Author(s): EBISUDA, Y., KITAOKA, Y., SUGIYAMA, F., TAKAHASHI, Y., YOSHIDA, T., MUKAI, K., Institution: JAPAN RACING ASSOCIATION EQUINE RESEARCH INSTITUTE, Country: JAPAN, Abstract-ID: 480

INTRODUCTION:
Heat acclimation, the repeated exposure to exercise in a hot environment, induces physiological adaptation with improved thermoregulatory and cardiovascular functions. In humans, daily training regimens in hot environments are recommended for improved adaptation and higher performance. While heat acclimation is also crucial for Thoroughbred horses, research on different training protocols for this species is scarce. This study compared the effects of consecutive and intermittent heat acclimation protocols in Thoroughbred horses.
METHODS:
This was a randomized crossover study, including eight trained Thoroughbred horses. The horses were assigned to either a consecutive (CONS, 9 consecutive days) or an intermittent protocol (INT, 3 days/week for 3 weeks), involving 30 min exercise in hot conditions (Wet Bulb Globe Temperature [WBGT] 29.5°C). Both training protocols comprised alternate sessions of high-intensity interval training (3 × 1 min bout at 100% VO2max with 2 min recovery) and moderate-intensity interval training (3 × 1 min bout at 70% VO2max with 2 min recovery). Before and after the heat acclimation period, incremental exercise tests (IETs) were conducted in hot conditions (WBGT 29.5°C), with collection of arterial and venous blood samples to measure blood gases and plasma lactate concentration. Pulmonary artery temperature and heart rate were also recorded. Prior to IETs, muscle biopsy samples were obtained from the middle gluteal muscle for enzyme activity analysis. Data were analyzed using mixed models, and pairwise comparisons were performed using Tukey’s test (P < 0.05).
RESULTS:
Both protocols decreased pulmonary artery temperature after heat acclimation, both at rest (CONS, −0.2°C; INT, −0.3°C) and during submaximal exercise (CONS, −0.3°C; INT, −0.3°C). For IETs, time to exhaustion increased in INT (+11.6%, 8 of 8 horses improved) but not in CONS (+5.7%, 5 of 8 horses improved). For both protocols VO2max remained unchanged (CONS, +7.5%; INT, +3.4%) whereas speed eliciting VO2max increased (CONS, +6.5%; INT, +10.4%). For CONS only, maximal stroke volume increased (CONS, +11.8%; INT, +3.9%) while heart rate during submaximal exercise decreased (CONS, −5.5%; INT, −3.7%). Moreover, weight loss was observed after heat acclimation for CONS only (CONS, −1.8%; INT, −0.6%). Regarding enzyme activity, citrate synthase remained unchanged in both protocols (CONS, +2.0%; INT, −3.6%) whereas 3-hydroxy acyl-CoA dehydrogenase decreased in INT only (CONS, −30.2%; INT, −42.8%).
CONCLUSION:
Both protocols elicited physiological adaptation to thermoregulation and aerobic energy metabolism. However, improved performance occurred only in INT, while cardiovascular improvement and weight loss occurred only in CONS. These findings suggest that consecutive heat acclimation training can induce robust physiological adaptations in a short period. However, care should be taken to prevent overreaching in Thoroughbred horses.