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 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,