Cutaneous Afferent Stimulation in the Human Lower Limb

42 A powerful early inhibition is seen in triceps surae following transcutaneous electrical stimulation 43 of the Achilles tendon (tendon electrical stimulation; TES). The aim of the current study was to 44 confirm results from surface electromyogram (SEMG) recordings that the inhibition is not wholly 45 or partly due to stimulation of cutaneous afferents that may lie within range of the tendon 46 electrodes. Due to methodological limitations, SEMG does not reliably identify the timecourse of 47 inhibitory and excitatory reflex components. This issue was revisited here using an analysis of 48 changes in single motor unit (SMU) firing rate (peristimulus frequencygram; PSF) and probability 49 (perstimulus time histogram; PSTH) to re-examine the timecourse of inhibitory SMU events that 50 follow purely cutaneous (superficial sural) nerve stimulation. Results were then compared with 51 similar data from TES. When compared with the reflex response to TES, sural nerve stimulation 52 resulted in a longer onset latency of the primary inhibition and a weaker effect on SMU firing 53 probability and rate. PSF also revealed that decreased SMU firing rates persisted during the 54 excitation phase in SEMG, suggesting that the initial inhibition was more prolonged than previously 55 reported. In a further study, the transcutaneous SEMG Achilles tendon response was compared with 56 that from direct intra-tendon stimulation with insulated needle electrodes. This method should 57 attenuate the SEMG response if it is wholly or partly dependent on cutaneous afferents. However 58 subcutaneous stimulation of the tendon produced similar components in the SEMG confirming that 59 cutaneous afferents made little or no contribution to the initial inhibition following TES. 60 61