Refining The Muscle-bone Unit: Independent Effects Of Muscle Size And Force, Body Weight, And Physical Activity

Abstract

PURPOSE: Muscle and bone form a functional unit. While muscle size is a useful surrogate of mechanical load on bone, the independent contributions to bone strength of muscle force, muscle size, gravitational load (body weight), and physical activity have not been assessed. METHODS: 321 healthy participants (32% black, 47% male), age 5 to 35 years were assessed. Peak dorsiflexion muscle torque (ft-lbs) of the ankle was assessed using isometric dynamometry. Tibia peripheral quantitative computed tomography measures included cortical bone polar section modulus (Zp, mm3), periosteal and endosteal circumference (mm), cortical area (mm2), and volumetric bone mineral density (vBMD, mg/cm3) at the 38% site, and muscle cross-sectional area (CSA, mm2) at the 66% site. Physical activity (average hours/week) was assessed by questionnaire. Log linear regression was used to assess determinants of muscle specific force (MSF; torque relative to muscle CSA) and Zp adjusted for age and tibia length. RESULTS: MSF was greater in blacks than whites (p < 0.05) and lower in females than males (p < 0.001). Zp was greater in blacks than whites (p = 0.002) in Tanner stages 1-4, but the difference was attenuated in Tanner 5 (interaction, p = 0.02), R2 = 0.87. Further adjustment for muscle CSA increased the R2 to 0.91. Muscle torque, body weight, and physical activity were added to the model and each load covariate was independently and significantly (all p < 0.02) associated with Zp (R2 = 0.92), periosteal circumference, and cortical area. Inclusion of these measures attenuated but did not eliminate the significant race differences. Only muscle CSA was positively associated with endosteal circumference, while none of the load covariates were associated with vBMD. CONCLUSIONS: Bone geometry is associated with several factors that define the mechanical load on bone, independent of age, tibia length, maturation, race, and sex. Race differences in Zp were not explained by these measures of mechanical load. Given that inclusion of muscle torque, body weight and physical activity resulted in a nominal increase in the R2, muscle size is an adequate surrogate for the mechanical load on bone in healthy participants.