Sequential Oxygen Mismatch from Skeletal Muscle to Prefrontal Cortex Underpins the Rate of Exhaustion During All-Out Exercise.

Purpose: We tested the overarching hypothesis that the expended rate of work above critical power (W' Balance) during all-out whole-body exercise is related to a decline in prefrontal cortex (PFC) oxygenation secondary to an organized systemic outstripping of muscle O2 supply relative to O2 demand.

Methods: We concomitantly measured (N = 16 males) skeletal muscle O2 saturation (vastus lateralis near infrared spectroscopy, NIRS; %SmO2), pulmonary O2 uptake (V̇O2), and Hb differential (∆[O2Hb-HHb]) as an index of PFC O2 mismatch (pfcO2) via functional NIRS bi-laterally in the ventrolateral (VLPFC), dorsolateral (DLPFC), and orbitofrontal (OFC) cortices during brief all-out cycling exercise (highest instantaneous power for three minutes).

Results: All-out exercise evoked significant changes in %SmO2 (∆-28.8 ± 14.1%), V̇O2 (∆27.7 ± 10.3%), and global pfcO2 (∆-7.6 ± 4.8%). Decreases in regional pfcO2 were greater in the VLPFC (∆-10.9 ± 6.1 μM) vs DLPFC (∆-4.8 ± 4.5 μM) or OFC (∆-5.9 ± 4.2 μM). Spatiotemporal analysis by O2 measurement location revealed a steep rate of change transition phase followed by a maximal sustaining plateau, and progression of this pattern occurred sequentially first in muscle (~13 sec) → pulmonary (~44 sec) → PFC (~80 sec). Transition phase O2 indices were strongly correlated to the rate of W' Balance expended (muscle, R2 = 0.91; pulmonary, R2 = 0.997; PFC, R2 = 0.968), with crossover between regional O2 mismatches occurring at the same %W' Balance (end muscle = 71% vs start pulmonary = 65%, P = 0.56; end pulmonary = 26% vs start PFC = 30%, P = 0.83) and end PFC transition phase occurring at complete depletion of W' (end PFC = -0.9%).

Conclusions: We conclude that whole-body all-out exercise tolerance may arise from a progressive O2 mismatch from skeletal muscle to the brain.