Mediating Role of Muscle Mass in the Relationship Between Physical Activity and Bone Mass in Female University Students in China

Objective We are aimed at investigating the relationships among muscle mass, physical activity, and calcaneal bone mineral density (BMD) in female university students, and exploring the mediating role of muscle mass in the association between physical activity and calcaneal BMD. Methods Using stratified cluster sampling, 593 female university students aged 19–25 of different grades were selected. Anthropocentric characteristics, body composition, and calcaneal BMD were measured. Statistical methods including multiple linear/binary logistic regression models, mediation analyses, sensitivity analyses, and subgroup analysis were performed. Results The normal and abnormal BMD groups significantly varied in both total skeletal muscle mass (TSM) and total skeletal muscle mass index (TSMI) (t[TSM] = 3.216, t[TSMI] = 3.304, p < 0.01), and also diverse in physical activity ( p < 0.05). The increment in muscle mass might be related to an increase in BMD ( β [TSM] = 0.089, β [TSMI] = 0.265, p < 0.01) and a decrease in the probability of abnormal BMD ( β [TSM] = −0.193, β [TSMI] = −0.682, p < 0.01). The effect of physical activity on calcaneal BMD is significant ( β = −0.109, p < 0.05). Muscle mass plays a mediating role in the relationship between physical activity and calcaneal BMD, and the subgroup mediation sensitivity analyses indicated that the mediation effect only in the overweight group had significant mediation effects and relatively high robustness (Rho = −0.2, R 2 _MR 2 _Y = 0.04). Conclusion Muscle mass and physical activity both present a significant positive correlation with calcaneal BMD in female university students, and high activity level is a protective factor for abnormal BMD. Muscle mass performs a mediating role in the relationship of physical activity and calcaneal BMD in female university students of the overweight group.