Physiological responses to incremental exercise in the heat following internal and external precooling

Twelve males completed three incremental, discontinuous treadmill tests in the heat [31.9(1.0)°C, 61.9(8.9)%] to determine speed at two fixed blood lactate concentrations (2 and 3.5mmol/L), running economy (RE), and maximum oxygen uptake ( V ˙ O 2 m a x ). Trials involved 20min of either internal cooling (ICE, 7.5g/kg ice slurry ingestion) or mixed-methods external cooling (EXT, cold towels, forearm immersion, ice vest, and cooling shorts), alongside no intervention (CON). Following precooling, participants ran 0.3km/h faster at 2mmol/L and 0.2km/h faster at 3.5mmol/L (P=0.04, partial η²=0.27). Statistical differences were observed vs CON for ICE (P=0.03, d=0.15), but not EXT (P=0.12, d=0.15). There was no effect of cooling on RE (P=0.81, partial η²=0.02), nor on V ˙ O 2 m a x (P=0.69, partial η²=0.04). An effect for cooling on physiological strain index was observed (P<0.01, partial η²=0.41), with differences vs CON for EXT (P=0.02, d=0.36), but not ICE (P=0.06, d=0.36). Precooling reduced thermal sensation (P<0.01, partial η²=0.66) in both cooling groups (P<0.01). Results indicate ICE and EXT provide similar physiological responses for exercise up to 30min duration in the heat. Differing thermoregulatory responses are suggestive of specific event characteristics determining the choice of cooling. Precooling appears to reduce blood lactate accumulation and reduce thermoregulatory and perceptual strain during incremental exercise.