Postural responses to unexpected perturbations of balance during reaching

Abstract

To study the interaction between feedforward and feedback modes of postural control, we investigated postural responses during unexpected perturbations of the support surface that occurred during forward reaching in a standing position. We examined postural responses in lower limb muscles of nine human subjects. Baseline measures were obtained when subjects executed reaching movements to a target placed in front of them (R condition) and during postural responses to forward and backward support-surface perturbations (no reaching, P condition) during quiet stance. Perturbations were also given at different delays after the onset of reaching movements (RP conditions) as well as with the arm extended in the direction of the target, but not reaching (P/AE condition). Results showed that during perturbations to reaching (RP), the initial automatic postural response, occurring around 100 ms after the onset of perturbations, was relatively unchanged in latency or amplitude compared to control conditions (P and P/AE). However, longer latency postural responses were modulated to aid in the reaching movements during forward perturbations but not during backward perturbations. Our results suggest that the nervous system prioritizes the maintenance of a stable postural base during reaching, and that later components of the postural responses can be modulated to ensure the performance of the voluntary task.

Figure 1

A. Average muscle activity (+/− 1SD) in the right vastus lateralis (VL), soleus (SOL), tibialis anterior (TA) and medial gastrocnemius (MG) during unperturbed reaches for subject 2. B. Average muscle activity of the MG for perturbation only (P), perturbations with the arm extended (P/AE) and reaching with perturbations occurring at delay 1, 25 ms after reach onset (RP1) for the same subject as in A (S2). C. As in B, but for the TA muscle. For B and C, traces are aligned to the onset of the perturbation in all 3 conditions and average reach end represents the average time taken to perform the reach for all trials in the RP1 condition for the subject shown.

Figure 2

Mean (+/− 1SD) automatic postural response (APR) latencies for backward (A) and forward perturbations (B) in SOL, MG, TA and VL muscles for the experimental conditions P, RP and P/AE. Statistical differences are shown between measures in the RP and P/AE with respect to the P condition (*=p<.05). Muscle abbreviations as in Fig. 1. Mean (+/− 1SD) EMG activity during two 100 ms periods after the onset of postural perturbations in the 3 experimental conditions (P, RP and P/AE) of the MG (C) and TA muscles (D) during backward and forward perturbations, respectively (*=p<.05, **=p<.01). End finger position for RP conditions in relation to the reach only (R) condition for all trials and all subjects studied during backward (E) and forward (F) perturbations. For each condition, end-point ellipses have been drawn representing the 95% confidence level.