oa Combined Temperature and Altitude Challenges do not Exacerbate the Degree of Muscle Fatigue Despite Shorter Cycling Time to Exhaustion

This study investigated the combined effect of environmental temperature [neutral (22°C/30%rH) vs. warm (35°C/40%rH)] and altitude challenge [sea level (FIO2 0.21) vs. reduced O2 content (FIO2 0.15)] on locomotor performance and the degree of end-exercise neuromuscular fatigue. Eleven physically active subjects cycled to exhaustion at constant workload (66% of their VO2max) in four different environmental conditions [Neutral/Sea level (Control), Warm/Sea level (Hot), Neutral/Reduced O2 content (Hypoxia) and Warm/Reduced O2 content (Hot+Hypoxia)]. The torque and EMG responses following electrical stimulation of the tibial nerve (plantar-flexion; soleus) were recorded before and 5 min after exercise. Time to exhaustion was reduced (P<0.05) in Hot (−35%) or Hypoxia (−36%) compared to Control, while Hot+Hypoxia (−51%) further decreased performance. There was no main effect of temperature or altitude on end-exercise core temperature (P=0.089 and P=0.070, respectively) and rating of perceived exertion (both P>0.05), nor any significant interaction. Reductions in maximal voluntary contraction (MVC) torque (−9%; P<0.001), voluntary activation (−4%; P<0.05) and peak twitch torque (−6%; P<0.05) from pre- to post-exercise were similar between all trials, independently of the environmental temperature or altitude. M-wave amplitude (at rest and during brief MVC) and RMS activity were reduced (P<0.05) in warm compared to neutral conditions, while altitude had no main effect on any measured parameters. Combining environmental temperature and altitude challenges further reduce cycling time to exhaustion but do not exacerbate the degree of end-exercise neuromuscular fatigue.