Early postural adjustments in preparation to whole-body voluntary sway

Abstract

We studied postural adjustments associated with a quick voluntary postural sway under two conditions, self-paced and simple reaction-time. Standing subjects were required to produce quick discrete shifts of the center of pressure (COP) forward. About 400-500ms prior to the instructed COP shift, there were deviations of the COP in the opposite direction (backwards) accompanied by changes in the activation levels of several postural muscles. Under the reaction-time conditions, the timing of those early postural adjustments did not change (repeated measures MANOVA: p>0.05) while its magnitude increased significantly (confirmed by repeated measures MANOVA: p<0.05). These observations are opposite to those reported for anticipatory postural adjustments under simple reaction time conditions (a significant change in the timing without major changes in the magnitude). We conclude that there are two types of feed-forward postural adjustments. Early postural adjustments prepare the body for the planned action and/or expected perturbation. Some of these preparatory actions may be mechanically necessary. Later, anticipatory postural adjustments generate net forces and moments of force acting against those associated with the expected perturbation. Both types of adjustments fit well the referent configuration hypothesis, which offers a unified view on movement-posture control.

Figure 1

Schematic representation of the experimental setup. The electromyographic activity of the following postural muscles was monitored: tibialis anterior (TA), soleus (S), gastrocnemius - medial head (GM), gastrocnemius – lateral head (GL), biceps femoris (BF), semitendinosus (ST), vastus lateralis (VL), rectus femoris (RF), vastus medialis (VM), lumbar erector spinae (ES), latissimus dorsi (LD) and rectus abdominis (RA).

Figure 2

Time profiles for ΔCOP_AP (dashed line) and EMG of M. Tibialis Anterior (solid line) averaged across trials for RT condition of a typical subject as well as the visual representation of the definitions for d_{EPA_ΔCOP} as the difference in ΔCOP_AP between steady-state and t₀, t_{EPA_ΔCOP} as the time of initiation of ΔCOP_AP shift calculated from the ΔCOP_AP averaged across trials and subjects, t_{EPA_EMG} as the time of initiation of the change in EMG, and A_{EPA_EMG} as the area under the graph from t_{EPA_EMG} to t₀. Vertical solid line corresponds to time zero, t₀.

Figure 3

EMG data averaged across trials for a typical subject, for reaction-time (RT) condition trials (solid line) and self-paced (SP) condition trials (dashed line). Vertical solid line corresponds to time zero, t₀. TA – tibialis anterior; RF – rectus femoris; RA – rectus abdominis; GM – gastrocnemius medialis; BF – biceps femoris; ES – erector spinae

Figure 4

Time profiles of TA – tibialis anterior; GL – gastrocnemius lateralis; VL – vastus lateralis, and the change in COP_AP (ΔCOP_AP) averaged across trials and subjects in both reaction-time (RT) condition trials (solid line) and self-paced (SP) condition trials (dashed line). EPAs were observed in each of these three muscles for each subject. Vertical solid line corresponds to time zero, t₀.

Figure 5

Magnitude of the change in muscle activity (A_{EPA_EMG}) of TA – tibialis anterior; GL – gastrocnemius lateralis; VL – vastus lateralis and the magnitude of the change in ΔCOP_AP (d_{EPA_ΔCOP}) for reaction-time (RT) condition (grey) and self-paced (SP) condition (white).