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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

Maximal isometric voluntary contraction torque of plantar flexors (PF MVC torque, a), voluntary activation (twitch interpolation technique, b) and peak twitch torque (Pt, c) before (pre-tests) and after (post-tests) cycling to exhaustion in various 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, significantly different from pre-tests. There was neither a main effect of temperature or altitude nor any interaction between temperature and altitude on either parameter
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Fig4: Maximal isometric voluntary contraction torque of plantar flexors (PF MVC torque, a), voluntary activation (twitch interpolation technique, b) and peak twitch torque (Pt, c) before (pre-tests) and after (post-tests) cycling to exhaustion in various 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, significantly different from pre-tests. There was neither a main effect of temperature or altitude nor any interaction between temperature and altitude on either parameter

Mentions: Significant reductions occurred from pre- to post-exercise for MVC torque, VA and Pt (−8.6 ± 1.5, −3.9 ± 1.5 and −6.1 ± 0.8 %, respectively; all trials compounded; P < 0.05), independently of the environmental conditions (P < 0.05; Fig. 4). Table 2 presents EMG-related variables determined during the neuromuscular assessment performed before and +5 min after the fatigue protocol in various conditions. Raw HMAX (P < 0.05) and HMAX/MMAX ratio values were significantly (P < 0.05) reduced from pre- to post-exercise, with no temperature or altitude main effects. Compared to temperate environments, the amplitude of both M-waves (at rest, during submaximal contraction and MVC) and V-waves, but not H-reflexes, together with raw soleus RMS activity was significantly reduced (P < 0.05) in hot conditions (Table 2). There was neither a significant main effect of altitude nor a significant interaction between exercise, altitude and/or temperature for any normalized EMG-related variables.Fig. 4


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)

Maximal isometric voluntary contraction torque of plantar flexors (PF MVC torque, a), voluntary activation (twitch interpolation technique, b) and peak twitch torque (Pt, c) before (pre-tests) and after (post-tests) cycling to exhaustion in various 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, significantly different from pre-tests. There was neither a main effect of temperature or altitude nor any interaction between temperature and altitude on either parameter
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

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

Fig4: Maximal isometric voluntary contraction torque of plantar flexors (PF MVC torque, a), voluntary activation (twitch interpolation technique, b) and peak twitch torque (Pt, c) before (pre-tests) and after (post-tests) cycling to exhaustion in various 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, significantly different from pre-tests. There was neither a main effect of temperature or altitude nor any interaction between temperature and altitude on either parameter
Mentions: Significant reductions occurred from pre- to post-exercise for MVC torque, VA and Pt (−8.6 ± 1.5, −3.9 ± 1.5 and −6.1 ± 0.8 %, respectively; all trials compounded; P < 0.05), independently of the environmental conditions (P < 0.05; Fig. 4). Table 2 presents EMG-related variables determined during the neuromuscular assessment performed before and +5 min after the fatigue protocol in various conditions. Raw HMAX (P < 0.05) and HMAX/MMAX ratio values were significantly (P < 0.05) reduced from pre- to post-exercise, with no temperature or altitude main effects. Compared to temperate environments, the amplitude of both M-waves (at rest, during submaximal contraction and MVC) and V-waves, but not H-reflexes, together with raw soleus RMS activity was significantly reduced (P < 0.05) in hot conditions (Table 2). There was neither a significant main effect of altitude nor a significant interaction between exercise, altitude and/or temperature for any normalized EMG-related variables.Fig. 4

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