The interaction between environmental temperature and hypoxia on central and peripheral fatigue during high-intensity dynamic knee extension

This study investigated causative factors behind the expression of different interaction types during exposure to multi-stressor environments. Neuromuscular fatigue rates and time to exhaustion (TTE) were investigated in active males (n=9) exposed to three climates (5°C,50%-rh/23°C,50%-rh/42°C,70%-rh) at two inspired oxygen fractions (0.209/0.125 FIO2; equivalent attitude=4100 m). After 40-mins rest in the environmental conditions, participants performed constant workload (high-intensity) knee extension exercise until exhaustion, with brief assessments of neuromuscular function every 110-s. Independent exposure to cold, heat and hypoxia significantly (p<0.01) reduced TTE from thermoneutral-normoxia (reduction of 190, 405, 505-s from 915-s respectively). The TTE decrease was consistent with a faster rate of peripheral fatigue development (p<0.01) compared with thermoneutral-normoxia (increase of 1.6, 3.1 and 4.9%.min-1 from 4.1%.min-1 respectively). Combined exposure to hypoxic-cold resulted in an even greater TTE reduction (-589-seconds), likely due to an increase in the rate of peripheral fatigue development (increased by 7.6%.min-1), but this without significant interaction between stressors (p>0.198). In contrast, combined exposure to hypoxic-heat reduced TTE by 609-s, showing a significant antagonistic interaction (p=0.003) similarly supported by an increased rate of peripheral fatigue development (increased by 8.3%.min-1). A small decline (<0.4%.min-1) in voluntary muscle activation was only observed in thermoneutral-normoxia. In conclusion, interaction type is influenced by the impact magnitude of the individual stressors' effect on exercise capacity, whereby the greater the stressors impact, the greater the probability that one stressor will be abolished by the other. This indicates humans respond to severe and simultaneous physiological strains based on a 'worst strain takes precedence' principle.