Abstract
Postural reflexes were recorded in healthy subjects (n = 17) using brief axial accelerations and tap stimuli applied at the vertebra prominens (C7) and manubrium sterni. Short latency (SL) responses were recorded from the soleus, hamstrings and tibialis anterior muscles and expressed as a percentage of the background EMG prior to stimulus onset. In the majority of postural conditions tested, subjects were recorded standing erect and leaning forward with their feet together. The SL response was larger for soleus than for the hamstrings during standing (soleus vs hamstrings; 70.4 vs 28.1 %), whereas the opposite occurred during kneeling (25.3 vs 127.3 %). Concordant head and trunk accelerations produced larger SL responses than discordant accelerations for soleus and hamstrings, but the evoked excitatory response was independent of head direction and as expected for the direction of truncal acceleration. Postural reflexes for soleus and tibialis anterior were strongly affected by conditions that posed a significant threat to postural stability; stimulation at C7 was associated with significant SL enhancement for soleus during anterior lean while sternal stimulation showed SL enhancement for tibialis anterior during posterior lean. Cutaneous anaesthesia applied over the C7 stimulation site had no significant effect on EMG responses, nor did vision or surface type (rigid or compliant). This study provides further evidence that postural reflexes produced by brief axial accelerations are independent of cutaneous receptors, vestibular afferents and ankle proprioceptors, and demonstrates that postural tasks and truncal orientation significantly affect the evoked response, consistent with a role in stabilising posture.Electronic supplementary materialThe online version of this article (doi:10.1007/s00221-014-4105-8) contains supplementary material, which is available to authorized users.
Highlights
Unexpected perturbations when standing present a threat to postural stability and short latency (SL) reflexes have an important role in countering such threats
This study provides further evidence that postural reflexes produced by brief axial accelerations are independent of cutaneous receptors, vestibular afferents and ankle proprioceptors, and demonstrates that postural tasks and truncal orientation significantly affect the evoked response, consistent with a role in stabilising posture
While these responses have been attributed to afferents excited by ankle movements (Fitzpatrick et al 1994), postural reflexes arising from truncal receptors have been reported (Gurfinkel et al 1981; Bloem et al 2000, 2002) but not widely known or accepted
Summary
Unexpected perturbations when standing present a threat to postural stability and short latency (SL) reflexes have an important role in countering such threats. Similar to a slip, evoke SL postural reflexes, which are prominent in trunk muscles (Horak et al 1994) While these responses have been attributed to afferents excited by ankle movements (Fitzpatrick et al 1994), postural reflexes arising from truncal receptors have been reported (Gurfinkel et al 1981; Bloem et al 2000, 2002) but not widely known or accepted. Graus et al (2013) presented further evidence in support of an axial source of postural reflexes These authors applied small perturbations to the head and trunk and showed that the responses in soleus were determined by the direction of the applied disturbance, that the upper trunk was the most effective site of stimulation and that the responses were not present with the
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