Effect of heat acclimation training frequency on the physiological adaptations of Thoroughbred horses

Heat acclimation enhances thermoregulation and cardiovascular function. While daily training protocols are typically recommended for humans, optimal training protocols for Thoroughbred horses remain unclear. Here, we compared the effects of two heat acclimation protocols, consecutive and intermittent, in Thoroughbred horses. In a randomized crossover study, eight trained Thoroughbred horses completed either a consecutive (CONS: 9 consecutive days) or an intermittent (INT: 3 days/week for 3 weeks) heat acclimation protocol, comprising 30 min of exercise in hot conditions (WBGT 30°C). Incremental exercise tests were performed before and after heat acclimation. Pulmonary artery temperature (T_PA), cardiovascular function, blood gases, plasma lactate, and HSP expression in skeletal muscle were assessed. Both protocols decreased resting T_PA (CONS, -0.24°C; INT, -0.25°C), increased the velocity at which T_PA reached 40°C (CONS, +14.4%; INT, +4.2%) and improved time to exhaustion (CONS +5.7%; INT +11.6%). Increased markers of aerobic performance were also observed, including V̇_O₂max (CONS, +7.5%; INT, +3.4%), the speed eliciting V̇_O₂max (CONS, +6.5%; INT, +10.4%) and HRmax (CONS, +10.6%; INT, +7.6%). The CONS group showed higher improvement of maximal stroke volume (CONS, +11.8%; INT, +3.9%) and body weight reduction (CONS, -1.8%; INT, -0.6%), whereas the INT group showed higher upregulation of HSP70 expression (CONS, +9.9%; INT, +21.5%). Both protocols elicited beneficial physiological adaptation, with differences in cardiovascular responses, body weight reduction, and HSP70 expression. These findings shed light on equine heat acclimation and may help to optimize training strategies for racehorses competing under heat stress.