Post-exercise facilitation of compound muscle action potentials evoked by transcranial magnetic stimulation in healthy subjects

Post-exercise facilitation (PEF) of motor evoked potentials (MEPs) was stud ied by transcranial magnetic stimulation in 15 healthy subjects following s tandardized and controlled isometric contraction of the biceps brachii musc le. PEF was highly dependent on the time delay (TD) from muscle relaxation to delivery of the magnetic stimulus and only to a minor degree on the dura tion of the maintained muscular contraction of 2, 4, and 6 s. In addition, PEF was unaffected by the contraction levels of 25%, 50%, and 100% of maxim al voluntary contraction (MVC). There was a linear relationship between the log amplitude of the post-exercise MEPs and the TD. The time point at whic h PEF had vanished was calculated to be 15.2 s. In order to challenge the q uestion whether segmental and/or suprasegmental mechanisms are primarily re sponsible for PEF, MEPs and I-I-reflexes were recorded from the soleus musc le following a sustained plantar flexion at the ankle joint in three health y subjects. PEF of MEPs was present at a TD of 1000 ms following a sustaine d contraction of 6 s at a level of 50% of MVC. It was accompanied by a pron ounced decrease in the soleus I-I-reflex amplitude at a TD of 1000 ms.