Viewpoint: Fatigue mechanisms determining exercise performance: integrative physiology is systems physiology.

TO THE EDITOR: The importance of considering the complex interaction of various biological responses to exercise in the process of explaining the limitation to human performance has previously been re-emphasized (5). In his timely Viewpoint, Hargreaves (4) now specifically highlights the possible link, or interaction, between feedback from fatiguing locomotor muscles, the “psyche,” and the degree of neuromuscular activation of the working limbs, i.e., central motor command (CMC), the ultimate determinant of exercise performance. Although inhibitory somatosensory feedback from the working/fatiguing limbs to the CNS is certainly only one of various factors influencing the ultimate determination of the magnitude of CMC during systemic endurance exercise, its negative impact on CMC—and consequently athletic performance—has been shown previously (1–3, 6). The authors of these papers have demonstrated the role of peripheral locomotor muscle fatigue as a significant determinant of exercise performance via its negative feedback effects on the CNS and consequently on the magnitude of CMC. In this context, it is very important to note that the inhibitory influence from the working/fatiguing limbs to the CNS is not the ultimate determinant of the magnitude of CMC and that it can voluntarily be “overridden,” although only for a very brief moment (1). However, research investigating the interaction between (fatiguing) locomotor muscles and CMC during whole body exercise is still in its infancy and has so far only scratched the surface. Therefore, additional experimental approaches, including the pharmacological modulation of the cortical projection of ascending sensory pathways originating in the locomotor muscles, are necessary to outline a more comprehensive model linking locomotor muscle fatigue and athletic performance.