Association of muscle quantity and quality with knee extension strength in knee osteoarthritis: the Nagahama study

Objective We aimed to examine whether lower‐limb muscle quantity and quality assessed by bioelectric impedance analysis (BIA) were associated with knee extension strength and whether the association differed with knee osteoarthritis (OA) severity. Methods We included 1,525 participants (63.6% women; mean ± SD age 68.0 ± 5.3 years) from the Nagahama Prospective Cohort. Knee extension strength was measured during maximum voluntary isometric contraction. Lower‐limb muscle mass and the extracellular water (ECW)–to–intracellular water (ICW) ratio were used as indicators of muscle quantity and quality, respectively, and assessed via a BIA device. We executed multiple linear regression analyses to investigate the association of muscle quantity and quality with knee extension strength. Additionally, participants were classified into three groups with respect to OA severity and symptoms: control, early OA, and advanced OA groups; subgroup analyses were also executed. Results The muscle mass ( P < 0.001) and ECW‐to‐ICW ratio ( P = 0.009) were significantly associated with knee extension strength. In the subgroup analysis, the muscle mass was significantly associated with knee extension strength ( P < 0.001), but there was no association between ECW‐to‐ICW ratio and knee extension strength ( P = 0.731) in the control group. In the early OA and advanced OA groups, the muscle mass (both P < 0.001) and ECW‐to‐ICW ratio (early OA, P = 0.034; advanced OA, P = 0.015) were significantly associated with knee extension strength. Conclusion Lower‐limb muscle quality was associated with knee extension strength, and the association was stronger in patients with knee OA. These findings suggest that both muscle quantity and quality should be assessed to better understand muscle function in patients with knee OA.