Neuromuscular Responses to Combined Heat Stress and Hypoxia During 20-km Cycling Time Trials

Abstract

The effect of combined heat stress and hypoxia on exercise performance may be dependent on the modality of exercise. In response to constant-work exercise, combined environmental stressors demonstrate an interaction effect on time to exhaustion; however, similar combined stressors have an independent influence on cycling time-trial (TT) performance. Investigation of the neuromuscular responses to combined environmental stressors may clarify the underlying mechanism(s) that contribute to apparent task-specific responses. PURPOSE:To examine the isolated and combined effects of ambient temperature [cool (18°C, 20% rh) vs hot (35°C, 20% rh)] and inspired oxygen content [normoxia (FiO2 0.21) vs hypoxia (FiO20.16)] on neuromuscular function in response to a cycling TT. METHODS:Five physically active male participants (23 ± 6 y) performed four 20-km cycling TTs in different environmental conditions [cool/normoxia (COOL); hot/normoxia (HOT); cool/hypoxia (HYPO); hot/hypoxia (H-H)]. Neuromuscular responses of the soleus, as indicated by changes in isometric MVC (iMVC), M-wave, twitch force (Qtw), and voluntary activation (VA), were assessed prior to and following each time-trial. Linear mixed model analyses were used to examine the neuromuscular responses, with fixed effects for each condition and a random intercept for participants. RESULTS:Time-trial performance was impaired during HOT (2211±85s; 192±18W), HYPO (2213±122s; 192±27W), and H-H (2214±117s; 192±27W) compared to COOL (2090±54s; 221±14W, p≤0.02). Similar reductions in iMVC (-9.0±12.0%) and VA (-14.0±9.6%) were observed across all conditions (p<0.05); however, no significant differences were observed in M-wave (p=0.09) or Qtw(p=0.43). CONCLUSION: Neuromuscular impairments following 20-km cycling TT are attributed to central mechanism(s) (i.e., VA); however, neuromuscular adaptations were similar in conditions where heat stress and hypoxia were combined, to conditions where each environmental stressor was examined in isolation.