Systemic hypoxia has a larger effect on reducing the external load at lower exercise intensity during heart rate clamped cycling.

The effects of acute hypoxic exposure on mechanical output and internal responses during cycling with heart rate (HR) clamped at lactate thresholds 1 and 2 (LT1 and LT2, respectively) were investigated. On separate days, 12 trained males cycled for 15 min at a clamped HR corresponding to LT1 and LT2 under normoxic or hypoxic conditions (simulated altitude of ∼3500 m and inspired oxygen fraction of 13.6%). Power output (PO), arterial oxygen saturation, ventilatory and perceptual responses were measured every 3 min, with metabolic response assessed pre- and post-exercise. At LT1, PO was consistently lower in hypoxia compared to normoxia (p < 0.01). At LT2, PO was not different between normoxia and hypoxia at 3 and 6 min (both p > 0.42) but was significantly lower in hypoxia at 9, 12 and 15 min (all p < 0.04). Overall, hypoxia induced a greater decrease in PO at LT1 (-33.3% ± 11.3%) than at LT2 (-18.0 ± 14.7%) compared to normoxia. Ventilatory, perceptual and metabolic responses were influenced by exercise intensity (all p < 0.01) but not environmental conditions (all p > 0.17). A simulated altitude of ∼3500 m is more effective in reducing cycling PO at LT1 than LT2 during HR clamped cycling while maintaining other internal loads. Therefore, normobaric hypoxia provides a greater benefit via a larger decrease in the mechanical constraints of exercise at lower exercise intensities.