Near-infrared spectroscopy (NIRS) provides a simple and reliable measure of skeletal muscle oxidative capacity; however, its relationship to aerobic fitness and sex are unclear. We hypothesized that NIRS-derived oxidative capacity in the vastus lateralis (VL) and medial gastrocnemius (MG) would be correlated to indices of aerobic fitness and independent of sex. Twenty-six participants (13 males, 13 females) performed ramp- and step-incremental tests to volitional exhaustion on separate days to establish maximal oxygen uptake (V̇O2max), peak power output (PPO), lactate threshold (LT), gas exchange threshold (GET), respiratory compensation point (RCP), and maximal fat oxidation (MFO). Data were normalized to lean body mass to account for sex-based differences in body composition. Exercise tests were preceded by duplicate measurements of NIRS-derived oxidative capacity on the VL and MG muscles (i.e., repeated arterial occlusions following a brief set of muscle contractions). Skeletal muscle oxidative capacity for the VL (mean±SD: 21.9±4.6s) and MG (22.5±6.1s) were similar but unrelated (r2=0.03, p=0.39). Skeletal muscle oxidative capacity for the VL, but not the MG (p>0.05 for all variables), was significantly correlated with V̇O2max (r2=0.24; p=0.01), PPO (r2=0.23; p=0.01), LT (r2=0.23; p=0.01), GET (r2=0.23; p=0.01), and RCP (r2=0.27; p=0.006). MFO was not correlated with VL or MG skeletal muscle oxidative capacity (p>0.05). Females (54.9±4.5mL/kg LBM/min) and males (56.0±6.2mL/kg LBM/min), matched for V̇O2max (p=0.62), had similar NIRS-derived oxidative capacities for VL (20.7±4.4s vs. 23.2±4.6s; p=0.18) and MG (24.4±6.8s vs. 20.5±4.8; p=0.10). Overall, NIRS-derived skeletal muscle oxidative capacity in VL is indicative of aerobic fitness and independent of sex in humans.

Hypoxia negatively impacts aerobic exercise, but exercise testing in hypoxia has not been studied comprehensively. To determine the effects of simulated altitude on the gas exchange threshold (GET), respiratory compensation point (RCP), and maximal oxygen uptake (V̇O2max), 24 participants (mean [SD]; 26 [4] years; 171.6 [9.7] cm; 69.2 [11.9] kg) acclimatized to mild altitude (~1100 m) performed three cycling ramp-incremental exercise tests (with verification stages performed at 110 % of peak power output (PPO)) in simulated altitudes of 0m (SL), 1111m (MILD), and 2222m (MOD), in a randomized order. There were significant effects of condition (i.e., fraction of inspired oxygen [(FIO2]) for GET (p=0.001), RCP (p<0.001), V̇O2max (p<0.001), and PPO (p<0.001). The V̇O2 corresponding to GET and RCP (mL·kg-1·min-1) in MOD (24.1 [4.3]; 37.3 [5.1]) were significantly lower (p<0.05) compared to SL (27.1 [4.4]; 41.8 [6.6]) and MILD (26.8 [5.7]; 40.7 [7.3]) but similar (p>0.05) between SL and MILD. For each increase in simulated altitude, V̇O2max (SL: 51.3 [7.4]; MILD: 50.0 [7.6]; MOD: 47.3 [7.1] mL·kg-1·min-1) and PPO (SL: 332 [80]; MILD: 327 [78]; SL: 316 [76] W) decreased significantly (p<0.05 for all comparisons). V̇O2max values from the verification stage were lower than those measured during the ramp-incremental test (p=0.017). Overall, a mild simulated altitude had a significant effect on V̇O2max and PPO but not GET and RCP, moderate altitude decreased all four variables, and the inclusion of a verification stage had little effect on the determination of V̇O2max in a group of young healthy adults regardless of the FIO2.