Author(s): MUKAI, K., TAKAHASHI, Y., EBISUDA, Y., SUGIYAMA, F., YOSHIDA, T., MIYATA, H., Institution: EQUINE RESEARCH INSTITUTE, JAPAN RACING ASSOCIATION, Country: JAPAN, Abstract-ID: 622

INTRODUCTION:
High-intensity interval training (HIIT) improves the exercise performance and aerobic and glycolytic metabolism of humans, but there have been few studies of HIIT in horses. The purpose of this study was to test the hypothesis that 6 weeks of HIIT would elicit greater skeletal muscle adaptation than moderate-intensity continuous training (MICT) in Thoroughbred horses.
METHODS:
We performed a randomized crossover trial using seven untrained Thoroughbred horses comprising 6-week (3 sessions/week) distance-matched training protocols on a treadmill, consisting of MICT (6 min at 70% VO2max) or HIIT (6 × 30 s at 100% VO2max, with 30 s recovery at 30% VO2max) with a 3-month washout period. Incremental exercise tests (IETs) were conducted before and after training, and biopsy samples were collected from the middle gluteal muscle before each IET. The muscle cross-sectional area, muscle fiber composition, capillary density, glycogen content, and succinate dehydrogenase (SDH) and phosphofructokinase (PFK) activities were measured. The effects of time and protocol were analyzed using mixed models and Tukey’s multiple comparisons test, and the relationships between the change in run distance and variables related to skeletal muscle were analyzed using Pearson’s correlation coefficient. Significant level was set at P < 0.05.
RESULTS:
After 6 weeks of training, the run distance to exhaustion at IET increased to a greater extent in HIIT (+29%) than in MICT (+5%). The cross-sectional areas of type IIA and IIX fibers increased significantly more following HIIT (+26% and +44%, respectively) than following MICT, whereas that of type I fibers increased following MICT (+59%) without a significant inter-protocol difference. There were no significant changes in muscle fiber composition or capillary density in either protocol, but the muscle glycogen content increased in both protocols (MICT, +32%; HIIT, +24%). The SDH activity increased only in HIIT (+61%), and the PFK activity did not change in either protocol. The change in run distance positively correlated with the change in the cross-sectional area of type IIX fibers following HIIT (r = 0.88), but there were no significant correlations associated with MICT.
CONCLUSION:
Despite the same training distance, 6 weeks of HIIT improved exercise performance and muscle hypertrophy, particularly in fast-twitch muscle, compared with MICT. These results indicate that high training intensity is essential for skeletal muscle adaptation and HIIT represents a promising training strategy for Thoroughbred horses.