Repeated muscle damage blunts the increase in heat strain during subsequent exercise heat stress.
OBJECTIVE
Exercise-induced muscle damage (EIMD) has recently been shown to increase heat strain during exercise heat stress (HS), and represents a risk factor for exertional heat illness (EHI). We hypothesised that a repeated bout of EIMD blunts the increase in rectal temperature (T re) during subsequent endurance exercise in the heat.


METHODS
Sixteen non-heat-acclimated males were randomly allocated to EIMD (n = 9) or control (CON, n = 7). EIMD performed a downhill running treatment at -10 % gradient for 60 min at 65 % [Formula: see text]O2max in 20 °C, 40 % RH. CON participants performed the same treatment but at +1 % gradient. Following treatment, participants rested for 30 min, then performed HS (+1 % gradient running for 40 min at 65 % [Formula: see text]O2max in 33 °C, 50 % RH) during which thermoregulatory measures were assessed. Both groups repeated the treatment and subsequent HS 14 days later. Isometric quadriceps strength was assessed at baseline, and 48 h post-treatment.


RESULTS
The decrease in leg strength 48 h post-EIMD trial 1 (-7.5 %) was absent 48 h post-EIMD trial 2 (+2.9 %) demonstrating a repeated bout effect. Final T re during HS was lower following EIMD trial 2 (39.25 ± 0.47 °C) compared with EIMD trial 1 (39.59 ± 0.49 °C, P < 0.01), with CON showing no difference. Thermal sensation and the T re threshold for sweating onset were also lower during HS on EIMD trial 2.


CONCLUSIONS
The repeated bout effect blunted the increase in heat strain during HS conducted after EIMD. Incorporating a muscle-damaging bout into training could be a strategy to reduce the risk of EHI and improve endurance performance in individuals undertaking heavy exercise with an eccentric component in the heat.
Extremes Research Group, School of Sport, Health and Exercise Sciences, College of Health and Behavioural Sciences, Bangor University, Bangor, Gwynedd, LL57 2PZ, UK.
A
A
Dolci
M B
MB
Fortes
F S
FS
Walker
A
A
Haq
T
T
Riddle
N P
NP
Walsh
D015444:Exercise / Q000502:physiology*; D005081:Exercise Therapy / Q000379:methods; D018882:Heat Stress Disorders / Q000209:etiology / Q000503:physiopathology* / Q000628:therapy; D006358:Hot Temperature; D006801:Humans; D008297:Male; D018482:Muscle, Skeletal / Q000502:physiology / Q000503:physiopathology*; D010101:Oxygen Consumption / Q000502:physiology; D012420:Running / Q000502:physiology*; D013546:Sweating / Q000502:physiology; D013697:Thermosensing / Q000502:physiology; D055815:Young Adult