Age-related deposition of fat in skeletal muscle is associated with functional limitations. Skeletal muscle fat may be present in people with preserved muscle mass or accompanied by muscle wasting. However, it is not clear if the association between muscle fat deposition and physical performance is moderated by muscle mass.
To determine whether the association between mid-thigh inter-muscular fat and physical performance is moderated by muscle area.
We performed a cross-sectional analysis of the Health ABC data collected in 2002-03 (n=1897, women:52.2%). Mid-thigh muscle cross-sectional area (by CT) and physical performance measures were compared across quartiles of inter-muscular fat absolute area. Moderation analysis was performed to determine the conditional effect of inter-muscular fat on physical performance as a function of muscle area. Conditional effects were evaluated at three levels of muscle area (mean and plus/minus 1 standard deviation (SD); 213.2±53.2 cm2).
Simple slope analysis showed that the negative association between inter-muscular fat area (cm2) and leg strength (N.m) was of greater magnitude [beta coefficient (b), 95% confidence interval (CI)=-0.288 (-0.427,-0.148)] in participants with greater muscle area (i.e., 1-SD above-the-mean) compared to those with lower muscle area [i.e., at mean (b=-0.12 (-0.248,0.008)) or 1-SD below-the-mean (b=0.048 (-0.122,0.217))]. Similarly, the negative association of inter-muscular fat with 400-m walk speed (meters/second) and chair stand (second) was greater in those with higher muscle areas (P<0.001) compared to those with lower muscle areas.
The association between higher inter-muscular fat area and impaired physical function in aging is moderated by muscle area.