Lower Physical Activity Is Associated with Skeletal Muscle Fat Content in Girls

FARR, J. N., M. D. VAN LOAN, T. G. LOHMAN, and S. B. GOING. Lower Physical Activity Is Associated with Skeletal Muscle Fat Content in Girls. Med. Sci. Sports Exerc., Vol. 44, No. 7, pp. 1375-1381, 2012. Fat contained within skeletal muscle is strongly associated with obesity, type 2 diabetes mellitus, and metabolic syndrome. Physical inactivity may be a risk factor for greater fat infiltration within skeletal muscle during growth. Purpose: We sought to examine the relationship between physical activity and skeletal muscle fat content of the calf and thigh in girls. Methods: Data from 464 girls, age 8-13 yr, were used to examine the relationship between physical activity and skeletal muscle fat content of the calf and thigh. Calf and thigh muscle density (mg.cm(-3)), an index of skeletal muscle fat content, was assessed at the 66% tibia and 20% femur sites relative to the respective distal growth plates of the non-dominant limb using peripheral quantitative computed tomography. Physical activity level was classified by past-year physical activity questionnaire score. Results: Muscle densities of the calf and thigh were inversely correlated with percent total body fat (r = -0.37 and -0.48, P values < 0.001) and total body fat mass (r = -0.33 and -0.40, P values < 0.001). Multiple linear regression with physical activity, ethnicity, maturity offset, and muscle cross-sectional area as independent variables showed that physical activity was independently associated with muscle densities of the calf (beta = 0.14, P = 0.002) and thigh (beta = 0.15, P < 0.001). Thus, lower physical activity was associated with higher skeletal muscle fat content. Conclusions: Our results suggest that a lower level of physical activity may lead to excess skeletal muscle fat content of the calf and thigh in girls.