Title : 1 The effects of postural threat on spinal stretch reflexes :

26 Standing balance is often threatened in everyday life. These threats typically involve scenarios where 27 either the likelihood or the consequence of falling is higher than normal. When cats are placed in these 28 scenarios they respond by increasing the sensitivity of muscle spindles imbedded in the leg muscles, 29 presumably to increase balance-relevant afferent information available to the nervous system. At present, 30 it is unknown if humans also respond to such postural threats by altering muscle spindle sensitivity. Here 31 we present two studies that probed the effects of postural threat on spinal stretch reflexes. In Study 1 we 32 manipulated the threat associated with an increased consequence of a fall by having subjects stand at the 33 edge of an elevated surface (3.2 m). In Study 2 we manipulated threat by increasing the likelihood of a 34 fall by occasionally tilting the support surface on which subjects stood. In both scenarios we used 35 Hoffmann (Hand nd tendon stretch (T-) reflexes to probe the spinal stretch reflex circuit of the soleus 36 muscle. We observed increased T-reflex and unchanged H-reflex amplitudes in both threat scenarios. 37 These results suggest that the synaptic state of the spinal stretch reflex is unaffected by postural threat, 38 and that therefore the muscle spindles activated in the T-reflexes must be more sensitive in the threatening 39 conditions. We propose that this increase in sensitivity may function to satisfy the conflicting needs to 40 restrict movement with threat, while maintaining a certain amount of sensory information related to 41 postural control.