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 O2 supply and metabolic demand may elucidate intensity domains, reveal a critical metabolic rate, and predict time to exhaustion. Seventeen active, healthy volunteers (12 male, 5 female; 32±2 years) 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 VO2 and local %SmO2 (via 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 3 continuous running trials to exhaustion and 3 intermittent intensity (ratio = 60s 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 (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 (SEE < 20.52sec for quad, < 44.03sec for forearm) vs running velocity (SEE < 65.76sec). 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.