title: Muscle-damaging exercise increases heat strain during subsequent exercise heat stress.
abstract: OBJECTIVE
  It remains unclear whether exercise-induced muscle damage (EIMD) increases heat strain during subsequent exercise heat stress, which in turn may increase the risk of exertional heat illness. We examined heat strain during exercise heat stress 30 min after EIMD to coincide with increases in circulating pyrogens (e.g., interleukin-6 [IL-6]) and 24 h after EIMD to coincide with the delayed muscle inflammatory response when a higher rate of metabolic energy expenditure (M˙) and thus decreased economy might also increase heat strain.
  
  
  METHODS
  Thirteen non-heat-acclimated males (mean ± SD, age = 20 ± 2 yr) performed exercise heat stress tests (running for 40 min at 65% V˙O2max in 33°C, 50% humidity) 30 min (HS1) and 24 h (HS2) after treatment, involving running for 60 min at 65% V˙O2max on either -10% gradient (EIMD) or +1% gradient (CON) in a crossover design. Rectal (Tre) and skin (Tsk) temperature, local sweating rate, and M˙ were measured throughout HS tests.
  
  
  RESULTS
  Compared with CON, EIMD evoked higher circulating IL-6 pre-HS1 (P < 0.01) and greater plasma creatine kinase and muscle soreness pre-HS2 (P < 0.01). The ΔTre was greater after EIMD than CON during HS1 (0.35°C, 95% confidence interval = 0.11°C-0.58°C, P < 0.01) and HS2 (0.17°C, 95% confidence interval = 0.07°C-0.28°C, P < 0.01). M˙ was higher on EIMD throughout HS1 and HS2 (P < 0.001). Thermoeffector responses (Tsk, sweating rate) were not altered by EIMD. Thermal sensation and RPE were higher on EIMD after 25 min during HS1 (P < 0.05). The final Tre during HS1 correlated with the pre-HS1 circulating IL-6 concentration (r = 0.67).
  
  
  CONCLUSIONS
  Heat strain was increased during endurance exercise in the heat conducted 30 min after and, to a much lesser extent, 24 h after muscle-damaging exercise. These data indicate that EIMD is a likely risk factor for exertional heat illness particularly during exercise heat stress when behavioral thermoregulation cues are ignored.
authors.affiliation: 1College of Health and Behavioural Sciences, Bangor University, Gwynedd, UNITED KINGDOM; and 2Department of Biomedical Sciences and Technologies, University of L'Aquila, Coppito, ITALY.
authors.forename: Matthew Benjamin
authors.initials: MB
authors.lastname: Fortes
authors.forename: Umberto
authors.initials: U
authors.lastname: Di Felice
authors.forename: Alberto
authors.initials: A
authors.lastname: Dolci
authors.forename: Naushad A
authors.initials: NA
authors.lastname: Junglee
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Crockford
authors.forename: Liam
authors.initials: L
authors.lastname: West
authors.forename: Ryan
authors.initials: R
authors.lastname: Hillier-Smith
authors.forename: Jamie Hugo
authors.initials: JH
authors.lastname: Macdonald
authors.forename: Neil Peter
authors.initials: NP
authors.lastname: Walsh
mesh_terms: D000293:Adolescent; D000328:Adult; D003402:Creatine Kinase / Q000097:blood; D018592:Cross-Over Studies; D004734:Energy Metabolism; D005080:Exercise Test; D018882:Heat Stress Disorders / Q000097:blood / Q000209:etiology / Q000503:physiopathology*; D006358:Hot Temperature; D006801:Humans; D015850:Interleukin-6 / Q000097:blood; D008297:Male; D018482:Muscle, Skeletal / Q000473:pathology* / Q000503:physiopathology*; D063806:Myalgia / Q000473:pathology / Q000503:physiopathology; D010101:Oxygen Consumption; D005082:Physical Exertion / Q000502:physiology*; D012420:Running / Q000502:physiology; D012881:Skin Temperature; D013546:Sweating; D013697:Thermosensing; D013997:Time Factors; D055815:Young Adult
chemical_list: D015850:Interleukin-6; D003402:Creatine Kinase