Rapidity Of Sensing Metabolic Disturbances During Exercise

Abstract

BACKGROUND: The ability to modify power output (PO) in response to a changing internal milieu during exercise is crucial for optimizing performance and involves an integration system consisting of a central performance template and feedback from peripheral receptors. The rapidity with which PO is modified has not been established. PURPOSE: To determine the rapidity of changes in muscular PO in response to a hypoxic challenge, and to determine if the change in PO is linked to changes in arterial O2 saturation (SaO2). We hypothesized that modifications in PO would occur after 45 s and PO will be linked to changes in SaO2. METHODS: Ten, well-trained cyclists performed two, randomly ordered 5 km time trials. Subjects began the trials breathing room air and switched to a hypoxic (HYPOXIC, FIO2=0.15) or room (CONTROL, FIO2=0.21) air mixture between 2-4 km, then back to room air for the final kilometer. The time delay to begin decreasing SaO2 and PO during hypoxia and to begin increasing SaO2 and PO during the return to room air were compared during the HYPOXIC trial. Additionally, the half time for changes in SaO2 and PO during hypoxia was compared. RESULTS: There was a significant difference in time trial performance between CONTROL and HYPOXIC (478.9 ± 37.5 s vs. 490.1 ± 33.8 s, p < 0.05). Mean SaO2 and PO between 2-4 km were significantly different between CONTROL and HYPOXIC (94 ± 2 vs. 83 ± 2% and 285 ± 16 vs. 245 ± 19 Watts, respectively, p < 0.05). There was no significant difference between the time delay for decreases in SaO2 (31.5 ± 12.8 s) and in PO (25.8 ± 14.4 s) or the recovery of SaO2 (29.0 ± 7.7 s) and PO (21.5 ± 12.4 s). The half time for decreases in SaO2 (56.6 ± 14.4 s) and in PO (62.7 ± 20.8 s) was not significantly different. There was a moderate correlation between combined temporal markers of changes in SaO2 and PO (r = 0.48) when subjects entered and exited the hypoxic interval. CONCLUSIONS: Modifications of power output due to the administration of hypoxic air are related to the development of arterial hypoxemia, and begin within ~30 s.