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 boundary and the highest steady-state metabolic rate. Thirteen participants (5 women) performed a graded exercise test (GXT) to determine peak oxygen consumption (V̇o_2peak) and lactate turn point (LTP). On a separate study day, a %SmO₂ zero-slope prediction trial included completing 5-min cycling bouts in an estimated heavy domain, at an estimated critical power, and in an estimated 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 (V̇o₂ and blood lactate) and the heavy-severe domain boundary. Our data suggest the %SmO₂ slope can identify the highest steady-state metabolic 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 exercise 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 dependent on muscle oxygen supply-demand balance.