Effects of acute simulated altitude on the maximal lactate steady state in humans.

We sought to determine the effects of acute simulated altitude on the maximal lactate steady state (MLSS) and physiological responses to cycling at and 10 W above the MLSS-associated power output (PO) (MLSSp and MLSSp+10, respectively). Eleven (4 female) participants (mean [SD]; 28 [4] years; V̇O2max: 54.3 [6.9] mL×kg-1×min-1) acclimatized to ~1100 m performed 30-min constant PO trials in simulated altitudes of 0 m (SL), 1111 m (MILD), and 2222 m (MOD). MLSSp, defined as the highest PO with stable (<1mM change) blood lactate concentration ([BLa]) between 10 and 30 min, was significantly lower in MOD (209 [54] W) compared to SL (230 [56] W; p<0.001) and MILD (225 [58] W; p=0.001), but MILD and SL were not different (p=0.12). V̇O2 and V̇CO2 decreased at higher simulated altitudes due to lower POs (p<0.05), but other end-exercise physiological responses (e.g., [BLa], ventilation (V̇E), heart rate (HR)) were not different between conditions at MLSSp or MLSSp+10 (p>0.05). At the same absolute intensity (MLSSp for MILD), [BLa], HR, and V̇E and all perceptual variables were exacerbated in MOD compared to SL and MILD (p<0.05). Maximum voluntary contraction, voluntary activation, and potentiated twitch forces were exacerbated at MLSSp+10 relative to MLSSp within conditions (p<0.05); however, condition did not affect performance fatiguability at the same relative or absolute intensity (p>0.05). As MLSSp decreased in hypoxia, adjustments in PO are needed to ensure the same relative intensity across altitudes, but common indices of exercise intensity may facilitate exercise prescription and monitoring in hypoxia.

Physiological responses to ramp-incremental cycling tests performed at three simulated altitudes: a randomized crossover trial.

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 (MILD; ∼1100 m) performed three cycling ramp-incremental exercise tests (with verification stages performed at 110% of peak power output (PPO)) in simulated altitudes of 0 m (sea level, SL), 1111 m (MILD), and 2222 m (moderate altitude, 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, MOD 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. Trial registration: Open Science Framework 10.17605/OSF.IO/ZTC9E.