Implications of evaluating leg muscle mass and fat mass separately for quadriceps strength in knee osteoarthritis: the SPSS-OK study.

Abstract

To examine the influence of obesity on quadriceps strength by separately analyzing body mass index (BMI) as fat mass and leg muscle mass in patients with knee osteoarthritis (KOA). The Screening for People Suffering Sarcopenia in Orthopedic cohort of Kobe (SPSS-OK) study was a single-center cross-sectional study that recruited 906 patients with KOA. Fat mass and leg muscle mass were measured by bio-impedance. Isometric knee extension torque (Nm) was measured as quadriceps strength. A series of general linear models were fitted to estimate the continuous associations of BMI and fat mass with quadriceps strength, with adjustment of confounders. In the fitted models, both BMI and fat mass were treated as restricted cubic spline functions. A continuous, non-linear relationship between BMI and quadriceps strength was found (P = 0.008 for non-linearity). In patients with a BMI of 16-25kg/m2, increasing quadriceps strength was observed. However, in patients with a BMI of 25-40kg/m2, quadriceps strength seemed similar. Additionally, an inverted U-shaped relationship between fat mass and quadriceps strength was demonstrated (P = 0.04 for non-linearity). In those with a fat mass of 10-20kg, increasing quadriceps strength was seen. However, in patients with a fat mass of 20-30kg, quadriceps strength showed a decreasing trend. Independent of fat mass, leg muscle mass was linearly associated with greater quadriceps strength. Our study suggests that there are independent associations between the leg muscle mass, fat mass, and quadriceps strength. It is difficult to easily predict quadriceps strength using only BMI. • An increase in body mass index (BMI) up to 25kg/m2was associated with increasing quadriceps strength. • Quadriceps strength remained almost unchanged among patients with a BMI of > 25kg/m2. • The association between fat mass and quadriceps strength had an inverted U-shaped relationship, suggesting the importance of the separate assessment of fat mass and muscle mass in patients with knee osteoarthritis, especially those who are overweight or obese.