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ENVIRONMENTAL, EXERCISE AND RESPIRATORY PHYSIOLOGY

Effect of arterial oxygenation on quadriceps fatigability during isolated muscle exercise

¹The John Rankin Laboratory of Pulmonary Medicine, Department of Health, Population Health Sciences, University of Wisconsin-Madison, Wisconsin; and ²Research Center of Health, Physical Fitness and Sports, Nagoya University, Nagoya, Japan

Submitted 3 August 2006 ; accepted in final form 15 November 2006

The effect of various levels of oxygenation on quadriceps muscle fatigability during isolated muscle exercise was assessed in six male subjects. Twitch force (Q_tw) was assessed using supramaximal magnetic femoral nerve stimulation. In experiment 1, maximal voluntary contraction (MVC) and Q_tw of resting quadriceps muscle were measured in normoxia [inspired O₂ fraction (FI_O2) = 0.21, percent arterial O₂ saturation (Sp) = 98.4%, estimated arterial O₂ content (Ca_O2) = 20.8 ml/dl], acute hypoxia (FI_O2 = 0.11, Sp = 74.6%, Ca_O2 = 15.7 ml/dl), and acute hyperoxia (FI_O2 = 1.0, Sp = 100%, Ca_O2 = 22.6 ml/dl). No significant differences were found for MVC and Q_tw among the three FI_O2 levels. In experiment 2, the subjects performed three sets of nine, intermittent, isometric, unilateral, submaximal quadriceps contractions (62% MVC followed by 1 MVC in each set) while breathing each FI_O2. Q_tw was assessed before and after exercise, and myoelectrical activity of the vastus lateralis was obtained during exercise. The percent reduction of twitch force (potentiated Q_tw) in hypoxia (–27.0%) was significantly (P < 0.05) greater than in normoxia (–21.4%) and hyperoxia (–19.9%), as were the changes in intratwitch measures of contractile properties. The increase in integrated electromyogram over the course of the nine contractions in hypoxia (15.4%) was higher (P < 0.05) than in normoxia (7.2%) or hyperoxia (6.7%). These results demonstrate that quadriceps muscle fatigability during isolated muscle exercise is exacerbated in acute hypoxia, and these effects are independent of the relative exercise intensity.

hypoxia; magnetic femoral nerve stimulation; hyperoxia; hypoxemia; quadriceps twitch force

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