Objective:
It is well documented that corticospinal projections to motoneurons of one muscle inhibit antagonist motoneurons through collaterals to reciprocally organized spinal inhibitory interneurons. During and just prior to dorsiflexion of the ankle, soleus motoneurons are thus inhibited as evidenced by a depression of the soleus H-reflex.
The objective of this study was to investigate if motor evoked potentials (MEPs) evoked by transcranial magnetic stimulation (TMS) show a similar depression prior to and at the onset of contraction of muscles that are antagonists to the muscle in which the MEP is evoked.

Methods:
Seated subjects (n=11) were instructed to react to an auditory cue by contracting either the tibialis anterior (TA) or soleus muscle of the left ankle to 30% of their maximal dorsiflexion voluntary contraction (MVC) or plantar flexion MVC, respectively.
Focal TMS at 1.2 x motor threshold was applied to the hotspot of TA and soleus muscles, respectively, prior to and at the beginning of the contraction. Peak to peak amplitudes of the MEPs were compared to MEPs elicited 100 ms prior to the auditory cue.

Results:
Soleus MEPs were facilitated when TMS was applied 50-75 ms prior to onset of plantar flexion. Surprisingly, soleus MEPs were also facilitated (although to a lesser extent) at a similar time in relation to the onset of dorsiflexion.
TA MEPs were facilitated 50-75 ms prior to onset of dorsiflexion. Prior to plantar flexion the behavior of TA MEPs was different. In 3 out of 9 subjects TA MEPs decreased 25-50 ms before onset of plantar flexion, while facilitation was observed in one subject.
A control experiment showed no difference between the facilitation of the MEP and cervicomedullary stimulation (CMEP) prior to dorsiflexion.

Conclusion:
The observation that soleus MEPs are facilitated prior to a contraction of the antagonist, while TA MEPs are not, may reflect differences in the central motor command for plantar- and dorsiflexion.
Our preliminary data from the CMEP experiments suggest that the facilitation of soleus MEPs prior to the onset of dorsiflexion is modulated at a subcortical level.