The mechanistic basis of the power–time relationship: potential role of the group III/IV muscle afferents

The power–time relationship for highintensity exercise is well known to be hyperbolic and generalizable to multiple exercise modalities in humans and other species. The critical power (CP) is mathematically defined as the asymptote of this hyperbola, while the curvature constant (W′) represents a fixed amount of work that can be performed above CP before reaching exhaustion. Importantly, CP is the highest intensity in which a steady state can be obtained for small muscle mass exercise, as assessed by intramuscular metabolic perturbation, and during wholebody exercise, assessed by oxygen uptake. However, experimental evidence demonstrating that CP represents a threshold for steady state ( CP) or non-steady state (> CP) intramuscular metabolic perturbation during whole-body exercise, a necessity for the validation of the CP concept, has remained elusive. In a recent article published in The Journal of Physiology, Vanhatalo and colleagues aimed to clarify the mechanistic bases of the power–time parameters (i.e. CP and W′) during whole-body exercise in relation to muscle metabolism and fibre type distribution (Vanhatalo et al. 2016). To this end, they performed two experimental protocols with multiple muscle biopsies prior to, and following, high-intensity cycling tests of varying duration. Notably, the authors present the first evidence demonstrating that CP demarcates intensities which result in steady-state ( CP) and non-steady state (> CP) intramuscular metabolic responses for wholebody exercise (i.e. phosphocreatine, creatine, pH, lactate, and glycogen). Moreover, Vanhatalo et al. (2016) documented that a greater CP was associated with a higher type I muscle fibre proportion and a lower type IIx proportion. These findings build upon previous work to further validate the CP concept and extend our understanding of the mechanisms determining the power–time relationship. The authors also determined that the size of the W′ is not proportional to any specific muscle fibre type population, further supporting the growing evidence that W′ is determined by the integration of a multitude of physiological mechanisms. Indeed, by compiling evidence from this study and several other recent publications, questions arise regarding the potential mechanistic role of the group III/IV muscle afferents in determining W′; however, little work has been performed with this focus.