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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

Protocol overview. General procedure (a) and neuromuscular assessment procedure (b). Tcore core temperature, Tskin skin temperature, HR heart rate, SpO2 arterial saturation percentage, RPE rating of perceived exertion, [La] blood lactate concentration, MVC maximal isometric voluntary contraction torque of plantar flexors. Straight arrows indicate the timing of motor nerve stimulations at submaximal (H-reflex, downwards arrow) or supra-maximal (M-wave, downwards arrow) intensities. Maximal H-reflex (HMAX) and M-wave (MMAX) were evoked on a relaxed muscle. The stimulation necessary to obtain HMAX at rest was superimposed to MVC to record HSUP. When supra-maximal stimulus was superimposed to MVC, superimposed M-wave (MSUP) and V-wave (VSUP) were recorded
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Fig1: Protocol overview. General procedure (a) and neuromuscular assessment procedure (b). Tcore core temperature, Tskin skin temperature, HR heart rate, SpO2 arterial saturation percentage, RPE rating of perceived exertion, [La] blood lactate concentration, MVC maximal isometric voluntary contraction torque of plantar flexors. Straight arrows indicate the timing of motor nerve stimulations at submaximal (H-reflex, downwards arrow) or supra-maximal (M-wave, downwards arrow) intensities. Maximal H-reflex (HMAX) and M-wave (MMAX) were evoked on a relaxed muscle. The stimulation necessary to obtain HMAX at rest was superimposed to MVC to record HSUP. When supra-maximal stimulus was superimposed to MVC, superimposed M-wave (MSUP) and V-wave (VSUP) were recorded

Mentions: Each participant completed a familiarization session and four experimental trials during which they cycled to exhaustion in four different environmental conditions [temperate/sea level (control), hot/sea level (hot), temperate/moderate altitude (hypoxia) and hot/moderate altitude (hot + hypoxia)]. Temperate and hot conditions were 22 °C/30 % rH and 35 °C/40 % rH, respectively. Sea level (FiO2 0.21) and moderate altitude (FiO2 0.15) corresponded to a simulated altitude of ~0 and ~2,500 m, respectively. The trials were randomized, separated by at least 5–7 days, and performed at the same time of the day (±2 h) with subjects wearing shorts and t-shirts. As depicted in Fig. 1a, each experimental session was conducted as follows: (1) rest in a seated position for 30 min inside the climatic chamber, while participants were instrumented; (2) 10-min warm-up on a computer-controlled electrically braked cycle ergometer (Excalibur Sport, Lode, Netherlands) at 75 W (pedalling rate 70–80 rpm); (3) neuromuscular tests (pre-tests; ~10 min); (4) 5-min rest; (5) time trial to the limit of exhaustion at a fixed workload, equal to 66 % of the power output associated with their maximal oxygen uptake (180.5 ± 23.6 W; pedalling rate 80–90 rpm); (6) 5-min recovery including 90 s of low-intensity (50 W, 60–70 rpm) pedalling followed by 3-min rest (time for the participants to be seated on the test ergometer) and (7) neuromuscular tests (post-tests; ~10 min). Cycling exhaustive bouts began exactly 20 min after the end of the warm-up and was preceded by a 2-min low-intensity phase at the same work rate as the one used for warm-up. Constant visual and vocal feedbacks were given to the subjects to avoid variations in pedal cadence. Exercise was terminated when pedal cadence dropped below 60 rpm for >5 s (exhaustion). The participants were unaware of the experimental hypotheses and naïve to the purpose of the study. All tests (cycling and neuromuscular assessment) were performed in an environmental chamber (Tescor, Warminster, PA, USA).Fig. 1


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)

Protocol overview. General procedure (a) and neuromuscular assessment procedure (b). Tcore core temperature, Tskin skin temperature, HR heart rate, SpO2 arterial saturation percentage, RPE rating of perceived exertion, [La] blood lactate concentration, MVC maximal isometric voluntary contraction torque of plantar flexors. Straight arrows indicate the timing of motor nerve stimulations at submaximal (H-reflex, downwards arrow) or supra-maximal (M-wave, downwards arrow) intensities. Maximal H-reflex (HMAX) and M-wave (MMAX) were evoked on a relaxed muscle. The stimulation necessary to obtain HMAX at rest was superimposed to MVC to record HSUP. When supra-maximal stimulus was superimposed to MVC, superimposed M-wave (MSUP) and V-wave (VSUP) were recorded
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

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Fig1: Protocol overview. General procedure (a) and neuromuscular assessment procedure (b). Tcore core temperature, Tskin skin temperature, HR heart rate, SpO2 arterial saturation percentage, RPE rating of perceived exertion, [La] blood lactate concentration, MVC maximal isometric voluntary contraction torque of plantar flexors. Straight arrows indicate the timing of motor nerve stimulations at submaximal (H-reflex, downwards arrow) or supra-maximal (M-wave, downwards arrow) intensities. Maximal H-reflex (HMAX) and M-wave (MMAX) were evoked on a relaxed muscle. The stimulation necessary to obtain HMAX at rest was superimposed to MVC to record HSUP. When supra-maximal stimulus was superimposed to MVC, superimposed M-wave (MSUP) and V-wave (VSUP) were recorded
Mentions: Each participant completed a familiarization session and four experimental trials during which they cycled to exhaustion in four different environmental conditions [temperate/sea level (control), hot/sea level (hot), temperate/moderate altitude (hypoxia) and hot/moderate altitude (hot + hypoxia)]. Temperate and hot conditions were 22 °C/30 % rH and 35 °C/40 % rH, respectively. Sea level (FiO2 0.21) and moderate altitude (FiO2 0.15) corresponded to a simulated altitude of ~0 and ~2,500 m, respectively. The trials were randomized, separated by at least 5–7 days, and performed at the same time of the day (±2 h) with subjects wearing shorts and t-shirts. As depicted in Fig. 1a, each experimental session was conducted as follows: (1) rest in a seated position for 30 min inside the climatic chamber, while participants were instrumented; (2) 10-min warm-up on a computer-controlled electrically braked cycle ergometer (Excalibur Sport, Lode, Netherlands) at 75 W (pedalling rate 70–80 rpm); (3) neuromuscular tests (pre-tests; ~10 min); (4) 5-min rest; (5) time trial to the limit of exhaustion at a fixed workload, equal to 66 % of the power output associated with their maximal oxygen uptake (180.5 ± 23.6 W; pedalling rate 80–90 rpm); (6) 5-min recovery including 90 s of low-intensity (50 W, 60–70 rpm) pedalling followed by 3-min rest (time for the participants to be seated on the test ergometer) and (7) neuromuscular tests (post-tests; ~10 min). Cycling exhaustive bouts began exactly 20 min after the end of the warm-up and was preceded by a 2-min low-intensity phase at the same work rate as the one used for warm-up. Constant visual and vocal feedbacks were given to the subjects to avoid variations in pedal cadence. Exercise was terminated when pedal cadence dropped below 60 rpm for >5 s (exhaustion). The participants were unaware of the experimental hypotheses and naïve to the purpose of the study. All tests (cycling and neuromuscular assessment) were performed in an environmental chamber (Tescor, Warminster, PA, USA).Fig. 1

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