Muscle contractions in response to foot stimulation as an inflight countermeasure

We propose that a passive countermeasure employing mechanical stimulation of the feet can be used during spaceflight to attenuate neuromotor degradation. The purpose of this investigation was to determine whether the time between consecutive stimuli to the human sole influences the subsequent muscle contraction response. Participants were exposed to mechanical stimulation of the right sole in a protocol of six conditions of stimulation couplets. Stimulation couplets consisted of two closely timed stimuli separated by an interstimulus interval (ISI) of either 100, 140, 180, 220, 260, or 300 ms. Electromyography of the leg musculature was collected. The focus of the analyses was the amplitude of the response to the second stimulus of each couplet relative to the isolated baseline response. The results indicate that the second response in a couplet was inhibited up to 220 ms ISI, but returned to baseline with greater ISIs. When developing an inflight dynamic foot stimulator it will be important to ensure that the timing between the stimulations is separated by ISIs that prevents the inhibition of neuromuscular activation.