Bending and torsion predominate the in vivo human tibia deformation regimes during locomotive activities and its relationship to muscular contractions

Introduction Bone deformation plays a decisive role in bone adaptation to the environment. Insufficient mechanical loading on bone under weightlessness may contribute to bone loss in astronauts. However, our understanding of the in vivo human bone deformation is limited due to technical difficulties. The contribution of muscular contractions on the bone loading is currently under debate. Taking the human tibia as a subject, the tibia deformation regimes and its relationship with muscular activities during different locomotor activities were investigated in the present study. Methods Tibia deformation was recorded in five subjects utilizing a novel optical approach established in our lab. Briefly, two marker clusters with three non-collinear retro-reflective markers on each cluster were affixed into the proximal and distal anterior-medial aspect of tibia cortex by bone screws. The markers trajectories were captured at 300 Hz with an optical system during different activities. Tibia deformations regimes, namely bending and torsion angles, were computed from the relative movement of the proximal cluster with respect to the distal cluster. Results The proximal tibia primarily bends to the posterior aspect (bending angle: 0.15° - 1.30°), medial aspect (bending angle: 0.38° - 0.90°) and twisted to the external aspect (torsion angle: 0.67° - 1.66°) during walking at between 2.5 and 6.1 km/h. Peak posterior bending and peak torsion occurred during the first (22%) and second half (76%) stance phase, respectively. Peak to peak (p2p) antero-posterior (AP) bending angles increased linearly with speed during walking and running, but p2p torsion angles remained constant. The largest p2p bending angle was observed during maximum single leg hopping, with p2p AP bending angle of 5.05 ± 0.33°. The p2p torsion angle was larger with forefoot than rear foot stair ascent and running. The tibia deformation regimes were characterized by torsion (1.35° ± 0.07°) rather than bending (0.52° ± 0.07°) during maximum isometric plantar flexion. Discussion and Conclusions Bending and torsion predominated the tibia deformation regimes during the investigated activities. Unexpected large torsion deformation, at least partially contributed by muscle contractions, might be another candidate to drive the long bone adaption. These findings therefore are relevant to the development of countermeasures, e.g. muscles training protocol, against bone loss in space