Thermal and cardiorespiratory responses of horses to submaximal exercise under hot and humid conditions.

The research investigates how exercise under different environmental conditions (cool, dry; hot, dry; hot, humid) affects the thermal and cardiorespiratory responses in horses. It concludes that high temperature and humidity increase the rate of heat storage during exercise and slow down heat loss during recovery.

Research Methodology

• The researchers engaged five thoroughbred horses (aged 3-6) in exercise tests under three different environmental conditions.
• These conditions varied in temperature and relative humidity (RH). The first setting was cool and dry with a room temperature of 20 degrees Celsius and a RH of 45-55%. The second setting was hot and dry with a temperature of 32-34 degrees Celsius and the same RH as the cool setting. The last setting was hot and humid with the same temperature as the hot setting but at 80-85% RH.
• Each horse was tasked with a workload equal to 50% VO2max (maximum oxygen consumption during exercise) on a treadmill.
• The exercise would continue until the pulmonary artery blood temperature reached 41.5 degrees Celsius. This was followed by a 30-minute walking recovery period and a 30-minute standing recovery, both undertaken in the same ambient conditions.
• Various temperatures (pulmonary artery blood, rectal, skin, and muscle) and heart rates were recorded before, during, and after the exercise. Respiratory rate was also measured before the exercise and throughout the 60-minute recovery.

Key Findings

• Exercise duration for both hot conditions was significantly less than in the cool environment.
• The rate of increase in pulmonary artery blood temperature was significantly higher in the hot, humid setting than the other two.
• The muscle temperature after 15 minutes of exercise was significantly higher in the hot, humid condition than in the other environments; whereas the rectal temperature at the end of exercise was significantly lower in the hot, humid condition.
• The difference between pulmonary artery blood and skin temperatures was significantly smaller in the two hot conditions than in the cold.
• Temperatures at all sites were higher in the two hot conditions during the 60-minute recovery period when compared with the cool condition.
• Throughout the exercise and recovery, the heart rate was significantly higher in the hot, humid condition compared with the others.
• Post exercise respiratory rate was significantly higher in the two hot conditions than in the cool setting throughout recovery.

Conclusion

The researchers conclude that high temperature and relative humidity contribute to an increased rate of heat storage during exercise and a delay in heat dissipation during recovery. This suggests a reduced ability to transfer heat from the horse’s skin to the surrounding environment. The implications of this research are particularly relevant for training and managing horses in different climate conditions, as well as for understanding the effects of global warming on the endurance and health of equine athletes.