Respiratory muscle demands during severe intensity exercise may constrain the blood flow increase to locomotory muscles, thereby limiting exercise tolerance. Muscle mitochondrial O2 delivery depends on both perfusive and diffusive O2 transport, the former reflecting the O2 delivery-utilization balance and the latter exercise-induced changes in muscle total hemoglobin (THb). Resolution of changes in deoxygenation and THb simultaneously within these two muscle groups will provide novel insights into vascular and metabolic control. PURPOSE: We investigated absolute deoxygenated [Hb+Mb] (HHb, index of fractional O2 extraction) and THb responses in respiratory and locomotor muscle using time-resolved near-infrared spectroscopy (TR-NIRS). METHODS: Ten males performed ramp incremental cycling (20 W·min-1) to exhaustion while measuring pulmonary VO2, HHb and THb (9-10th intercostal space for intercostal muscle and diaphragm, IC-DP, and vastus lateralis, VL). HHb and THb were corrected for adipose tissue thickness (ATT) at each optode site, using the THb-ATT regression. RESULTS: HHb increased systematically with power output (PO) in VL and plateaued above ~80% POmax (Figure 1A). In contrast, HHb in IC-DP was unchanged from rest. THb in VL increased modestly and plateaued above ~50% POmax, whereas THb in IC-DP declined gently from rest such that THb was greater in VL than IC-DP from 30 to 100 % POmax (Figure 1B). CONCLUSIONS: These disparate HHb and THb profiles imply a muscle-specific regulation of perfusive and diffusive O2 fluxes for respiratory versus locomotory muscle(s). Supported by JSPS-17J09854, 18K17875