Ischaemia after exercise does not reduce responses of human motoneurones to cortical or corticospinal tract stimulation

1. Motor unit firing rates and voluntary activation of muscle decline durin g sustained isometric contractions. After exercise, the responses to motor cortical and corticospinal stimulation are reduced. These changes may refle ct motoneuronal inhibition mediated by group III and IV muscle afferents. T o determine whether the post-contraction depression of the responses to cor ticospinal or motor cortical stimulation could be maintained by continued f iring of ischaemically sensitive group III and IV muscle afferents, we exam ined responses in muscles that were held ischaemic after exercise. 2. Following a sustained maximal voluntary contraction (MVC) of the elbow f lexors lasting 2 min, the response to stimulation of the corticospinal trac t was reduced but the usual recovery lover,2 min) was not delayed when the muscles were maintained ischaemic for 2 min after the contraction. 3. Following a sustained MVC, the time course of the reduction in the respo nse to motor cortical stimulation (a gradual decrease over similar to 2 min , maintained for > 10 min) was also not altered if the muscle was held isch aemic. 4. Mean arterial blood pressure rose to 155 +/- 12 mmHg during the 2 min MV C, declined to 125 +/- 9 mmHg immediately after it, but remained at this le vel without returning to preexercise levels (102 +/- 10 mmHg) until circula tion to the arm was restored. This confirms that the sustained MVC activate d a reflex dependent oil group III and IV muscle afferents. 5. This study shows that ischaemically sensitive group III and IV muscle af ferents do not mediate depression of responses to motor cortical or cortico spinal stimulation after fatiguing exercise. It also suggests that firing o f such afferents does not directly inhibit motoneurones or motor cortical o utput cells.