Peculiarity of soleus motor potentials evoked by transcranial magnetic stimulation and electrical stimulation of tibial nerve

Objective: To reveal and discuss the peculiarities of soleus muscle in comparison with electrophysiological features of other leg muscles.

Methods: Vastus lateralis (L3), tibialis anterior (L4), extensor digitorum brevis (L5) and soleus (S1) muscles were tested at rest. Transcranial magnetic stimulation (TMS) combined with electrical stimulation of relevant peripheral nerves were applied. Cortically evoked motor potentials (C-MEP), peripheral compound muscle action potentials (CMAP) and F-waves were recorded. Estimating F-wave conduction time allowed calculation of the central conduction time (CCT-F) within the cortex-spinal motoneurones segment for each muscle.

Results: One could expect that the lower spinal metameric representation of the muscle the longer a corresponding CCT-F. However, a study of 30 healthy subject (60 right and left muscles) reveals a relatively short CCT-F for the soleus muscle. Moreover, the mean amplitude of soleus C-MEP is the lowest, CMAP and F-wave amplitude are the highest and standardised distal motor latency is the longest compared to the analogous parameters for the other muscles.

Conclusions: The reason for these special features can be attributed probably to a different structure and innervation of the soleus (mainly red, slow, tonic muscle) in contrast to the tibialis anterior and extensor digitorum brevis (mainly white, fast, phasic muscle).