Identification of maximal steady-state metabolic rate by the change in muscle oxygen saturation

We tested the hypothesis that a %SmO2 (muscle O2 saturation) slope can distinguish the heavy-severe exercise domain bound-ary and the highest steady-state metabolic rate. Thirteen participants (5 women) performed a graded exercise test (GXT) to determine peak oxygen consumption (V_ O2peak) and lactate turn point (LTP). On a separate study day, a %SmO2 zero-slope pre-diction trial included completing 5-min cycling bouts in an estimated heavy domain, at an estimated critical power, and in an esti-mated severe domain. Linear regression then determined the work rate at the predicted %SmO2 zero-slope, before a fourth 5-min confirmation trial. Two separate validation study days included confirmed steady-state (heavy domain) and nonsteady-state (severe domain) constant work rate trials. The power at the predicted %SmO2 zero-slope was 204 +/- 36 W and occurred at a % SmO2 slope of 0.7 +/- 1.4%/min (P = 0.12 relative to zero). There was no difference between the power at LTP (via GXT) and the predicted %SmO2 zero-slope linked power (P = 0.74). From validation study days, the %SmO2 slope was 0.32 +/- 0.73%/min during confirmed heavy-domain constant work rate exercise and-0.75 +/- 1.94%/min during confirmed severe-domain exercise (P < 0.05). The %SmO2 zero-slope consistently delineated steady state from nonsteady-state metabolic parameters (V_O2 and blood lactate) and the heavy-severe domain boundary. Our data suggest the %SmO2 slope can identify the highest steady-state meta-bolic rate and the physiological boundary between the heavy-severe domain, independent of work rate.NEW & NOTEWORTHY Muscle O2 saturation (%SmO2) rate can be used to not only identify sustainable from unsustainable exer-cise intensities but also delineate the transition from heavy to severe exercise domains. This report is the first to identify, and then validate, that the highest steady-state metabolic rate is related to a zero-slope muscle O2 saturation and is therefore de-pendent on muscle oxygen supply-demand balance.