Comparison of internal and external precooling on the determinants of endurance running performance in hot and humid conditions: Training and Competing in the Heat Conference, Aspetar, Doha

Aim: Precooling techniques are acute interventions that ameliorate exertional heat stress and mediate a decline in endurance running performance in the heat. This study compared two prominent approaches to precooling on the determinants of endurance performance under heat stress. Methods: Twelve males (age 38.6±11.1 years, body mass 77.3±7.1 kg) completed three incremental, discontinuous treadmill tests in the heat (31.9±1°C, 61.9±8.9% relative humidity) to determine speed at lactate threshold at two fixed blood lactate concentrations (2 & 3.5 mmol. L-1), running economy and VO2max. Trials involved 20 minutes of either internal cooling (INT, 7.5 g.kg-1 ice slurry ingestion) or mixed-methods external cooling (EXT, cold towels, forearm immersion, ice vest and cooling shorts), alongside no intervention (CON). Results: An effect on running speed was observed at fixed lactate concentrations (F=3.78, p=0.04, partial η2 = 0.27). Mean values at 2 mmol.L-1 were INT 12.3±1.1 km.h-1, EXT 12.3±1.1 km.h-1, CON 12±1.1 km.h-1 and at 3.5 mmol.L-1 INT 13.8±1 km.h-1, EXT 13.8±1 km.h-1, CON 13.6±1 km.h-1. Bonferroni pairwise comparisons identified a difference between INT and CON (p=0.03), but not for EXT (p=0.12). There was no effect of cooling on running economy across six exercise stages (INT 269.5±36.5 ml.km-1.min-1, EXT 269.7±37.9 ml.km-1.min-1, CON 266.5±35.2 ml.km-1.min-1, p=0.82), nor on VO2max (INT 57.5±5.6 ml.kg-1.min-1, EXT 58.4±4.7 ml.kg-1.min-1, CON 57.3±4.9 ml.kg-1.min-1, p=0.69). An effect for cooling on physiological strain index was observed (F=4.85, p=0.02, partial η2 = 0.38), with differences between EXT and CON (CON 4.8±1.6 vs. EXT 4.3±1.5, p=0.02), but not for INT (4.2±1.8, p=0.15). Precooling reduced thermal sensation (F=20.98, p<0.01, partial η2 = 0.66) with differences in both cooling methods (INT 5.7±0.9, EXT 5.4±0.8, CON 6.2±0.8, p<0.01). Conclusions: Beneficial effects of precooling on endurance performance have been widely evidenced during free-paced and open-ended trials. Results from this study may indicate that modest changes in lactate threshold or physiological strain, alongside a diminished perceptual strain, could help explain the better maintenance of endurance running performance in the heat from precooling.