Dynamic Biomechanical Model of Foot Forces During Hypergravity Resistance Training

Abstract

Spaceflight has been shown to cause atrophy, reduced functional capacity, and increased fatigue in limb skeletal muscles, with the greatest change observed in antigravity muscles. The mechanisms of these losses are not fully understood but can be attributed to altered gene expression of myofibrillar proteins that is closely related to the muscle usage. During long duration space flight, periods of hypergravity may induce heightened G-like forces that could prevent muscle atrophy. PURPOSE: Determine if hypergravity is an effective resistive load to create significant ground reaction forces when performing squats in microgravity. METHODS: The Space Cycle is a human powered short arm centrifuge that can generate varying levels of hypergravity. The Space Cycle is configured with two arms, and active arm and passive arm, vertically hanging from a rotational axel. When a rider on the active arm peddles, the rotational rate of the centrifuge increases causing both arms swing upward towards a horizontal orientation. The hanging-arm design allows a subject standing in the passive arm (gondola) to experience hypergravity along their Gz axis. Experimental testing with this ground based Space Cycle has allowed the comparison of foot forces between squats performed in hypergravity and in earth gravitation. To test the effectiveness of the induced hypergravity in a microgravity environment a dynamic biomechanical model has been developed representing the mechanical Space Cycle and human subject. The subject is modeled using 12 rigid bodies with specified inertial properties representing different limb segments. Joints were modeled with kinematic constraints and viscoelastic properties. Input torques were prescribed for both the rotational speed of the Space Cycle and subject joint trajectories such that the rotational velocity and joint angles matched measured values. RESULTS: During squats on ground and while riding the Space Cycle: insole pressure sensors recorded foot forces, goniometers recorded the subject joint angles and hall-effect sensors recorded rotational velocity. Foot forces were simulated for squats performed on the Space Cycle both in microgravity (0G) and on earth (1G) environments. CONCLUSIONS: The dynamic model of the Space Cycle captures the major characteristics of the foot forces during squats performed in hypergravity on earth and significant foot forces have been predicted for squats in microgravity.