One of the determining factors for muscular endurance is oxygen supply to the exercising muscles. To the best of our knowledge, no reports have investigated the effects of hyperoxia on low-intensity repetitive muscle contractions. Although these previous studies used maximum voluntary contractions as the muscular endurance exercise, low-intensity repetitive exercise is recommended for appropriate muscular endurance training. Meanwhile, only a few studies have evaluated the effects of hyperoxia on local muscular endurance, and these studies reported that the mean value of peak torque during 60 maximal knee extensions was higher under hyperoxic conditions than normoxic conditions, thus suggesting positive effects of hyperoxia on muscular endurance. These advantages of hyperoxic conditions are provided mainly by higher oxygen delivery to muscles and changes in muscle metabolism compared to normoxic conditions. Thus, many previous studies have reported positive effects of hyperoxia during whole-body aerobic and anaerobic exercise. During sub-maximal aerobic exercise, the heart rate (HR), blood lactate concentration, and rating of perceived exertion (RPE) are lower under hyperoxic conditions than normoxic conditions. Hyperoxic conditions also increase maximal anaerobic power. Measures of maximal aerobic exercise performance, such as maximal oxygen uptake ( ) and maximum exercise duration in high-intensity aerobic exercise, increase when exercise is performed under hyperoxic conditions. Several studies have investigated the effects of hyperoxia on exercise performance. As a way to increase the number of repetitions until exhaustion, we focused on exercise performed under hyperoxic conditions. Most previous studies explaining the rationale for low-intensity training involving high repetitions for muscular endurance used repetitions to the point of exhaustion therefore, increasing the number of repetitions performed to the point of exhaustion might be effective for improving muscular endurance. Low-intensity training involving high repetitions is generally recommended to enhance muscular endurance. Muscular endurance is defined as the ability of a muscle group to execute repeated contractions over a period of time sufficient to cause muscular fatigue, or to maintain a specific percentage of maximum voluntary contraction for a prolonged period of time. The results suggest that hyperoxia can acutely enhance dynamic muscular endurance, with delayed elevation of EMG amplitude due to fatigue, and the effects are associated with individual whole-body endurance capacity. The 95% CI for the correlation coefficient between the percent change in the maximum number of repetitions and muscular endurance included 0 ( τ = 0.284, 95% CI = −0.003–0.565, P = 0.047) this indicated no significant correlation between the two parameters. The percent change in the maximum number of repetitions between hyperoxic and normoxic conditions had significant positive correlations with individual maximal oxygen uptake measured using an incremental cycle ergometer test ( r = 0.562, 95% confidence intervals = 0.213–0.783, P = 0.003), but not with muscle strength ( τ = −0.124, 95% CI = −0.424–0.170, P = 0.387). The root mean square amplitude of EMG and oxygenated hemoglobin concentration for the last five repetitions under normoxic conditions were greater than those under hyperoxic conditions ( P = 0.015 and P = 0.003, respectively). The maximum number of repetitions was greater ( P < 0.001) under hyperoxic conditions (132 ± 59 repetitions) than under normoxic conditions (114 ± 40 repetitions). Electromyogram (EMG) and near-infrared spectroscopy parameters were measured in the biceps brachii. The maximum number of repetitions was recorded as an index of muscular endurance. Twenty-five young men performed repetitions of the one-arm preacher curl at 30% of their 1-repetition maximum until exhaustion under hyperoxic and normoxic conditions. A single-blinded, counterbalanced crossover design was used. This study aimed to investigate the acute effects of hyperoxia on dynamic muscular endurance, and determine individual factors that may be related to these effects. Hyperoxic conditions could increase the number of repetitions until exhaustion and thereby improve the results of muscular endurance training. Low-intensity training involving high repetitions is recommended to enhance muscular endurance.
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