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John Rankin Laboratory of Pulmonary Medicine, University of Wisconsin Medical School, Madison, Wisconsin
Submitted 20 December 2005 ; accepted in final form 15 February 2006
The effect of arterial O2 content (CaO2) on quadriceps fatigue was assessed in healthy, trained male athletes. On separate days, eight participants completed three constant-workload trials on a bicycle ergometer at fixed workloads (314 ± 13 W). The first trial was performed while the subjects breathed a hypoxic gas mixture [inspired O2 fraction (FIO2) = 0.15, Hb saturation = 81.6%, CaO2 = 18.2 ml O2/dl blood; Hypo] until exhaustion (4.5 ± 0.4 min). The remaining two trials were randomized and time matched with Hypo. The second and third trials were performed while the subjects breathed a normoxic (FIO2 = 0.21, Hb saturation = 95.0%, CaO2 = 21.3 ml O2/dl blood; Norm) and a hyperoxic (FIO2 = 1.0, Hb saturation = 100%, CaO2 = 23.8 ml O2/dl blood; Hyper) gas mixture, respectively. Quadriceps muscle fatigue was assessed via magnetic femoral nerve stimulation (1–100 Hz) before and 2.5 min after exercise. Myoelectrical activity of the vastus lateralis was obtained from surface electrodes throughout exercise. Immediately after exercise, the mean force response across 1–100 Hz decreased from preexercise values (P < 0.01) by –26 ± 2, –17 ± 2, and –13 ± 2% for Hypo, Norm, and Hyper, respectively; each of the decrements differed significantly (P < 0.05). Integrated electromyogram increased significantly throughout exercise (P < 0.01) by 23 ± 3, 10 ± 1, and 6 ± 1% for Hypo, Norm, and Hyper, respectively; each of the increments differed significantly (P < 0.05). Mean power frequency fell more (P < 0.05) during Hypo (–15 ± 2%); the difference between Norm (–7 ± 1%) and Hyper (–6 ± 1%) was not significant (P = 0.32). We conclude that 
CaO2 during strenuous systemic exercise at equal workloads and durations affects the rate of locomotor muscle fatigue development.
hypoxia; hyperoxia; hemoglobin saturation; magnetic femoral nerve stimulation
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