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

Girard O, Racinais S - Eur. J. Appl. Physiol. (2014)

Bottom Line: 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.Altitude had no effect on any measured parameters.Moderate hypoxia in combination with heat stress reduces cycling time to exhaustion without modifying neuromuscular fatigue characteristics.

View Article: PubMed Central - PubMed

Affiliation: Athlete Health and Performance Research Centre, Aspetar, Qatar Orthopaedic and Sports Medicine Hospital, PO Box 29222, Doha, Qatar, oliv.girard@gmail.com.

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.

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Related in: MedlinePlus

Exercise responses in four different environmental conditions. CON temperate and sea level, HOT hot and sea level, HYP temperate and moderate altitude, H + H hot and moderate altitude. See text for details. At the onset of exercise, HR and Tskin were higher (P < 0.05) in warm conditions, whereas altitude exposure reduced SpO2 and increased HR (both P < 0.05). Warm environments increased Tskin and HR at exhaustion (P < 0.05), whereas SpO2 (P < 0.05) was lowered with altitude exposure. The rate of rise in Tcore, HR and RPE was speeded under warm conditions (P < 0.05), whereas the rate of HR and RPE increase and SpO2 decrease was faster with altitude exposure (both P < 0.05). There was no interaction effect between temperature and altitude on either parameter
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Fig3: Exercise responses in four different environmental conditions. CON temperate and sea level, HOT hot and sea level, HYP temperate and moderate altitude, H + H hot and moderate altitude. See text for details. At the onset of exercise, HR and Tskin were higher (P < 0.05) in warm conditions, whereas altitude exposure reduced SpO2 and increased HR (both P < 0.05). Warm environments increased Tskin and HR at exhaustion (P < 0.05), whereas SpO2 (P < 0.05) was lowered with altitude exposure. The rate of rise in Tcore, HR and RPE was speeded under warm conditions (P < 0.05), whereas the rate of HR and RPE increase and SpO2 decrease was faster with altitude exposure (both P < 0.05). There was no interaction effect between temperature and altitude on either parameter

Mentions: Hot conditions significantly (P < 0.05) increased HR (+9 ± 5 bpm) and Tskin (+5 ± 1 °C) at exhaustion independently of the simulated altitude, whereas altitude exposure reduced SpO2 independently of the environmental temperature (96 ± 1 vs. 88 ± 4 % in sea level and moderate altitude conditions, P < 0.05) (Fig. 3). There was no main effect of temperature and altitude on Tcore (P = 0.089 and P = 0.070, respectively) or RPE (P > 0.49).Fig. 3


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

Girard O, Racinais S - Eur. J. Appl. Physiol. (2014)

Exercise responses in four different environmental conditions. CON temperate and sea level, HOT hot and sea level, HYP temperate and moderate altitude, H + H hot and moderate altitude. See text for details. At the onset of exercise, HR and Tskin were higher (P < 0.05) in warm conditions, whereas altitude exposure reduced SpO2 and increased HR (both P < 0.05). Warm environments increased Tskin and HR at exhaustion (P < 0.05), whereas SpO2 (P < 0.05) was lowered with altitude exposure. The rate of rise in Tcore, HR and RPE was speeded under warm conditions (P < 0.05), whereas the rate of HR and RPE increase and SpO2 decrease was faster with altitude exposure (both P < 0.05). There was no interaction effect between temperature and altitude on either parameter
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC4048668&req=5

Fig3: Exercise responses in four different environmental conditions. CON temperate and sea level, HOT hot and sea level, HYP temperate and moderate altitude, H + H hot and moderate altitude. See text for details. At the onset of exercise, HR and Tskin were higher (P < 0.05) in warm conditions, whereas altitude exposure reduced SpO2 and increased HR (both P < 0.05). Warm environments increased Tskin and HR at exhaustion (P < 0.05), whereas SpO2 (P < 0.05) was lowered with altitude exposure. The rate of rise in Tcore, HR and RPE was speeded under warm conditions (P < 0.05), whereas the rate of HR and RPE increase and SpO2 decrease was faster with altitude exposure (both P < 0.05). There was no interaction effect between temperature and altitude on either parameter
Mentions: Hot conditions significantly (P < 0.05) increased HR (+9 ± 5 bpm) and Tskin (+5 ± 1 °C) at exhaustion independently of the simulated altitude, whereas altitude exposure reduced SpO2 independently of the environmental temperature (96 ± 1 vs. 88 ± 4 % in sea level and moderate altitude conditions, P < 0.05) (Fig. 3). There was no main effect of temperature and altitude on Tcore (P = 0.089 and P = 0.070, respectively) or RPE (P > 0.49).Fig. 3

Bottom Line: 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.Altitude had no effect on any measured parameters.Moderate hypoxia in combination with heat stress reduces cycling time to exhaustion without modifying neuromuscular fatigue characteristics.

View Article: PubMed Central - PubMed

Affiliation: Athlete Health and Performance Research Centre, Aspetar, Qatar Orthopaedic and Sports Medicine Hospital, PO Box 29222, Doha, Qatar, oliv.girard@gmail.com.

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.

Show MeSH
Related in: MedlinePlus