The purpose of this study was to assess corticospinal excitability of soleus (SOL) and tibialis anterior (TA) at a segmental level during passive ankle movement. Four experimental components were performed to assess the effects of passive ankle movement and muscle length on corticospinal excitability (MEP/Mmax) at different muscle lengths, subcortical excitability at the level of lumbar spinal segments (LEP/Mmax), intracortical inhibition (SICI) and facilitation (ICF), and H-reflex in SOL and TA. In addition, the degree of fascicle length changes between SOL and TA was assessed in a subpopulation during passive ankle movement. Fascicles shortened and lengthened with joint movement during passive shortening and lengthening of SOL and TA to a similar degree (p < 0.001). Resting motor threshold was greater in SOL compared to TA (p ≤ 0.014). MEP/Mmax was facilitated in TA during passive shortening relative to the static position (p ≤ 0.023) and passive lengthening (p ≤ 0.001), but remained similar during passive ankle movement in SOL (p ≥ 0.497), regardless of muscle length at the point of stimulus (p = 0.922). LEP/Mmax (SOL: p = 0.075, TA: p = 0.071), SICI (SOL: p = 0.427, TA: p = 0.540), and ICF (SOL: p = 0.177, TA: p = 0.777) remained similar during passive ankle movement. H-reflex was not different across conditions in TA (p = 0.258), but was reduced during passive lengthening compared to shortening in SOL (p = 0.048). These results suggest a differential modulation of corticospinal excitability between plantar and dorsiflexors during passive movement. The corticospinal behaviour observed might be mediated by an increase in corticospinal drive as a result of reduced afferent input during muscle shortening and appears to be flexor-biased.
Keywords: Fascicle length; H-reflex; Ia afferent; Transcranial magnetic stimulation.