Short latency crossed inhibitory responses in the human 1 soleus muscle

22 Even though interlimb coordination is critical in bipedal locomotion, the role of muscle 23 afferent mediated feedback is unknown. The aim of this study was to establish if ipsilateral 24 muscle generated afferent feedback can influence contralateral muscle activation patterns in 25 the human lower limb and to elucidate the mechanisms involved. 26 The effect of ipsilateral tibial nerve stimulation on contralateral soleus (cSOL) responses 27 were quantified. Three interventions were investigated (1) Electrical stimulation applied to 28 the tibial nerve at stimulation intensities from 0–100% of maximal M-wave (M-max) with 29 the cSOL contracted from 5–15% of maximal voluntary contraction (MVC) and 15–30% 30 MVC, (2) Ipsilateral tibial nerve stimulation at 75% M-max prior to, during and following 31 the application of ischemia to the ipsilateral thigh. (3) Electrical stimulation applied to the 32 ipsilateral sural (SuN) and medial plantar nerves at stimulation intensities from 1–3 × 33 perceptual threshold. 34 A short latency depression in the cSOL EMG (onset; 37–41ms) was observed following 35 ipsilateral tibial nerve stimulation. The magnitude of this depression increased (P=.0005 36 and P=.000001) with increasing stimulus intensities. Ischemia delayed the time of the 37 minimum of the cSOL depression (P=.04). SuN and medial plantar nerve stimulation 38 evoked a longer latency depression (average; 91.2ms (SuN); 142ms (medial plantar nerve)) 39 and therefore do not contribute to the response. 40 This is the first study to demonstrate a short latency depression in the cSOL following 41 ipsilateral tibial nerve stimulation. Due to its short latency, the response is spinally 42 mediated. The involvement of crossed spinal interneurons receiving input from low 43