Matching oxygen delivery to the muscle's oxygen demand: has contractile function been overlooked?

The current line of reasoning within the field of cardiovascular exercise physiology is that skeletal muscle oxygen delivery is tightly matched to oxygen demand. However, this relationship is conditional: holding true at rest and for exercise below one’s critical power (CP) – the threshold for severe-intensity exercise. When considering exercise performed within the severe-intensity domain, this matching becomes uncoupled. That being said, if oxygen delivery ismatched to oxygen demand and is already sufficient to support a given rate of oxygen consumption (V̇O2 ), then further increases in oxygen delivery will not yield an elevation in V̇O2 (Nyberg & Jones, 2022). However, the current belief that further increases in oxygen delivery will not elevate V̇O2 fails to consider that the maintenance of oxygen uptake is feasible despite a reduction in oxygen delivery through adjustments of redox and phosphorylation potentials (Wilson & Erecińska, 1985). Adjustments in redox and phosphorylation potentials are mediated by an accumulation of metabolites that increase oxidative phosphorylation to compensate and maintain mitochondrial oxygen uptake. The caveat to this consideration is that the increasedmetabolic disturbance at the same V̇O2 affects the muscle’s ability to generate force for a given muscle activation. Despite maintaining V̇O2 , this decreased oxygen deliverymay not be optimal formaintaining the active muscle’s contractile function. In a recent issue of The Journal of Physiology, Drouin et al. (2023) explored whether the ‘normal’ adjustment of oxygen delivery in small muscle mass exercise below CP was optimal for muscle performance. To test the hypothesis that the ‘normal’ adjustment of oxygen delivery is optimal for muscle performance, Drouin et al. (2023) activated the metaboreflex in a remote skeletal muscle to stimulate an increase in mean arterial blood pressure (MAP) and subsequent muscle perfusion to a separate active muscle of interest. The authors hypothesized that if the ‘normal’ adjustment of oxygen delivery was not matched to demand, an improvement in muscle force relative to electromyography (EMG), as a measure of excitation–contraction coupling, would be observed when forearm skeletal muscle perfusion is increased. The authors tested if ‘normal’ oxygen delivery is optimal for muscle performance by increasing MAP, and as a result, blood flow to an exercising forearm muscle during a rhythmic handgrip exercise protocol, targeting 90% CP in 20 healthy participants (n = 10 female). The incited change in blood pressure and flow was achieved by using remote skeletal muscle activation of the bilateral tibialis anterior during dorsiflexion, with the metaboreflex condition achieved via suprasystolic cuff inflation. The experimental design required the collection of forearm muscle activation, oxygenation and blood flow (FBF), as well as MAP and handgrip force. The hyperperfusion induced by the metaboreflex effectively increased forearm oxygen delivery which in turn improved the muscle activation (EMG) to force of contraction ratio. After 30 s of remote skeletal muscle metaboreflex activation, MAP, FBF and muscle oxygenation were increased and remained increased until metaboreflex discontinuation. Upon cessation of the metaboreflex, forearm oxygendelivery and themuscle activation to force of contraction ratio rapidly returned to control levels. We applaud the authors for conducting a rigorous study which involved an equal distribution of male and female participants alongside a within-participant design to minimize individual physiological variability by having participants serve as their own controls. The use of near-infrared spectroscopy (NIRS) signals to measure muscle oxygenation was well done with physiological calibration. The authors should also be commended for creating a target muscle activation as well as a personalized target force of contraction according to an individual’s CP, enabling either muscle activation or force of contraction to adjust to alterations in oxygen delivery. The authors were wise to screen participants to ensure they could tolerate the discomfort of the metaboreflex and coordinate the handgrip and dorsiflexion exercises. Drouin et al. (2023) stated that handgrip exercise was always performed with the participants’ right hand. We assume this was done because the majority of individuals are right-handed; however, if left-handed individuals participated, this may have affected the force of their muscle contraction and determination of CP due to potential variation in neuromuscular activation and recruitment of motor units/muscle fibres in the non-dominant forearm. While the authors were wise to select an exercise intensity just below CP in order to create a consistent relative intensity and cellular environment across individuals, without blood lactate and metabolism measures, it is challenging to conclude with certainty that cellular disturbances were truly matched and attenuated across individuals. The addition of blood samples would also allow for a measure of arterial oxygen content to quantify oxygen delivery. Furthermore, since the exercise protocol in their study was completed in the heavy-intensity domain, future research would benefit from investigating whether exercise outside of this domain and/or above CP would elicit similar results. With the forearm being the muscle of interest, which is rich in type I (slow-twitch) fibres, future research would also benefit from applying similar methodologies to type II (fast-twitch) fibres since fibre type distribution has been found to affect oxygen uptake kinetics during exercise in the heavy intensity domain (Barstow et al., 1996). The use of a micropuncture biopsy may provide important fibre type information to establish whether this phenomenon persists across varying fibre type distributions. Matching oxygen delivery to the muscle’s oxygen demand during exercise is a critical determinant of our capacity to both perform and tolerate physical activity. However, whether oxygen delivery is