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1 Research Center of Health, Physical Fitness and Sports, Nagoya University, Nagoya, Japan
2 The John Rankin Laboratory of Pulmonary Medicine, University of Wisconsin-Madison, United States
3 The John Rankin Laboratory of Pulmonary Medicine, University of Wisconsin-Madison, Madison, Wisconsin, United States
* To whom correspondence should be addressed. E-mail: katayama{at}htc.nagoya-u.ac.jp.
The effect of various levels of oxygenation on quadriceps muscle fatigability during isolated muscle exercise was assessed in six male subjects. Twitch force (Qtw) was assessed using supra-maximal magnetic femoral nerve stimulation. In Experiment I, maximal voluntary contraction (MVC) and Qtw of resting quadriceps muscle were measured in nomoxia [inspired O2 fraction (FIO2) = 0.21, arterial O2 saturation (SpO2) = 98.4%, estimated arterial O2 content (CaO2) = 20.8 ml·dl-1; NORM], acute-hypoxia (FIO2 = 0.11, SpO2 = 74.6%, CaO2 = 15.7 ml·dl-1; HYPO), and acute-hyperoxia (FIO2= 1.0, SpO2 = 100%, CaO2 = 22.6 ml·dl-1; HYPER). No significant differences were found for MVC and Qtw among the three FIO2s. In Experiment II, the subjects performed three sets of nine, intermittent, isometric, unilateral, submaximal quadriceps contractions (62% MVC followed by one MVC in each set) while breathing each FIO2. Qtw was assessed before and after exercise, and myoelectrical activity of the vastus lateralis was obtained during exercise. The percent reduction of twitch force (potentiated Qtw) in HYPO (-27.0%) was significantly (P<0.05) greater than those in NORM (-21.4%) and HYPER (-19.9%); as were the changes in intra-twitch measures of contractile properties. The increase in integrated electromyogram over the course of the nine contractions in HYPO (15.4%) was higher (P<0.05) than in NORM (7.2%) or HYPER (6.7%). These results demonstrate that quadriceps muscle fatigability during isolated muscle exercise is exacerbated in acute hypoxia, and these effects were independent of the relative exercise intensity.
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