Article

We investigated the adaptation of balancing behavior during a continuous,
predictable perturbation of stance consisting of 3-min backward and forward
horizontal sinusoidal oscillations of the support base. Two visual conditions
(eyes-open, EO; eyes-closed, EC) and two oscillation frequencies (LF, 0.2 Hz; HF,
0.6 Hz) were used. Center of Mass (CoM) and Center of Pressure (CoP) oscillations
and EMG of Soleus (Sol) and Tibialis Anterior (TA) were recorded. The time course
of each variable was estimated through an exponential model. An adaptation index
allowed comparison of the degree of adaptation of different variables. Muscle
activity pattern was initially prominent under the more challenging conditions
(HF, EC and EO; LF, EC) and diminished progressively to reach a steady state. At
HF, the behavior of CoM and CoP was almost invariant. The time-constant of EMG
adaptation was shorter for TA than for Sol. With EC, the adaptation index showed
a larger decay in the TA than Sol activity at the end of the balancing trial,
pointing to a different role of the two muscles in the adaptation process. At LF,
CoM and CoP oscillations increased during the balancing trial to match the
platform translations. This occurred regardless of the different EMG patterns
under EO and EC. Contrary to CoM and CoP, the adaptation of the muscle activities
had a similar time-course at both HF and LF, in spite of the two frequencies
implying a different number of oscillation cycles. During adaptation, under
critical balancing conditions (HF), postural muscle activity is tuned to that
sufficient for keeping CoM within narrow limits. On the contrary, at LF, when
vision permits, a similar decreasing pattern of muscle activity parallels a
progressive increase in CoM oscillation amplitude, and the adaptive balancing
behavior shifts from the initially reactive behavior to one of passive riding the
platform. Adaptive balance control would rely on on-line computation of risk of
falling and sensory inflow, while minimizing balance challenge and muscle effort.
The results from this study contribute to the understanding of plasticity of the
balance control mechanisms under posture-challenging conditions.
CI - Copyright (c) 2011 Elsevier B.V. All rights reserved.

Langue : ANGLAIS