BACKGROUND: Climbing and trekking at altitude are common recreational and military activities. Physiological effects of altitude are hypoxia and hyperventilation. The hyperventilatory response to altitude may cause respiratory muscle fatigue and reduce sustained submaximal exercise. Voluntary isocapnic hyperpnea respiratory muscle training (VIHT) improves exercise endurance at sea level and at depth. The purpose of this study was to test the hypothesis that VIHT would improve exercise time at altitude [3600 m (11,811 ft)] compared to control and placebo groups.METHODS: Subjects pedaled an ergometer until exhaustion at simulated altitude in a hypobaric chamber while noninvasive arterial saturation (S_ao₂), ventilation (V_E), and oxygen consumption (Vo₂) were measured.RESULTS: As expected, S_ao₂ decreased to 88 ± 4% saturation at rest and to 81 ± 2% during exercise, and was not affected by VIHT. VIHT resulted in a 40% increase in maximal training V_E compared to pre-VIHT. Exercise endurance significantly increased 44% after VIHT (P = <0.001). Vo₂ (30 ± 3 ml · kg⁻¹ · min⁻¹) and heart rate (177 ± 10 bpm) did not change during exercise and were not affected by VIHT (P = 0.531). Pre-VIHT V_E increased 21–27% during the initial 12 min of exercise, after which it decreased 17% at 17.7 ± 6.0 min. V_E at altitude post-VIHT increased more (49%) for longer (21 min) and decreased less (11% at 25.4 ± 6.7 min).DISCUSSION: VIHT improved exercise time at altitude and sustained V_E. This suggests that VIHT reduced respiratory muscle fatigue and would be useful to trekkers and military personnel working at altitude.Helfer S, Quackenbush J, Fletcher M, Pendergast DR. Respiratory muscle training and exercise endurance at altitutde. Aerosp Med Hum Perform. 2016; 87(8):704–711.