A whole-body inverted pendulum model was used to investigate the control of balance and posture in the frontal plane during human walking. The model assessed the effects of net joint moments, joint accelerations and gravitational forces acting about the supporting foot and hip. Three video cameras and two force platforms were used to collect kinematic and kinetic data from repeat trials on four subjects during natural walking. An inverse solution was used to calculate net joint moments and powers. Whole body balance was ensured by the centre of mass (CM) passing medial to the supporting foot, thus creating a continual state of dynamic imbalance towards the centreline of the plane of progression. The medial acceleration of the CM was primarily generated by a gravitational moment about the supporting foot, whose magnitude was established at initial contact by the lateral placement of the new supporting foot relative to the horizontal location of the CM. Balance of the trunk and swing leg about the supporting hip was maintained by an active hip abduction moment, which recognized the contribution of the passive accelerational moment, and countered a large destabilizing gravitational moment. Posture of the upper trunk was regulated by the spinal lateral flexors. Interactions between the supporting foot and hip musculature to permit variability in strategies used to maintain balance were identified. Possible control strategies and muscle activatin synergies are discussed.
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The resultso f this study demonstratedth at whole body balance and posture in the frontal plane is preciselyr egulatedb y an interactiveh, ierarchicacl on-trol systemC. ontrol over the lateralt rajectoryo f the CM is achievedb y regulatingt he angularr otation of thew holeb ody CM aboutt heb aseo f supporta nd the trunk CM about the hips during single support.I n summary, frontal balance is accomplishedb y the following: (1) Total body medial/lateralb alancei s achieved primarily by the placemento f the supporting foot relativet o the horizontal location of the CM. (2) Errors in foot placemenat rec orrectede ithera t thel evelo f thes upportings ubtalaro r hip joints which work in synergy. (3) Control of HAT balancea bout the supporting hip is achievedb y the hip abductors and passive assistancefr om medial accelerationo f the hip joint which countert he largeg ravitationalm omento f the HAT. (4) Posture of the trunk is regulatedb y the spinal musculaturea nd the coupling betweent he pelvisa nd uppert runk.