Postural responses to anterior and posterior perturbations applied to the upper trunk of standing human subjects.

James G Colebatch,Danielle L Dennis,Sendhil Govender

doi:10.1007/s00221-015-4442-2

James G Colebatch, Danielle L Dennis + Show 1 more

Open Access

https://doi.org/10.1007/s00221-015-4442-2

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Abstract

This study concerned the effects of brisk perturbations applied to the shoulders of standing subjects to displace them either forwards or backwards, our aim being to characterise the responses to these disturbances. Subjects stood on a force platform, and acceleration was measured at the level of C7, the sacrum and both tibial tuberosities. Surface EMG was measured from soleus (SOL), tibialis anterior (TA), the hamstrings (HS), quadriceps (QUAD), rectus abdominis (RA) and lumbar paraspinal (PS) muscles. Trials were recorded for each of four conditions: subjects’ eyes open (reference) or closed and on a firm (reference) or compliant surface. Observations were also made of voluntary postural reactions to a tap over the deltoid. Anterior perturbations (mean C7 acceleration 251.7 mg) evoked activity within the dorsal muscles (SOL, HS, PS) with a similar latency to voluntary responses to shoulder tapping. Responses to posterior perturbations (mean C7 acceleration −240.4 mg) were more complex beginning, on average, at shorter latency than voluntary activity (median TA 78.0 ms). There was activation of TA, QUAD and SOL associated with initial forward acceleration of the lower legs. The EMG responses consisted of an initial phasic discharge followed by a more prolonged one. These responses differ from the pattern of automatic postural responses that follow displacements at the level of the ankles, and it is unlikely that proprioceptive afferents excited by ankle movement had a role in the initial responses. Vision and surface properties had only minor effects. Perturbations of the upper trunk evoke stereotyped compensatory postural responses for each direction of perturbation. For posterior perturbations, EMG onset occurs earlier than for voluntary responses.Electronic supplementary materialThe online version of this article (doi:10.1007/s00221-015-4442-2) contains supplementary material, which is available to authorized users.

Highlights

Human upright stance must be maintained despite challenges from unexpected perturbations
The responses shown in this study differ substantially from those we previously reported for taps applied at a similar truncal level (Govender et al 2015)
The body did not behave like a rigid object; rather the acceleration wave propagated down the axial structures and was both delayed and attenuated by the time it reached the sacrum

Summary

Introduction

Human upright stance must be maintained despite challenges from unexpected perturbations. Larger postural perturbations require more complex responses, many “automatic”, and these have been explored using moving and rotating platforms. Such studies have shown the shortest latencies for calf muscles and longer latencies for more proximal muscles (Horak and Nashner 1986) as well as a role for central “set” in determining the size of responses (Horak et al 1989). Normal subjects are known to have less stability for AP displacements than for lateral ones (Horak et al 2005). Previous evidence existed for a reflex originating from axial structures, possibly muscle spindles within truncal muscles, and independent of any effects of ankle proprioceptors

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