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Effects of acute hypoxia on cerebral and muscle oxygenation during incremental exercise

University of Colorado Altitude Research Center, Denver Health Science Center and Colorado Springs Campuses, Colorado

Submitted 29 December 2006 ; accepted in final form 11 April 2007

To determine if fatigue at maximal aerobic power output was associated with a critical decrease in cerebral oxygenation, 13 male cyclists performed incremental maximal exercise tests (25 W/min ramp) under normoxic (Norm: 21% FI_O2) and acute hypoxic (Hypox: 12% FI_O2) conditions. Near-infrared spectroscopy (NIRS) was used to monitor concentration (µM) changes of oxy- and deoxyhemoglobin ([O₂Hb], [HHb]) in the left vastus lateralis muscle and frontal cerebral cortex. Changes in total Hb were calculated ([THb] = [O₂Hb] + [HHb]) and used as an index of change in regional blood volume. Repeated-measures ANOVA were performed across treatments and work rates ( = 0.05). During Norm, cerebral oxygenation rose between 25 and 75% peak power output {Power_peak; increased (inc) [O₂Hb], inc. [HHb], inc. [THb]}, but fell from 75 to 100% Power_peak {decreased (dec) [O₂Hb], inc. [HHb], no change [THb]}. In contrast, during Hypox, cerebral oxygenation dropped progressively across all work rates (dec. [O₂Hb], inc. [HHb]), whereas [THb] again rose up to 75% Power_peak and remained constant thereafter. Changes in cerebral oxygenation during Hypox were larger than Norm. In muscle, oxygenation decreased progressively throughout exercise in both Norm and Hypox (dec. [O₂Hb], inc. [HHb], inc. [THb]), although [O₂Hb] was unchanged between 75 and 100% Power_peak. Changes in muscle oxygenation were also greater in Hypox compared with Norm. On the basis of these findings, it is unlikely that changes in cerebral oxygenation limit incremental exercise performance in normoxia, yet it is possible that such changes play a more pivotal role in hypoxia.

near-infrared spectroscopy; fatigue; altitude

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