Phase modulation of the short - latency crossed spinal 1 response in the human soleus muscle

24 Short-latency spinally mediated interlimb reflex pathways were recently reported between 25 the left and right soleus muscles in the human lower-limb during sitting. The aim of the 26 current study was to establish if these pathways were observed during a functional motor 27 task such as human gait and modulated by the gait cycle phase and/or electrical stimulation 28 intensity. The second aim was to elucidate on the afferents involved. Two interventions 29 were investigated. 1) Ipsilateral tibial nerve (iTN) stimulation at motor threshold (MT), 30 35% of the maximal peak-to-peak M-wave (M-Max) and 85% M-Max (85M-Max) with 31 stimuli applied at 60%, 70%, 80%, 90% and 100% of the gait cycle of the ipsilateral leg. 2) 32 Ipsilateral sural nerve (SuN) and medial plantar nerve (MpN) stimulation at 1×, 2× and 3× 33 perceptual threshold at 90% of the gait cycle. The root mean squared (RMS) of the 34 contralateral soleus (cSOL) responses were analysed in a time window, 40–55ms (or 45– 35 60ms for subjects >50y/o) following iTN stimulation. The most consistent responses 36 occurred at 90% and 100% of the gait cycle at higher stimulation intensities of the iTN. 37 Significantly inhibitory responses (P=.006) were reported at 60% vs. 80% (P=.03), 90% 38 (P=.006) and 100% (P=.002) and 70% vs. 90% (P=.02) and 100% (P=.009) of the gait 39 cycle at 85M-Max. The responses became more inhibitory with increasing stimulation 40 intensities at 80% (P=.01), 90% (P=.001) and 100% (P=.004) of the gait cycle. Stimulation 41 of the MpN and SuN at all stimulation intensities demonstrated no short-latency responses. 42 Therefore, it is unlikely that afferents within these nerves contribute to the response. This is 43 the first study to show short-latency spinally mediated responses in the cSOL following 44 iTN stimulation, during walking. It provides evidence for a new spinal pathway 45