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INTEGRATIVE |
1 University of Wisconsin Medical School, John Rankin Laboratory of Pulmonary Medicine, Madison, WI, USA
2 Centre for Sports Medicine and Human Performance, Brunel University, Middlesex, UK
3 University of Colorado Altitude Research Center, Denver Health Science Center and Colorado Springs campuses, CO, USA
We examined the effects of hypoxia severity on peripheral versus central determinants of exercise performance. Eight cyclists performed constant-load exercise to exhaustion at various fractions of inspired O2 fraction (FIO2 0.21/0.15/0.10). At task failure (pedal frequency < 70% target) arterial hypoxaemia was surreptitiously reversed via acute O2 supplementation (FIO2
= 0.30) and subjects were encouraged to continue exercising. Peripheral fatigue was assessed via changes in potentiated quadriceps twitch force (
Qtw,pot) as measured pre- versus post-exercise in response to supramaximal femoral nerve stimulation. At task failure in normoxia (haemoglobin saturation (SpO2) 
94%, 656 ± 82 s) and moderate hypoxia (SpO2
82%, 278 ± 16 s), hyperoxygenation had no significant effect on prolonging endurance time. However, following task failure in severe hypoxia (SpO2
67%; 125 ± 6 s), hyperoxygenation elicited a significant prolongation of time to exhaustion (171 ± 61%). The magnitude of Qtw,pot at exhaustion was not different among the three trials (–35% to –36%, P
= 0.8). Furthermore, quadriceps integrated EMG, blood lactate, heart rate, and effort perceptions all rose significantly throughout exercise, and to a similar extent at exhaustion following hyperoxygenation at all levels of arterial oxygenation. Since hyperoxygenation prolonged exercise time only in severe hypoxia, we repeated this trial and assessed peripheral fatigue following task failure prior to hyperoxygenation (125 ± 6 s). Although Qtw,pot was reduced from pre-exercise baseline (–23%; P < 0.01), peripheral fatigue was substantially less (P < 0.01) than that observed at task failure in normoxia and moderate hypoxia. We conclude that across the range of normoxia to severe hypoxia, the major determinants of central motor output and exercise performance switches from a predominantly peripheral origin of fatigue to a hypoxia-sensitive central component of fatigue, probably involving brain hypoxic effects on effort perception.
(Received 1 February 2007;
accepted after revision 16 February 2007;
first published online 22 February 2007)
Corresponding author M. Amann: The John Rankin Laboratory of Pulmonary Medicine, 4245 Medical Science Center, 1300 University Avenue, Madison, WI 53706, USA. Email: amann{at}wisc.edu
This article has been cited by other articles:
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  M. Amann, S. M. Marcora, L. Nybo, T. A. Duhamel, T. D. Noakes, V. Jaquinandi, J. L. Saumet, P. Abraham, B. T. Ameredes, M. Burnley, et al. Viewpoint: Fatigue mechanisms determining exercise performance: integrative physiology is systems physiology. J Appl Physiol, May 1, 2008; 104(5): 1543 - 1544. [Full Text] [PDF]
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 M. Amann and J. A. Dempsey Reply from Markus Amann and Jerome A. Dempsey J. Physiol., April 1, 2008; 586(7): 2029 - 2030. [Full Text] [PDF]
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M. Amann and J. A. L. Calbet Convective oxygen transport and fatigue J Appl Physiol, March 1, 2008; 104(3): 861 - 870. [Abstract] [Full Text] [PDF]
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B. D. Levine : what do we know, and what do we still need to know? J. Physiol., January 1, 2008; 586(1): 25 - 34. [Abstract] [Full Text] [PDF]
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M. Amann and J. A. Dempsey Locomotor muscle fatigue modifies central motor drive in healthy humans and imposes a limitation to exercise performance J. Physiol., January 1, 2008; 586(1): 161 - 173. [Abstract] [Full Text] [PDF]
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T. D. Noakes and F. E. Marino Arterial oxygenation, central motor output and exercise performance in humans J. Physiol., December 15, 2007; 585(3): 919 - 921. [Full Text] [PDF]
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M. Amann, L. M. Romer, and J. A. Dempsey Reply from Markus Amann, Lee M. Romer and Jerome A. Dempsey J. Physiol., December 15, 2007; 585(3): 923 - 924. [Full Text] [PDF]
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M. Amann, D. F. Pegelow, A. J. Jacques, and J. A. Dempsey Inspiratory muscle work in acute hypoxia influences locomotor muscle fatigue and exercise performance of healthy humans Am J Physiol Regulatory Integrative Comp Physiol, November 1, 2007; 293(5): R2036 - R2045. [Abstract] [Full Text] [PDF]
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