It is well known that hypoxia decreases exercise capacity but less is known about how acute changes in ambient oxygen concentration (FIO2) affect pacing profiles. The pacing profile during a time trial usually includes an initial surge, a middle plateau and an end-spurt (Amann et al. 2006; Noakes et al., 2008; Billat et al., 2006). The purpose of this study was to describe changes in pacing profile and physiology in response to a mid-exercise change in FIO2. Methods: Eight male cyclists completed two 20 km time trials in random order. In one trial, the 1st 10 km was normoxic (FIO2=21%) and the 2nd 10 km was hypoxic (FIO2=15% ~2700 m). The order was reversed in trial two. Subjects were blinded to the conditions. The trials were done inside a chamber set at normobaric pressure. Subjects acclimatised to the initial FIO2 for 10 min before each trial. Expired gases (4, 14 km), cardiac output (Q) (continuous), and blood gases (0, 4, 9, 14, 19 km) were measured. Subjects used their own bicycles on an electrically braked cycle trainer. PO was averaged over 2 km. Analysis was by a Students` t-test and repeated-measures ANOVA with Bonferroni post-hoc. Results: Power output (PO) was lower in hypoxia at every interval except 12 and 14 km (p<0.01). PO for the final 2 km was 116% of the mean PO for the 2nd 10 km in normoxia, but only 102% in hypoxia (p<0.01). VE during hypoxia was higher at 14 km (p<0.05) and pO2 during hypoxia was lower at every interval (p<0.001). Q for the final 2 km was lower in hypoxia (p<0.001). Discussion: PO changed less than 210 sec after switching FIO2. Descriptively, pacing during the 1st 10 km was similar in both conditions, but absolute PO was lower in hypoxia. The attenuated hypoxic end-spurt was consistent with Amann et al. (2006). Q during the end-spurt was lower during hypoxia and thus was not limiting. Instead, the end-spurt seemed to be related to the altered VE and pO2. However, altered PO during the first 500 m (~60 sec) and a quick PO response after changing FIO2 (cf., Joseph et al., 2008) suggests that pacing changes are not merely a result of peripheral fatigue. Indeed, the initial PO was likely determined during the 10 min pre-trial equilibration period and/or from sensations in the first minute of exercise. Changes in PO with altered FIO2 may have involved a rapid oxygen sensing response. The data support the notion of an initial anticipatory component to pacing based on sensation.
© Copyright 2009 14th annual Congress of the European College of Sport Science, Oslo/Norway, June 24-27, 2009, Book of Abstracts. Published by The Norwegian School of Sport Sciences. All rights reserved.