To cross-sectionally determine the quantitative relationship of age-adjusted, sex-specific isometric knee extensor and flexor strength to patient-reported knee pain.
Difference of thigh muscle strength by age, and that of age-adjusted strength per unit increase on the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) knee pain scale, was estimated from linear regression analysis of 4553 Osteoarthritis Initiative (OAI) participants (58% women). Strata encompassing the minimal clinically important difference (MCID) in knee pain were compared to evaluate a potentially non-linear relationship between WOMAC pain levels and muscle strength.
In OAI participants without pain, the age-related difference in isometric knee extensor strength was -9.0%/-8.2% (women/men) per decade, and that of flexor strength was -11%/-6.9%. Differences in age-adjusted strength values for each unit of WOMAC pain (1/20) amounted to -1.9%/-1.6% for extensor and -2.5%/-1.7% for flexor strength. Differences in torque/weight for each unit of WOMAC pain ranged from -3.3 to -2.1%. There was no indication of a non-linear relationship between pain and strength across the range of observed WOMAC values, and similar results were observed in women and men.
Each increase by 1/20 units in WOMAC pain was associated with a ∼2% lower age-adjusted isometric extensor and flexor strength in either sex. As a reduction in muscle strength is known to prospectively increase symptoms in knee osteoarthritis (KOA) and as pain appears to reduce thigh muscle strength, adequate therapy of pain and muscle strength is required in KOA patients to avoid a vicious circle of self-sustaining clinical deterioration.