Assessing the relationship among physical activity, muscle force capacity, and cortical diaphyseal bone status: the muscle-bone unit in young adults.
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Muscular forces associated with physical activity (PA) are the largest applied to the skeleton. However, their relationship with bone status is unclear as proxy measures such as muscle cross sectional area (MCSA) are typically assessed rather than direct measures of force. These proxy measures poorly characterize the effect of muscle force and require participants to undergo costly and radiative methodologies. There is a need to define the relationship between muscle force and bone status, and to identify non-invasive, field-based measures of muscle force to be used for osteoporosis assessment and research. Emerging research has used estimated power from vertical jump as a predictor of bone status; however, this was not done in the context of PA, and did not assess sex-differences. Thus, this study aimed to: 1) examine whether muscle force mediates the relationship between PA and bone status at the mid-tibia in young adults (n=144, 18-20 yo), and whether this relationship is moderated by sex, and 2) determine the utility of several lab-based and field-based measures of muscle force as predictors of bone status compared to a common muscle force proxy. Bone status and MCSA were assessed via peripheral quantitative computed tomography at the mid-tibia. Muscle force was estimated using dynamometry, leg extension power, and vertical jump. PA was measured over ≥7-days via a waist-worn accelerometer. Moderated mediation analyses revealed that sex moderates the relationship between PA and bone status (Cortical Thickness; Coeff.(SE)=-.0088±.0039, LLCI -.0166, ULCI -.0010), with a positive relationship existing in females (Cortical Thickness; Coeff.(SE)=.0088±.0027, LLCI .0034, ULCI .0142) but not males. However, ankle dorsiflexor force did not mediate the relationship between PA and bone status (all p>.05). In further analyses knee extension peak torque and peak anaerobic power estimated from vertical jump emerged as the strongest predictors of bone status, independent of MCSA, with standardized effects ranging from β=-.38 to .57 (all p<.05). Measures of muscle force vary greatly in their utility as predictors of bone status, as such, future research should assess other methodologies such as knee extension torque and the field-based peak power estimate from a vertical jump as predictors of skeletal health outcomes.