatology and 2 orthopedic surgery clinics. All participants met the clinical diagnostic criteria established by the American College of Rheumatology. These criteria include having knee pain present on most days of the month and at least 3 of the following 6 criteria: 50 years of age or older, knee stiffness lasting longer than 30 minutes, crepitus, bony tenderness, bony enlargement, or no warmth to the touch.10 Participants were excluded if they had alternate forms of arthritis (eg, rheumatoid arthritis) or active nonarthritic disease, any present or past use of intra-articular injections, previous knee surgeries, or any conditions that may have been exacerbated by the protocol (eg, unstable angina). In addition, screened participants were excluded if they reported that they required an assistive walking aid, such as a cane or a walker; could not ascend 2 flights of 9 steps; sustained lower extremity trauma within the previous 3 months; had ipsilateral hip or ankle conditions; had radiation therapy; or were pregnant. All participants provided written informed consent. We aimed to recruit a sample of 40 participants to accommodate adjusted analyses that included 4 predictors. A sample of 55 participants (12 men and 43 women, mean [SD] age=60.9 [6.9] years) met all criteria. The Knee Injury and Osteoarthritis Outcome score (KOOS) was collected to describe the sample. All 5 subscales of the KOOS were recorded and normalized out of 100. Higher scores for each subscale reflected less severe symptoms. The KOOS produces both highly reliable11 and valid12 data on knee OA.
Dependent Variables
The main variable of interest was stair climbing. This task is a common activity of daily living, requires greater biomechanical demand than walking,13 and is recommended in the core set of a test battery of physical function in knee OA.14 We assessed stair ascent and stair descent separately because the biomechanical demands placed on the knee differ between these tasks.14 A staircase of 9 steps, with a standard rise and run and with railings on both sides, was used. Participants were asked to climb the staircase as quickly and as safely as possible without running or jogging. Once they reached the top of the staircase, they were asked to descend the stairs as quickly and safely as possible. For both ascent and descent, participants were advised they could use the handrails. A stopwatch was used to record the time for each ascent and descent trial separately. Time was started when the participant's lead foot left the ground, and time was stopped when both feet were planted firmly on the final step. Both of the ascent and descent tasks were completed 2 times. The average time (in seconds) for each of stair ascent and stair descent was recorded to the nearest tenth of a second.
The second dependent variable was the 6MWT. The 6MWT is a mobility and submaximal exercise test used in clinical populations.15 The 6MWT produces valid data in the assessment of both pain and physical function in people with knee OA.16 For this test, participants were instructed to walk at a self-selected pace with the aim of covering as much distance as possible in 6 minutes. Participants were allowed to slow down, stop, or sit but were reminded that the time would continue running. Verbal encouragement was provided at 1-minute intervals throughout the test in accordance with the 6MWT guidelines.17 The 6MWT score was the distance walked to the nearest tenth of a meter.
Independent Variables
Independent variables were knee extensor strength and power of the most painful knee measured on an isokinetic dynamometer (Biodex Medical Systems Inc, Shirley, New York).
Strength was recorded as the maximal voluntary isometric contraction (MVIC) achieved for the knee extensor muscles. Each participant was positioned in the Biodex to ensure the knee joint center of rotation was in line with the dynamometer's axis of rotation. The participant was instructed to complete a submaximal warm-up by flexing and extending the knee with minimal resistance. Then, MVICs were recorded at 60 degrees of knee flexion.18 Five knee extension trials were recorded. Each isometric contraction was 5 seconds in length, with 5 seconds of rest between contractions. Verbal encouragement was provided. Raw time, torque, and velocity data from the isometric MVIC contractions were downloaded from the Biodex. A peak torque value (in newton-meters) was extracted and divided by body mass as a representative of peak knee extensor strength (in newton-meters per kilogram).
Knee extensor power was measured during isotonic contractions where resistance was scaled to each participant. Knee extensor power was assessed on the same dynamometer using an isotonic mode. Following MVICs, resistances of 25%, 50%, and 75% of MVIC torque were calculated. Participants completed 10 consecutive knee extension and flexion motions at each of these 3 resistances. A 1-minute rest period was provided after each set of isotonic contractions. The participants were provided with verbal encouragement to move their knee as quickly and forcefully as possible. Raw time, torque, and velocity data also were extracted for the isotonic contractions at 25%, 50%, and 75% MVIC. Power was calculated by first converting velocity from degrees per second to radians per second (1°/s × 0.0175). This velocity was multiplied by torque (in newton-meters) to yield a measurement of power (in watts). A single peak value for extensor power at each of 25%, 50%, and 75% MVIC was extracted from the middle 5 extensor power peaks (peaks 3–7) and was normalized to body mass (in watts per kilogram) to account for body size. Some participants did not complete 10 consecutive knee extensions and flexions with resistance set at 25% MVIC (n=1), 50% MVIC (n=11), and 75% MVIC (n=24). Thus, power was represented as the peak value of extensor power (in watts per kilogram) with resistance set at 25% MVIC.
Covariates
Age, self-efficacy beliefs, and BMI were chosen as covariates based on previous work that has shown these variables to be determinants of the outcomes of interest. Specifically, age-related declines have been demonstrated for the 6MWT19 and for stair climbing.20 The Arthritis Self-Efficacy Scale (ASES) was used to assess self-efficacy beliefs because self-efficacy is a strong determinant of mobility performance in people with knee OA.21 The ASES is a self-report questionnaire used to measure a person's self-efficacy to manage pain, physical function, and other health-related variables associated with arthritis. The ASES has 3 subscales: pain (5 items), function (9 items), and other symptoms (6 items). Each item was marked by the participant along a visual analog scale and subsequently scored from 10 to 100. Higher scores corresponded to greater self-efficacy beliefs (a positive result). The ASES produces consistent data in knee OA, with a Cronbach alpha of .75 to .89.22
Body mass index shares an inverse relationship with mobility performance.23–25 Body mass and height were measured, with participants barefoot and wearing T-shirts and shorts, using a platform scale stadiometer. Body mass was measured to the nearest 0.1 kg, and height was measured to the nearest 0.001 m.
Data Analysis
Descriptive statistics were calculated. Independent t tests were completed on all demographic, dependent, independent, covariate, and descriptive variables to assess the mean differences between sexes. In these comparisons between men and women, the Levene test for equality of variances determined whether the assumption for equal variances was met. If the Levene test result was not significant, the null hypothesis was kept, and equal variances between groups were assumed. If the test was significant, equal variances were not assumed, and the accompanying P value for unequal variances was used.
We performed unadjusted and adjusted analyses to examine the association between the performance measures (dependent variables) and strength and power (independent variables). For the unadjusted analysis, we calculated the percentage of variance explained by a simple regression analysis. Six simple regression models were constructed that consisted of the 3 dependent variables (6MWT, stair ascent test, stair descent test) and the 2 independent variables (knee extensor strength and knee extensor power).
The adjusted analysis applied multiple linear regression models where each model included the covariates identified previously and either knee extensor strength or power. The models were constructed in a stepwise manner. The 3 covariates were entered as a block (step 1), and the independent variable (strength or power) was entered (step 2). Separate models were created for strength and power. For each dependent variable model, we examined whether the addition of strength or power, given the covariates were already in the model, significantly improved the predictive ability of the model. All statistical tests were completed in SPSS version 21 (IBM Corp, Armonk, New York) and STATA version 12 (StataCorp LP, College Station, Texas).
Role of the Funding Source
This study was funded by the Canadian Institutes of Health Research (#102643 M.R.M.). Dr Maly holds a New Investigator Award from the Canadian Institutes of Health Research.
Results
A comparison of independent, dependent, covariate, and descriptive variables between women and men is presented in Table 1. This comparison was completed because of the imbalanced number of women (n=43) to men (n=12) within the sample. Women did not have higher ASES pain scores than men when the P value was adjusted for unequal variances.
Mean (SD) Values for Women (n=43) and Men (n=12) for All Dependent Variables, Independent Variables, and Covariates Used in the Analysisa
Table 2 provides a summary of the results. For the unadjusted models, all R2 values were statistically greater than zero. Moreover, for each performance measure, the power point estimate R2 value was greater than the strength point estimate R2 value. Similar results were obtained for adjusted analysis. Specifically, models containing strength did not significantly increase the predictive ability over covariate models. In contrast, adjusted models that included power significantly increased prediction compared with covariate alone models in the 6MWT, the timed stair ascent task, and the timed stair descent task. For the 6MWT, the addition of power over and above the covariate model increased the R2 from .33 to .39. Similarly, the addition of power to the model of the timed stair ascent task increased the R2 from .52 to .60, and the addition of power to the model for the timed stair descent task increased the R2 from .44 to .47.
Unadjusted and Adjusted Model Comparisons for Knee Extensor Strength and Power for 54 Casesa
Discussion
Knee extensor power was related to mobility performance in knee OA for the 6MWT, timed stair ascent task, and timed stair descent task. Knee extensor strength was unrelated to the performance of stair or walking tasks. Thus, knee extensor power explained greater variance than knee extensor strength in the performance of all 3 tasks, contributing an additional 6%, 8%, and 3% of the variance in the 6MWT, timed stair ascent task, and timed stair descent task, respectively. The relative contribution of determinants of mobility performance varied based on task; however, BMI consistently contributed to the explained variance during all 3 tasks. These findings provide further support that nutritional programs aimed at decreasing body mass play a critical role in managing immobility caused by knee OA.26 The findings are consistent with evidence from large clinical trials, systematic reviews, and clinical practice guidelines that emphasize the role of diet in combination with strengthening exercise26–28 in managing knee OA. The findings also suggest that physical therapists may want to consider the addition of training to increase knee extensor power when aiming to improve mobility in people with knee OA.
The relatively greater importance of knee extensor power than strength matches results presented in the literature on healthy older adults.7,29,30 In a sample of mobility-limited older adults (n=45), Bean and colleagues30 demonstrated that power explained up to 8% more of the total variance compared with strength in stair climbing, chair rise time, maximum gait speed, and balance. The current study demonstrated that in people with knee OA, mean peak extensor power explained up to 6%, 8%, and 3% more of the total variance of scores from the 6MWT, timed stair ascent task, and timed stair descent task, respectively, compared with their respective mean extensor strength model. Therefore, knee extensor power may be an important determinant for physical performance of everyday tasks in an older adult knee OA population. There are greater age-related power deficits than age-related strength deficits.7 It is possible that a loss of type IIA fibers31 and changes in agonist and antagonist neural activation32 related to aging muscle negatively affect speed of contraction more so than the ability to reach maximum force output. Considering that all of the dependent variables investigated in this study were timed tasks, the velocity or force development as dictated by measures of muscle power may be more important factors of mobility performance than peak strength capacity alone.
Although relatively few exercise intervention studies focused on knee OA utilize power or high-speed resistance training, pilot work suggests power training for knee OA may yield benefits beyond traditional strengthening programs.9 Power training protocols for knee OA in the literature emphasize that participants focus effort on enhancing their speed of movement. Examples of well-tolerated power exercises for older adults with knee OA include aquatic power training (eg, moving as quickly as possible during walking forward and backward, step-ups, hip abduction, extension and adduction, and ankle dorsiflexion and plantar flexion in the pool)33 and low-resistance, high-repetition training that requires participants to move “as fast as possible” on pneumatic leg press and standard knee extension machines.9 Even though the benefits of power training in knee OA appear to be no more efficacious than traditional “slow” resistance exercise in terms of improving strength and mobility performance, only power training appears to improve the rate of force development.9 Findings from these early studies of power training, combined with the small amount of variance in mobility attributed to knee extensor power in the current study, suggest that more work is necessary to further explore whether benefits exist as a result of power training in knee OA in larger samples.
Methodological differences exist between the current study and previous work examining knee power. We examined knee power using the isotonic protocol on a dynamometer. This protocol enabled participants to vary speed of contraction while working against a resistance scaled to their maximal strength output. By comparison, a double-leg press machine or a power rig with pneumatic scaling is often used to assess leg power in older adults.7,29,30,34 The power rig can be scaled to preset resistance levels similar to the 25%, 50%, and 75% protocol used in the current study.27 However, the power rig protocol requires one maximal power output effort at each preset resistance; the peak power achieved among any resistance level is chosen to represent power.30 The assumption that true peak power occurs with one maximal repetition is a potential limitation. Reports in the literature have often stated that multiple single-repetition exertions were collected with ample rest between exertions to obtain a peak in power output.30,34 In the current sample, peak power was not achieved on the first repetition, presenting a valid argument for selecting a maximum from the middle 5 peaks of a 10-repetition protocol when assessing leg power. Nonetheless, though methodological differences existed between the current study and previous literature, the studies consistently demonstrated the relatively greater importance of power versus strength in mobility performance among older adults with or without knee OA.
This study had limitations. First, the sample demonstrated very good scores on the KOOS, mobility performance, and muscle function measures. An older sample or a sample of people with more advanced knee OA may produce different results. Second, the predominance of women in the current study limits the generalizability of results to men with knee OA. Although no statistical differences in means were observed between women (n=43) and men (n=12), there exists a possibility that a larger sample of men would elicit different scores than those of the women in terms of self-reported function, mobility, muscle strength, and muscle power. Third, the stair ascent and stair descent tasks are not as challenging as some of the measures used to fully capture extensor power capabilities of the knee. Choosing a more challenging power measurement, such as jumping onto boxes as previous power protocols have described,35–37 was avoided to minimize pain and risk for injury. Nonetheless, increasing the number of stairs climbed or increasing the number of stair climbing trials completed may have altered the results.
In conclusion, knee extensor power was related to the performance of walking and stair ascent in people with knee OA. Knee power performed better than strength in explaining mobility in people with knee OA. Physical therapists should consider whether the addition of power training could be a useful adjunct to traditional physical therapy for knee OA. Additional research needs to examine a broader range of physical tasks in older populations and in populations of adults with more severe knee OA. Whether power training yields benefits beyond traditional strength training in larger samples of patients with knee OA also warrants further research.
Footnotes
Ms Accettura and Dr Maly provided concept/idea/research design, data collection, and project management. All authors provided writing and data analysis. Dr Maly provided fund procurement and facilities/equipment. Ms Accettura and Mr Stratford provided consultation (including review of manuscript before submission).
The study was approved by the Hamilton Integrated Research Ethics Board.
This study was funded by the Canadian Institutes of Health Research (#102643 M.R.M.). Dr Maly holds a New Investigator Award from the Canadian Institutes of Health Research.
- Received August 14, 2014.
- Accepted January 28, 2015.
- © 2015 American Physical Therapy Association
References
Issue highlights
