Role of trunk inertia in non-stepping balance recovery

Previous research has identified two major non-stepping strategies used to recover balance following mechanical perturbations: ankle and hip strategy [1, 2]. These strategies are selected depending on eg the perturbation magnitude, prior experience, and configuration of the support surface [2] in order to control the posture (upright trunk and leg orientation) and angular momentum [3, 4]. Following an external mechanical perturbation, both body posture and angular momentum depend, in part, on passive properties of the body, such as the amount and distribution of mass. Simple mechanical models, like the inverted pendulum (IP)[4, 5] or the double IP [6] suggest an approximately linear inverse relationship between the inertia of a perturbed body segment and the resultant acceleration and, presumably, also the segment deflection.