Physical Activity and Sedentary Behavior are Independently Associated with Combined Skeletal Muscle Index and Bone Mineral Density Status among Community-Dwelling Middle- Aged Women

Background Age-related decline in skeletal muscle mass and bone mineral density (BMD) frequently coexist, contributing to adverse health outcomes in aging populations. Although physical activity (PA) and sedentary behavior (SB) are important modifiable movement behaviors related to musculoskeletal health, their association with combined skeletal muscle mass and BMD status remains unclear. We aimed to investigate the associations of PA and SB with muscle–bone status among community-dwelling middle-aged women. Methods This cross-sectional observational study included 199 community-dwelling women aged 40–64 years in Aomori Prefecture, Japan. Skeletal muscle index (SMI) was assessed using bioelectrical impedance analysis, and BMD was evaluated via quantitative ultrasonography of the calcaneus. Participants were categorized into four groups according to the combination of SMI and BMD status: non-low BMD/non-low SMI, low BMD/non-low SMI, non-low BMD/low SMI, and low BMD/low SMI. PA and SB were objectively assessed using a triaxial accelerometer-based activity tracker. Multinomial logistic regression analysis was performed to examine the associations of PA and SB with muscle–bone status, adjusting for age, body fat mass, smoking status, and the interaction between PA and SB. Results Participants were classified as follows: Groups 1 (non-low BMD/non-low SMI, n = 139), 2 (low BMD/non-low SMI, n = 35), 3 (non-low BMD/low SMI, n = 12), and 4 (low BMD/low SMI, n = 13). Increased PA was independently associated with a greater likelihood of belonging to Groups 1 (odds ratio [OR]: 2.368, 95% confidence interval [CI]: 1.323–4.239; p = 0.004) and 2 (OR: 2.133, 95% CI: 1.174–3.876; p = 0.013) than to Group 4. Short sedentary time was independently associated with the likelihood of belonging to Groups 1 (OR: 0.992, 95% CI: 0.986–0.999; p = 0.023) and 2 (OR: 0.993, 95% CI: 0.987–0.999; p = 0.040). No significant interactions were observed between PA and SB. Furthermore, participants with combined low BMD and low SMI showed the lowest PA and longest sedentary time. Conclusions Increased PA and short sedentary time were independently associated with favorable combinations of skeletal muscle mass and BMD among community-dwelling middle-aged women. Therefore, increasing PA and reducing SB may be potential targets for preserving musculoskeletal health in midlife.