The effects of changing bone and muscle size on limb inertial properties and limb dynamics: a computer simulation

The magnitude and distribution of bone and muscle mass within limbs affect limb inertial properties, maximum movement speed and the energy required to maintain submaximal movements. Musculoskeletal modeling and movement simulations were used to determine how changes in bone and muscle cross-sectional area (and thus mass) affect human thigh and shank inertial properties, the maximum speed of unloaded single-leg cycling and the energy required to sustain submaximal single-leg cycling. Depending on initial conditions, shank moments of inertia increased 61-72 kg cm2 per kg added bone and 72-100 kg cm2 per kg added muscle. Thigh moments of inertia increased 46-63 kg cm2 per kg bone and 180-225 kg cm2 per kg muscle. Maximum unloaded cycling velocity increased with increased muscle mass (approximately 2.2-2.9 rpm/kg muscle), but decreased with increased cortical bone mass (approximately 2.0-2.8 rpm/kg bone). The internal work associated with unloaded submaximal cycling increased with increased muscle mass (approximately 0.42-0.48 J/kg muscle) and bone mass (approximately 0.18-0.22 J/kg bone).