Muscle-damaging exercise increases heat strain during subsequent exercise heat stress

Area of Science:

• Exercise Physiology
• Environmental Physiology
• Sports Medicine

Background:

• Exercise-induced muscle damage (EIMD) is common after strenuous physical activity.
• The impact of EIMD on heat strain during subsequent exercise in hot environments remains poorly understood.
• EIMD may influence thermoregulation through inflammatory responses and altered metabolic efficiency.

Purpose of the Study:

• To investigate the effect of EIMD on physiological heat strain during exercise in a hot environment.
• To assess heat strain at two time points post-EIMD: 30 minutes (acute inflammation) and 24 hours (delayed inflammation and metabolic changes).
• To determine if EIMD increases the risk of exertional heat illness.

Main Methods:

• Thirteen non-heat-acclimated males underwent exercise heat stress tests (40 min at 65% V˙O2max in 33°C, 50% humidity).
• Tests were conducted 30 min (HS1) and 24 h (HS2) after either EIMD (running at -10% gradient) or a control condition (CON, running at +1% gradient) in a crossover design.
• Measurements included rectal and skin temperature, local sweating rate, and metabolic energy expenditure (M˙).

Main Results:

• EIMD led to significantly greater increases in rectal temperature (ΔTre) during both HS1 and HS2 compared to CON.
• Metabolic energy expenditure (M˙) was higher following EIMD during both heat stress tests.
• While thermoeffector responses were unchanged, thermal sensation and perceived exertion were elevated after EIMD during HS1.

Conclusions:

• EIMD increases heat strain during exercise in hot conditions, both acutely (30 min post-EIMD) and to a lesser extent 24 hours later.
• Elevated heat strain post-EIMD suggests a potential increased risk for exertional heat illness, especially when behavioral thermoregulation is disregarded.
• Circulating interleukin-6 (IL-6) levels post-EIMD correlated with increased final rectal temperature during heat stress.