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 9 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.

Postural adjustments to support surface perturbations during reaching depend upon body–target reference frame

“…6a, consistent with Trivedi et al 2010). Likely because the rSOL was not the muscle principally responsible for restoring balance after a forward perturbation of the surface, the activity was affected overall to a much lesser extent for Bins 2-4 than the rTA muscle, but interesting 20mm Y X trends nonetheless emerged with respect to reference frame and time of perturbation during a reaching movement.…”

“…A ratio of one indicates equal time spent in the two phases, <1 = more time spent decelerating than accelerating. To be comparable with our previous study which used slightly higher perturbation speeds of 0.6 g (Trivedi et al 2010) and other commonly used postural perturbation studies (TorresOviedo and Ting 2007;Welch and Ting 2009), in this study we only analysed RP trials collected at the highest platform speed (velocity of 40 cm/s, or 0.5 g). Following data collection, we sorted all RP trials into two groups depending on whether platform movements occurred during the acceleration or deceleration phase of the reach movement.…”

“…Custom-written routines using the NextMove ESB Workbench software (Baldor Electric Co., Fort Smith, AR, USA) were used to control platform motion. The moveable platform was the same as that used in previous studies to investigate various aspects of feedback-based postural control (Torres-Oviedo and Ting 2007;Ting 2008, 2009;Trivedi et al 2010). Participants wore a chest band containing a modified gaming switch (model 459512, RP Electronics, Burnaby, BC) that emitted a 5-V signal when pressed.…”

“…Our previous results (Trivedi et al 2010) were obtained using a paradigm in which the target was anchored to the moving surface for all experimental trials. Thus, regardless of the direction or speed of support surface movement, the approximate distance between the body and the target remained constant, and both travelled at a similar rate of displacement.…”

“…In view of this apparent flexibility of the integration of muscle activity produced from different modes of postural control, we have previously used unexpected disturbances of balance during ongoing reach movements to visual targets in the standing position to explore the integration of postural activity under behaviourally relevant conditions of reaching when the support base is disturbed (e.g., reaching for the handrail on a bus when it suddenly moves; Trivedi et al 2010). We showed that reaching had little effect upon the short-latency component (≤100 ms) of postural responses following the perturbation onset, regardless of the direction of disturbance.…”