Combining heat stress and moderate hypoxia reduces cycling time to exhaustion without modifying neuromuscular fatigue characteristics

Purpose: This study investigated the isolated and combined effects of heat [temperate (22°C/30% rH) vs. hot (35°C/40% rH)] and hypoxia [sea level (FiO2 0.21) vs. moderate altitude (FiO2 0.15)] on exercise capacity and neuromuscular fatigue characteristics. Methods: Eleven physically active subjects cycled to exhaustion at constant workload (66% of the power output associated with their maximal oxygen uptake in temperate conditions) in four different environmental conditions [temperate/sea level (control), hot/sea level (hot), temperate/moderate altitude (hypoxia) and hot/moderate altitude (hot+hypoxia)]. Torque and electromyography (EMG) responses following electrical stimulation of the tibial nerve (plantar-flexion; soleus) were recorded before and 5min after exercise. Results: Time to exhaustion was reduced (P0.05) did not reach significance. Maximal voluntary contraction torque, voluntary activation (twitch interpolation) and peak twitch torque decreased from pre- to post-exercise (−9±1, −4±1 and −6±1% all trials compounded, respectively; P<0.05), with no effect of the temperature or altitude. M-wave amplitude and root mean square activity were reduced (P