The balance of muscle oxygen supply and demand reveals critical metabolic rate and predicts time to exhaustion

We tested the hypothesis that during whole body exercise, the balance between muscle O-2 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 <(V)over dot>o(2) 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: 30s 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 (R-2 = 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 (SEE < 65.76 s). Muscle O-2 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. NEW & NOTEWORTHY Dynamic muscle O-2 saturation discriminates boundaries between exercise intensity domains, exposes a critical metabolic rate as the highest rate of steady state O-2 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 O-2 supply and demand as a primary determinant for sustainable exercise intensities from those that are unsustainable and lead to exhaustion.

Automated summary

The study found that during whole body exercise, the balance between muscle oxygen supply and metabolic demand can elucidate intensity domains, reveal a critical metabolic rate, and predict time to exhaustion. Measurement of muscle oxygen saturation percentage accurately predicts depletion and repletion of work capacity and time to exhaustion, providing a physiological boundary between sustainable and unsustainable exercise intensities.