Effects of short-term training on thermoregulatory and sweat responses during exercise in hot conditions

There is evidence that extensive training in cool conditions results in improvements to heat dissipation that contribute to successful acclimatisation. In horses, the effects of a less extensive training regimen have not been determined.

This study investigated whether 10 consecutive days of moderate intensity treadmill training in cool conditions would improve thermoregulatory and sweating responses of horses to exercise in the heat.

Six unfit Thoroughbred horses completed a standardised treadmill exercise test (SET) in hot, dry conditions (32-34 °C, 45-55% RH) before (SET1) and after (SET2) 10 consecutive days of running at 55% VO2max for 60 min in cool conditions (19-21°C, 45-55% RH). Each SET consisted of a 5 min warm-up and cool down at a walk, 40 min of trotting (50% VO2max), 7 min at 75% VO2max and a 30 min standing recovery. Bodyweight was determined pre- and post SET. Heart rate, rectal, skin, pulmonary artery and muscle temperatures were measured throughout the SETs and sweating rate (SR) and sweat ion losses determined for each 5 min interval.

Following training, mean VO2max increased by 8.9% (P < 0.05). In SET2, PCV was lower during the last 30 min of exercise and end-exercise rectal, muscle and pulmonary artery temperatures were decreased by 1.5 ± 0.2, 0.8 ± 0.1 and 1.0 ± 0.2 °C, respectively (P < 0.05). Peak SR and the pattern of sweat ion losses during exercise was unchanged post training whereas SR and sweat losses during recovery were decreased (P < 0.05).

Similar SRs for a given core temperature during exercise but a more rapid decrease in recovery resulted in an overall reduction in sweat fluid losses with no change in sweat ion losses after training.

The results provide insight into the extent to which short-term training can improve the capacity of horses to exercise in hot conditions.