Muscle O2 extraction reserve during intense cycling is site-specific.

The present study compared peak muscle deoxygenation ([HHb]_peak) responses at three quadriceps sites during occlusion (OCC), ramp incremental (RI), severe- (SVR) and moderate-intensity (MOD) exercise. Seven healthy men (25 ± 4 yr) each completed a stationary cycling RI (20 W/min) test to determine [HHb]peak [at distal and proximal vastus lateralis (VLD and VLP) and rectus femoris (RF)], peak V̇o₂ (V̇o_2peak), gas exchange threshold (GET), and peak work rate (WR_peak). Subjects also completed MOD (WR = 80% GET) and SVR exercise (WR corresponding to 120% V̇o_2peak) with absolute [HHb] (quantified by multichannel, time-resolved near-infrared spectroscopy) and pulmonary VO₂ (V̇o_2p) monitored continuously. Additionally, [HHb] and total hemoglobin ([Hb]_tot) were monitored at rest and during subsequent OCC (250 mmHg). Site-specific adipose tissue thickness was assessed (B-mode ultrasound), and its relationship with resting [Hb]_tot was used to correct absolute [HHb]. For VLD and RF, [HHb]_peak was higher (P > 0.05) during OCC (VLD = 111 ± 38, RF = 114 ± 26 µM) than RI (VLD 64 ± 14, RF = 85 ± 20) and SVR (VLD = 63 ± 13, RF = 81 ± 18). [HHb]_peak was similar (P > 0.05) across these conditions at the VLP (OCC = 67 ± 17, RI = 69 ± 17, SVR = 63 ± 17 µM). [HHb] peaked and then decreased prior to exercise cessation during SVR at all three muscle sites. [HHb]_peak during MOD was consistently lower than other conditions at all sites. A “[HHb] reserve” exists during intense cycling at the VLD and RF, likely implying either sufficient blood flow to meet oxidative demands or insufficient diffusion time for complete equilibration. In VLP this [HHb] reserve was absent, suggesting that a critical PO₂ may be challenged during intense cycling. [ABSTRACT FROM AUTHOR]