title: Combining heat stress and moderate hypoxia reduces cycling time to exhaustion without modifying neuromuscular fatigue characteristics.
abstract: OBJECTIVE
  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.
  
  
  CONCLUSIONS
  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.affiliation: Athlete Health and Performance Research Centre, Aspetar, Qatar Orthopaedic and Sports Medicine Hospital, PO Box 29222, Doha, Qatar, oliv.girard@gmail.com.
authors.forename: Olivier
authors.initials: O
authors.lastname: Girard
authors.forename: Sébastien
authors.initials: S
authors.lastname: Racinais
mesh_terms: D000328:Adult; D000531:Altitude; D001642:Bicycling; D001696:Biomechanical Phenomena; D004558:Electric Stimulation; D004576:Electromyography; D005080:Exercise Test; D017079:Exercise Tolerance; D018882:Heat Stress Disorders / Q000503:physiopathology*; D006358:Hot Temperature; D006801:Humans; D006813:Humidity; D000860:Hypoxia / Q000503:physiopathology*; D008297:Male; D009119:Muscle Contraction; D018763:Muscle Fatigue; D018482:Muscle, Skeletal / Q000294:innervation / Q000503:physiopathology*; D010101:Oxygen Consumption; D010807:Physical Endurance; D011780:Qatar; D013997:Time Factors; D019415:Torque
references.citation: Scand J Med Sci Sports. 2011 Jun;21(3):455-64
references.pmid: 20030784
references.citation: Eur J Appl Physiol. 2010 Aug;109(6):1027-36
references.pmid: 20358218
references.citation: J Appl Physiol (1985). 2001 Sep;91(3):1055-60
references.pmid: 11509498
references.citation: J Physiol. 2013 Mar 1;591(5):1339-46
references.pmid: 23230235
references.citation: Neuroscience. 2013 Feb 12;231:384-99
references.pmid: 23131709
references.citation: J Neurophysiol. 2007 Jan;97(1):596-603
references.pmid: 17093121
references.citation: J Appl Physiol (1985). 2008 Mar;104(3):871-8
references.pmid: 17962572
references.citation: J Appl Physiol (1985). 2006 Apr;100(4):1361-9
references.pmid: 16339343
references.citation: Appl Ergon. 2010 Oct;41(6):806-11
references.pmid: 20206916
references.citation: Respir Res. 2010 Aug 11;11:109
references.pmid: 20701769
references.citation: Am J Physiol Regul Integr Comp Physiol. 2007 Jan;292(1):R598-606
references.pmid: 16959862
references.citation: Int J Sports Med. 2007 Jan;28(1):26-32
references.pmid: 17213963
references.citation: J Physiol. 2012 Jun 1;590(11):2767-82
references.pmid: 22473785
references.citation: Scand J Med Sci Sports. 2010 Oct;20 Suppl 3:1-18
references.pmid: 21029186
references.citation: J Physiol. 2007 May 15;581(Pt 1):389-403
references.pmid: 17317739
references.citation: High Alt Med Biol. 2009 Summer;10(2):175-82
references.pmid: 19519224
references.citation: Med Sci Sports Exerc. 2011 Nov;43(11):2039-45
references.pmid: 21502884
references.citation: Med Sci Sports Exerc. 2010 Jan;42(1):135-41
references.pmid: 20010120
references.citation: Eur J Appl Physiol Occup Physiol. 1990;60(2):155-61
references.pmid: 2335174
references.citation: Med Sci Sports Exerc. 1997 Sep;29(9):1240-9
references.pmid: 9309637
references.citation: J Physiol. 2008 Jan 1;586(1):45-53
references.pmid: 17855754
references.citation: Eur J Appl Physiol. 2004 May;91(5-6):729-36
references.pmid: 15015001
references.citation: Physiol Rev. 2008 Jan;88(1):287-332
references.pmid: 18195089
references.citation: J Neurophysiol. 2005 Nov;94(5):3555-62
references.pmid: 16049144
references.citation: Muscle Nerve. 2010 May;41(5):642-50
references.pmid: 19918764
references.citation: Eur J Appl Physiol. 2012 Jun;112(6):2323-30
references.pmid: 22012541
references.citation: J Appl Physiol (1985). 2009 Jul;107(1):355-7; discussion 357-8
references.pmid: 19567806
references.citation: J Clin Neurophysiol. 2013 Aug;30(4):415-21
references.pmid: 23912583
references.citation: PLoS One. 2011 Feb 22;6(2):e17059
references.pmid: 21364944
references.citation: Psychophysiology. 2012 Jun;49(6):853-65
references.pmid: 22416901
references.citation: J Physiol. 2008 Oct 1;586(19):4751-62
references.pmid: 18703579
references.citation: J Appl Physiol (1985). 2007 Jul;103(1):402-11
references.pmid: 17446407
references.citation: Exp Physiol. 2011 Feb;96(2):134-44
references.pmid: 20851861
references.citation: J Physiol. 2010 Jun 1;588(Pt 11):1985-95
references.pmid: 20403976
references.citation: J Physiol. 2006 Sep 15;575(Pt 3):937-52
references.pmid: 16793898
references.citation: J Strength Cond Res. 2014 Jul;28(7):1906-14
references.pmid: 24276293