Staying upright by shutting down? Evidence for global suppression of the motor system when recovering balance

Background: When automatic, yet unwanted action is quickly inhibited, short-lived suppression throughout the motor system ensues. This effect is referred to as global suppression. Although response inhibition is essential for behavioral flexibility, widespread motor suppression may delay action reprogramming. In reactive balance control, even fleeting suppression of the motor system could interfere with our ability to adapt compensatory reactions quickly enough to avoid a fall. Research Question: Is muscle activity in the hand suppressed when a prepotent compensatory step becomes suddenly blocked in a balance recovery task? Methods: Nineteen young adults were tested using a lean and release apparatus. Participants were occasionally released from a support cable resulting in forward body displacement. At the start of each trial, vision was occluded and a leg block was either placed in front of the legs or removed to allow a forward step. After goggles opened, the cable was released to cause a postural perturbation and participants had to either quickly step forward (STEP) or use a feet-in-place reaction to regain stability (NO-STEP). Step trials were much more frequent to promote stepping. Transcranial magnetic stimulation (TMS) was delivered shortly after receving vision (but before postural perturbation) to assess corticospinal excitability in an intrinsic hand muscle that was irrelevant to the balance recovery task. Results: Repeated measures ANOVA compared motor-evoked potentials across two step conditions (STEP, NOSTEP) and two TMS latencies (100 ms, 200 ms). The resultant interaction provided evidence of motor suppression in the hand when a forward step was blocked. Significance: Inhibition of a hand muscle uninvolved in a compensatory leg response provided evidence of global suppression in a whole-body, reactive balance context. Such widespread suppression of the motor system has implications for maintaining postural equilibrium, where even a momentary shutdown across body regions could interfere with the ability to adapt corrective balance reactions.