The balance of muscle oxygen supply and demand reveals critical metabolic rate and predicts time to exhaustion, 2025, Brett S Kirby et al

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NEW & NOTEWORTHY Dynamic muscle O2 saturation discriminates boundaries between exercise intensity domains, exposes a critical metabolic rate as the highest rate of steady state O2 supply and demand, describes time series depletion and repletion for work above critical power, and predicts time to exhaustion during severe domain whole body exercise.

These results highlight the matching of O2 supply and demand as a primary determinant for sustainable exercise intensities from those that are unsustainable and lead to exhaustion.

Abstract​

We tested the hypothesis that during whole body exercise, the balance between muscle O2 supply and metabolic demand may elucidate intensity domains, reveal a critical metabolic rate, and predict time to exhaustion.

Seventeen active, healthy volunteers (12 males, 5 females; 32 ± 2 yr) participated in two distinct protocols. Study 1 (n = 7) consisted of constant work rate cycling in the moderate, heavy, and severe exercise intensity domains with concurrent measures of pulmonary V̇o2 and local %SmO2 [via near-infrared spectroscopy (NIRS)] on quadriceps and forearm sites. Average %SmO2 at both sites displayed a domain-dependent response (P < 0.05). A negative %SmO2 slope was evident during severe-domain exercise but was positive during exercise below critical power (CP) at both muscle sites. In study 2 (n = 10), quadriceps and forearm site %SmO2 was measured during three continuous running trials to exhaustion and three intermittent intensity (ratio = 60 s severe: 30 s lower intensity) trials to exhaustion. Intensity-dependent negative %SmO2 slopes were observed for all trials (P < 0.05) and predicted zero slope at critical velocity. %SmO2 accurately predicted depletion and repletion of %D′ balance on a second-by-second basis (R2 = 0.99, P < 0.05; both sites).

Time to exhaustion predictions during continuous and intermittent exercise were either not different or better with %SmO2 [standard error of the estimate (SEE) < 20.52 s for quad, <44.03 s for forearm] versus running velocity.

Muscle O2 balance provides a dynamic physiological delineation between sustainable and unsustainable exercise (consistent with a “critical metabolic rate”) and predicts real-time depletion and repletion of finite work capacity and time to exhaustion.
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Conclusion​

The present data suggest that %SmO2, and its derivatives, can be leveraged to differentiate sustainable from unsustainable exercise intensity, describe and predict the depletion and repletion of the finite work capacity above CP (i.e., W′), and accurately predict the time to exhaustion.

Our findings are consistent with the idea that the balance between local muscle O2 supply and metabolic O2 demand provides a useful physiological surrogate of critical metabolic rate and W′ balance. Combined, these data extend current understanding of factors contributing to the maximal steady-state metabolic rate and, importantly, provide a useful approach to describe and predict human exercise limitations.