In vitro modeling of human tibial strains during exercise in micro-gravity

Prolonged exposure to micro-gravity causes substantial bone loss (Leblanc e t al., Journal of Bone Mineral Research 11 (1996) S323) and treadmill exerc ise under gravity replacement loads (GRLs) has been advocated as a counterm easure. To date, the magnitudes of GRLs employed for locomotion in space ha ve been substantially less than the loads imposed in the earthbound 1G envi ronment, which may account for the poor performance of locomotion as an int ervention. The success of future treadmill interventions will likely requir e GRLs of greater magnitude. It is widely held that mechanical tissue strai n is an important intermediary signal in the transduction pathway linking t he external loading environment to bone maintenance and functional adaptati on; yet, to our knowledge, no data exist linking alterations in external sk eletal loading to alterations in bone strain. In this preliminary study, we used unique cadaver simulations of micro-gravity locomotion to determine r elationships between localized tibial bone strains and external loading as a means to better predict the efficacy of future exercise interventions pro posed for bone maintenance on orbit. Bone strain magnitudes in the distal t ibia were found to be linearly related to ground reaction force magnitude ( R-2 > 0.7), Strain distributions indicated that the primary mode of tibial loading was in bending, with little variation in the neutral axis over the stance phase of gait, The greatest strains, as well as the greatest strain sensitivity to altered external loading, occurred within the anterior crest and posterior aspect of the tibia, the sites furthest removed from the neu tral axis of bending. We established a technique for estimating local strai n magnitudes from external loads, and equations for predicting strain durin g simulated micro-gravity walking are presented. (C) 2001 Elsevier Science Ltd. All rights reserved.