Specificity and functional impact of post-exercise depression of cortically evoked motor potentials in man

Magnetic stimulation of the motor cortex with electromyographic recordings from exercising muscles has shown corticospinal excitability to be depressed following exercise. We now investigate whether this depression spreads to non-exercising muscles and its influence on performance. Healthy volunteers made unilateral biceps curls to exhaustion and, in another later session, for 25% of the time to exhaustion. Bilateral motor-evoked potentials (MEPs) in biceps brachii and first dorsal interosseus muscles were measured at 2-min intervals before and after exercise. In another experiment, subjects performed exhaustive curls and, in addition to MEP areas, force production in biceps, hand-grip force, simple reaction times and movement times were measured bilaterally. MEPs were depressed after exhaustive exercise in the exercising biceps for over 60 min; depression was also observed 10-15 min after exercise in the non-exercising biceps but not in the first dorsal interosseus of either hand. The shorter exercise period produced depression of MEPs only in the exercising muscle. After exhaustive exercise maximum voluntary contraction fell in the exercising biceps and this correlated with MEP areas. No reduction in force was seen in the non-exercising biceps but hand-grip force fell slightly in both arms. There was no change in reaction times or movement times. Depression of MEPs can occur in non-exercising homonymous muscles but not in heteronymous muscles and only when exercise levels are high. There was no measurable functional deficit in the non-exercising limb, so we conclude that the reduced corticospinal excitability observed in this limb has little or no consequence on the performance parameters measured.