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 (V O_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 (V O₂ 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. [ABSTRACT FROM AUTHOR]