Primary factors causing the rearfoot external eversion moment during walking
The purpose of this study was to determine the primary factors causing the rearfoot external eversion moment during walking. We hypothesised that there is some factors. Twenty-nine healthy adult males participated in this study. The foot motion during the support phase was collected by the motion capture system (Vicon Motion Systems, Oxford, UK) at 500 Hz, and the force platform data (Type 9281E; Kistler Instruments, Winterthur, Switzerland) were collected at 1000 Hz. Participants were asked to walk barefoot along the walkway and across the force platform. The resultant moment vector acting on the ankle joint center was computed using the cross product of the vector from the ankle joint center to the center of pressure (COP) and the vector of the ground reaction force (GRF). From the moment vector, the parallel component to the longitudinal foot axis (heel to toe) was computed as the anatomically relevant total rearfoot external inversion/eversion moment due to the GRF (Mtot). The Mtot was decomposed into the moment components derived from the medio-lateral GRF (Mml) and from the vertical GRF (Mv). The Mml and Mv moment components were then decomposed into each force and moment arm. For Mml, the medio-lateral GRF (Fml) was the force, and the height of the ankle joint center (aH) was the moment arm. For Mv, the vertical GRF (Fv) was the force and the medio-lateral distance of the COP relative to the ankle joint center in the transverse plane (a_cop) was the moment arm. To visualize moment changes, we first normalized the data of each participant with the entire support phase as 100% (101 points). The ensemble change of all participants was obtained as the average waveform of each participant. The average values of each moment component, force, and moment arm in Phase I and Phase II for each participant were calculated (Phase I was defined until 10% of the support phase and Phase II was defined from 10% to 60% of the support phase). To investigate the contribution of force and moment arm to each moment component, a correlation analysis was conducted. Rearfoot eversion moment was dominated by the external moment due to the medio-lateral component of GRF during Phase I, and then the external moment due to the vertical component of GRF came to dominate the rearfoot eversion moment during Phase II (sheet normalized_Mtot,Mml,Mv). Correlation analysis revealed that Fml was strongly correlated with Mml, and also a_cop was strongly correlated with Mv in both phases (sheet various parameters). We found that the magnitude of medio-lateral GRF contributed to the rearfoot external eversion moment just after foot contact. During the subsequent phase, we found that the medio-lateral distance of the COP relative to the ankle joint center strongly contributed to the rearfoot external eversion moment.