An experimental paradigm to assess postural stabilization: no more movement and not yet posture.

A ground reaction based method is proposed to evaluate the hypothesis that a stabilization phase occurs in transitions towards erect posture, following the macroscopic movement and preceding the quiet final erect posture, whose aim is to control and dissipate the residual inertial unbalancing forces occurring at the transition end. The experimental protocol considers three tasks leading to the final erect posture: taking a step forward (F), sit-to-stand (S), and bending the trunk forward (B), The method mainly consists of the fitting of a negative exponential function on the instability time profile following the end of the transition movement. The model parameters Y(0), T, and Y(inf), respectively, quantify the initial instability rate, a time duration related to the stabilization, and the final asymptotic instability rate. Results from a sample of 40 adult able bodied subjects demonstrated that a postural stabilization phase actually occurs: Y(inf) is smaller (0.010, 0.010, and 0.008 m/s(2) for, respectively, F, S, and B tasks) than Y(0) (0.081, 0.137, and 0.057 m/s(2)). Tis in the order of seconds (0.95, 0.51, and 1.00 s). No trial with large values of both Y(0) and T was observed, evidencing that large initial instability rates are quickly controlled and reduced. The Y(0) and T parameters distribution are discussed according to the possible underlying active and/or passive stabilization mechanisms. The test-retest reliability overall figure (mean ICC 0.45 for 12 indexes) increased, when dropping the indexes related to the less reliable B task, to values (mean ICC 0.56 for eight indexes) comparable to published posturographic data.