Swimmers can train in hypoxia at sea level through voluntary hypoventilation.

•We measured blood oxygenation in swimmers thanks to a waterproofed forehead sensor.•Voluntary hypoventilation at low lung volume induced severe hypoxemia in swimmers.•Hypoventilation at low lung volume increased the solicitation of anaerobic glycolysis.•Voluntary hypoventilation at high lung volume did not lead to hypoxemia in swimmers.•Hypoventilation at high lung volume decreased the glycolytic stimulus in swimmers.