Effects of pre-exercise cooling in hot environments on performance and physiological responses in Thoroughbred horses

Pre-exercise cooling may prevent exertional heat illness in horses. We hypothesized that pre-exercise cooling before warm-up in a hot environment would not affect performance but would mitigate reductions in body weight and increases in body temperature following exercise. Six trained Thoroughbred horses were studied using a randomized, crossover design with three pre-treatments: 30-min walk on a treadmill at 1.7 m/sec (WALK), 30 min of standing (REST), and a 10-min pre-cooling shower at 26.2 ± 0.8°C (SHOWER). All horses underwent each pre-treatment, followed by a warm-up and main exercise in a hot environment (wet-bulb globe temperature: 32-33°C). After warming up by cantering at 10.0 m/sec for 30 sec, horses exercised on a treadmill with a 6% incline and a speed eliciting exhaustion within 2 min, which was approximately 115% V̇O₂max (relative intensity; 13.5-14.3 m/sec). Run time to exhaustion was recorded, with body weight measured before pre-treatment and after main exercise to calculate weight loss. Heart rate was measured from before pre-treatment to after the main exercise. Plasma lactate concentration (Lac) and pulmonary arterial temperature (a measure of body temperature) were assessed before and after pre-treatment and after the warm-up and main exercise. Weight loss in SHOWER was significantly reduced compared with the other treatments. Pulmonary artery temperatures in WALK after pre-treatment were significantly higher than in the other treatments. No significant differences were observed in heart rate, Lac, or run time. These results suggest that pre-cooling mitigates reductions in body weight and increases in body temperature without affecting performance.