Dynamic leg muscle power is an independent determinant of pain and quality of life in knee osteoarthritis (OA), and it appears to outperform muscle strength as a measure of muscle performance, according to researchers from Tufts University and Tufts Medical Center, Boston.
Skeletal muscle power, distinct from muscle strength, is defined as the product of dynamic muscular force and muscle contraction velocity. Compared with muscle strength, lower extremity muscle power is a better predictor of performance on tasks such as walking, rising from a chair, or climbing a flight of stairs.
“Compared to traditional measurements of muscle strength, our findings emphasize that muscle power is a more clinically important measure of muscle performance in knee OA,” the investigators wrote online in Arthritis and Rheumatology.
Data from 190 patients with knee OA also suggest that “individuals with established knee OA have substantial and accelerated impairments in muscle power,” they added.
“Indeed, the muscle power values in the present study are equivalent to levels generated in a specific group of mobility-limited older adults with a mean age of approximately 80 years.”
The findings also raise the possibility that training interventions specifically designed to improve muscle power may improve performance, pain, and quality of life in patients with knee OA, and merits investigation.
The investigators, led by Kieran F. Reid, PhD, MPH, analyzed baseline data of 190 adults with knee OA who participated in a randomized, controlled trial of tai chi versus physical therapy. Knee pain was measured using the Western Ontario and McMaster Osteoarthritis Index (WOMAC) pain subscale, which scores pain on a scale of 0 to 55, with higher scores indicating more severe disease.
Quality of life was assessed using the Short Form-36 (SF-36) questionnaire. Scores on each of the eight domains of the SF-36 were combined to obtain a physical component summary score and a mental component summary score.
Leg extensor muscle strength was measured via bilateral leg press, using a one-repetition maximum through a full range of motion. After a 5-minute rest period, peak power was measured as power output and contraction velocity during five bilateral leg press repetitions at 40% and 70% of the one-repetition maximum.
WOMAC pain was significantly and inversely associated with all measures of muscle strength, peak muscle power, and peak contraction velocity. In univariate analysis, greater peak muscle power at both 40% (low external resistance) and 70% (high external resistance) of the one-repetition maximum was significantly and negatively associated with WOMAC pain (r=-0.17, P≤ 0.05), and was significantly and positively associated with the SF-36 physical component score (r= 0.16, P≤0.05).
In separate multiple regression analyses, peak muscle power evaluated at high external resistance and peak contraction velocity measured at low external resistance were significantly and independently associated with WOMAC pain (all P≤0.05). Muscle strength, however, was not significantly associated with WOMAC pain (P=0.13).
Peak muscle power evaluated at both low and high external resistances were significant independent predictors of the SF-36 physical component score (P=0.04 and P=0.003, respectively).
Muscle power training at higher external resistances should be examined in knee OA, according to the investigators, “since an intervention of high intensity power training may portend greater reductions in disease severity or comparatively greater improvements in physical functioning, compared to low resistance power training.”
Two small pilot studies have demonstrated that low resistance power training (training performed at 40% of one-repetition maximum) improves muscle power, pain, and function in patients with knee OA. “However, additional, larger-scale randomized controlled trials of resistance training interventions to improve lower extremity muscle power across a variety of training intensities, and their subsequent impact on pain, quality of life, and functional outcomes in knee OA, are necessary,” the authors wrote.
Study limitations include the cross-sectional nature of the analysis and the lack of accounting for knee alignment (and other individual differences) that might contribute to variability in the relationship between disease severity and muscle performance. In addition, the influence of well-established risk factors for knee OA, such as adipose mass, were not assessed.
The study was supported by the National Institutes of Health, the Boston Claude D. Pepper Older Americans Independence Center, and the Boston Rehabilitation Outcomes Center.
The authors disclosed no financial relationships.
Reference: Reid K, et al “Muscle power is an independent determinant of pain and quality of life in knee osteoarthritis” Arthritis Rheum 2015; DOI: 10.1002/art.39336.