Differences in motor cortical control of the Soleus and Tibialis

The tibialis anterior (TA) and the soleus (SOL) are ankle joint muscles with functionally very different tasks. Thus, differences in motor cortical control between the TA and the SOL have been debated. This study compared the activity of the primary motor cortex during dynamic plantarflexions and dorsiflexions and compared this with measures obtained during rest. Single- and paired-pulse transcranial magnetic stimulations known as short-interval intracortical inhibition (SICI) were applied to the cortical representation of either the SOL or the TA muscle. The results show that the range of SICI from rest to activity is significantly greater in the TA than in the SOL. Furthermore, when the TA acts as the agonist during dorsiflexions of the ankle, SICI is almost absent (2.9%). When acting as the antagonist during plantarflexions, intracortical inhibition is significantly increased (28.7%). This task-specific modulation is far less pronounced in the SOL, which displayed higher levels of SICI when acting as the agonist (10.9%) during plantarflexion, but there was no significant inhibition (6.5%) as the antagonist during dorsiflexion. Furthermore, the cortical silent period (CSP) during plantarflexions was significantly longer in the SOL than in the TA during dorsiflexions, accompanied by a greater corticospinal excitability in the TA. Thus, cortical control considerably differs between the SOL and the TA in a way that inhibitory cortical control (SICI and CSP) of the TA is task-specifically adapted in a broader range of movements, whereas inhibition in the SOL muscle is less specific and more limited in its magnitude of modulation.