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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

Time to exhaustion 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. *P < 0.05. Temperature and altitude had a main effect (P < 0.05) on time to exhaustion
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Fig2: Time to exhaustion 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. *P < 0.05. Temperature and altitude had a main effect (P < 0.05) on time to exhaustion

Mentions: Temperature (P < 0.05) and altitude (P < 0.05) both reduced time to exhaustion (Fig. 2), while there was also a significant interaction effect (P < 0.05) between these two factors. Hot + hypoxia was shorter than hot or hypoxia alone (P < 0.05). There was an antagonistic effect of either a hot (−22 ± 4 % from control to hot vs. −11 ± 3 % from hypoxia to hot + hypoxia; P < 0.05) or an altitude (−24 ± 6 % from control to hypoxia vs. −13 ± 5 % from hot to hot + hypoxia; P < 0.05) exposure when the other stressor was already present.Fig. 2


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)

Time to exhaustion 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. *P < 0.05. Temperature and altitude had a main effect (P < 0.05) on time to exhaustion
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

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

Fig2: Time to exhaustion 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. *P < 0.05. Temperature and altitude had a main effect (P < 0.05) on time to exhaustion
Mentions: Temperature (P < 0.05) and altitude (P < 0.05) both reduced time to exhaustion (Fig. 2), while there was also a significant interaction effect (P < 0.05) between these two factors. Hot + hypoxia was shorter than hot or hypoxia alone (P < 0.05). There was an antagonistic effect of either a hot (−22 ± 4 % from control to hot vs. −11 ± 3 % from hypoxia to hot + hypoxia; P < 0.05) or an altitude (−24 ± 6 % from control to hypoxia vs. −13 ± 5 % from hot to hot + hypoxia; P < 0.05) exposure when the other stressor was already present.Fig. 2

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