The Future is NIRS: Muscle Oxygen Saturation as an Estimation of The Power-Duration Relationship

The relationship between power output and exercise tolerance is well described by a hyperbolic equation, establishing the critical power (CP) and the curvature constant (W’). Critical power represents the greatest metabolic rate that results in ‘wholly-oxidative’ energy provision. Wholly-oxidative considers the active organism in toto and means that energy supply through substrate-level phosphorylation reaches a steadystate, indicated by a lack of progressive intramuscular phosphocreatine breakdown. W’ was initially described as an anaerobic work capacity but has subsequently been shown to be associated with the depletion of intramuscular energy stores as well as to be sensitive to alterations in oxygen delivery. Taken together, these two findings are consistent with the hypothesis that muscle oxygen saturation (SmO2), as measured by near-infrared spectroscopy (NIRS), is a viable means of estimating the power-duration relationship, and subsequently, time to exhaustion at a fixed pace.