Lower physical activity is associated with skeletal muscle fat content in girls.

UNLABELLED: 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 nondominant 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 (β = 0.14, P = 0.002) and thigh (β = 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.