Leg extensor muscle power is an independent determinant of pain and quality of life in knee osteoarthritis, according to a cross-sectional analysis of data from a trial.
“Compared to strength, muscle power may be a more clinically important measure of muscle function within this population. New trials to systematically examine the impact of muscle power training interventions on disease severity in knee OA are particularly warranted,” concluded the investigators, led by Kieran Reid, Ph.D., of Tufts University in Boston (Arthritis Rheumatol. 2015 Aug 28. doi: 10.1002/art.39336).
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The conclusion comes from their analysis of baseline data for 190 subjects participating in a trial comparing Tai Chi to standard physical therapy for knee OA. Their mean age was 60 years, and they had a mean body mass index of 32.7 kg/m2. The majority of participants were women.
On univariate analysis, greater muscle power – defined as the product of dynamic muscular force and muscle contraction velocity – was significantly associated with pain (r = –0.17, P less than .02), as assessed by the Western Ontario and McMaster Osteoarthritis Index. It was also significantly and positively associated with Short Form 36 physical component scores (r = 0.16, P less than .05), a measure of health-related quality of life.
After adjusting for multiple covariates, muscle power was a significant independent predictor of pain (P less than or equal to .05) and physical component scores (P less than or equal to .04). Strength, a far more common assessment in knee OA, was not an independent determinant of either pain or quality of life (P greater than or equal to .06).
To measure leg extension muscle power, subjects were asked to do two sets of five bilateral leg press repetitions, one set at 40% of their maximum strength and one set at 70%. Each repetition within a set was performed as quickly as possible, with 30 seconds rest between each repetition.
Knee OA patients “have substantial and accelerated impairments in muscle power,” and might benefit from training to improve it. Power generated at 70% resistance in the study seemed to be “a marginally greater determinant of pain and quality of life in knee OA” than power at 40%, suggesting that higher intensity training might be more helpful.
The investigators had several ideas about how power relates to knee pain. Power, rather than strength alone, helps keep the knee stable when in motion. Impairment could contribute to “inadequate control of tibial translation during ambulation, leading to damage, and ultimately knee pain.” Also, impairment probably limits the ability to dissipate knee joint loads, increasing “the risk of articular contact stress, which leads to pain,” they said.
“Muscle strength, the maximal force generating capacity of skeletal muscle, has been the focus of numerous investigations. However, studies examining muscle strength in the etiology of knee OA have presented inconsistent and controversial findings.” Dynamic leg extensor power “represents a more functionally relevant assessment of muscle performance and ... a more clinically important correlate of disease burden in knee OA,” they said.
The study was supported by grants from the National Institutes of Health. The investigators have no disclosures.