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Combining heat stress and moderate hypoxia reduces cycling time to exhaustion without modifying neuromuscular fatigue characteristics.
Eur J Appl Physiol 2014; 114(7):1521-32EJ

Abstract

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 5 min after exercise.

RESULTS

Time to exhaustion was reduced (P < 0.05) in hot (-35 ± 15 %) or hypoxia (-36 ± 14 %) compared to control (61 ± 28 min), while hot + hypoxia (-51 ± 20 %) further compromised exercise capacity (P < 0.05). However, the effect of temperature or altitude on end-exercise core temperature (P = 0.089 and P = 0.070, respectively) and rating of perceived exertion (P > 0.05) 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 < 0.05) in hot compared to temperate conditions, while normalized maximal EMG activity did not change. Altitude had no effect on any measured parameters.

CONCLUSION

Moderate hypoxia in combination with heat stress reduces cycling time to exhaustion without modifying neuromuscular fatigue characteristics. Impaired oxygen delivery or increased cardiovascular strain, increasing relative exercise intensity, may have also contributed to earlier exercise cessation.

Authors+Show Affiliations

Athlete Health and Performance Research Centre, Aspetar, Qatar Orthopaedic and Sports Medicine Hospital, PO Box 29222, Doha, Qatar, oliv.girard@gmail.com.No affiliation info available

Pub Type(s)

Comparative Study
Journal Article
Randomized Controlled Trial

Language

eng

PubMed ID

24748530

Citation

Girard, Olivier, and Sébastien Racinais. "Combining Heat Stress and Moderate Hypoxia Reduces Cycling Time to Exhaustion Without Modifying Neuromuscular Fatigue Characteristics." European Journal of Applied Physiology, vol. 114, no. 7, 2014, pp. 1521-32.
Girard O, Racinais S. Combining heat stress and moderate hypoxia reduces cycling time to exhaustion without modifying neuromuscular fatigue characteristics. Eur J Appl Physiol. 2014;114(7):1521-32.
Girard, O., & Racinais, S. (2014). Combining heat stress and moderate hypoxia reduces cycling time to exhaustion without modifying neuromuscular fatigue characteristics. European Journal of Applied Physiology, 114(7), pp. 1521-32. doi:10.1007/s00421-014-2883-0.
Girard O, Racinais S. Combining Heat Stress and Moderate Hypoxia Reduces Cycling Time to Exhaustion Without Modifying Neuromuscular Fatigue Characteristics. Eur J Appl Physiol. 2014;114(7):1521-32. PubMed PMID: 24748530.
* Article titles in AMA citation format should be in sentence-case
TY - JOUR T1 - Combining heat stress and moderate hypoxia reduces cycling time to exhaustion without modifying neuromuscular fatigue characteristics. AU - Girard,Olivier, AU - Racinais,Sébastien, Y1 - 2014/04/19/ PY - 2013/09/23/received PY - 2014/04/01/accepted PY - 2014/4/22/entrez PY - 2014/4/22/pubmed PY - 2015/1/24/medline SP - 1521 EP - 32 JF - European journal of applied physiology JO - Eur. J. Appl. Physiol. VL - 114 IS - 7 N2 - 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 5 min after exercise. RESULTS: Time to exhaustion was reduced (P < 0.05) in hot (-35 ± 15 %) or hypoxia (-36 ± 14 %) compared to control (61 ± 28 min), while hot + hypoxia (-51 ± 20 %) further compromised exercise capacity (P < 0.05). However, the effect of temperature or altitude on end-exercise core temperature (P = 0.089 and P = 0.070, respectively) and rating of perceived exertion (P > 0.05) 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 < 0.05) in hot compared to temperate conditions, while normalized maximal EMG activity did not change. Altitude had no effect on any measured parameters. CONCLUSION: Moderate hypoxia in combination with heat stress reduces cycling time to exhaustion without modifying neuromuscular fatigue characteristics. Impaired oxygen delivery or increased cardiovascular strain, increasing relative exercise intensity, may have also contributed to earlier exercise cessation. SN - 1439-6327 UR - https://www.unboundmedicine.com/medline/citation/24748530/Combining_heat_stress_and_moderate_hypoxia_reduces_cycling_time_to_exhaustion_without_modifying_neuromuscular_fatigue_characteristics_ L2 - https://dx.doi.org/10.1007/s00421-014-2883-0 DB - PRIME DP - Unbound Medicine ER -