Significance of Group III and IV muscle afferents for the endurance exercising human.

1. With the onset of dynamic whole-body exercise, contraction-induced mechanical and biochemical stimuli within locomotor muscle cause an increase in the discharge frequency of thinly myelinated (Group III) and unmyelinated (Group IV) nerve fibres located within the muscle. 2. These thin fibre muscle afferents project to various sites within the central nervous system and thereby substantially influence the exercising human. 3. First, Group III/IV muscle afferents are the afferent arm of cardiovascular and ventilatory reflex responses that are mediated in the nucleus tractus solitarius and the ventrolateral medulla. Therefore, neural feedback from working skeletal muscle is a vital component in providing a high capacity for endurance exercise because muscle perfusion and O₂ delivery determine the fatigability of skeletal muscle. 4. Second, Group III/IV muscle afferents facilitate 'central fatigue' (failure, or unwillingness, of the central nervous system to 'drive' motoneurons) by exerting inhibitory influences on central motor drive during exercise. 5. Thus, Group III/IV muscle afferents play a substantial role in a human's susceptibility to fatigue and capacity for endurance exercise. Clinical and Experimental Pharmacology and Physiology