The Impact of Track Incline on Musculoskeletal Injuries in Thoroughbred Racehorses in Training: A Retrospective Cohort Study of Fractures on Flat and Uphill Woodchip Surfaces.

BACKGROUND: While race-day musculoskeletal injuries are well-documented, a considerable gap exists regarding fractures sustained during training, where Thoroughbred racehorses accumulate the majority of their high-speed exercise. In Japan, compliant woodchip tracks are frequently used, and uphill track geometries are a foundational element of conditioning. OBJECTIVE: To quantify and describe the incidence and anatomical distribution of fractures occurring during training on flat versus uphill woodchip tracks over a 15-year period (2008-2022). METHODS: A retrospective cohort study was conducted using centralized records from the Japan Racing Association's Miho and Ritto Training Centers. Daily horse exposure was quantified utilizing an automated lap time measurement system and standardized visual counts. Fracture incidence was evaluated using a quasi-Poisson regression model and Relative Risk analyses. RESULTS: The quasi-Poisson regression model identified a lower incidence of fractures on the uphill track compared to the flat track (Incidence Rate Ratio = 0.359; 95% CI: 0.329-0.392; P < 0.001). When stratified by injury type, forelimb injuries (such as carpal chip and third metacarpal fractures) and hindquarter injuries (such as third metatarsal and pelvic fractures) demonstrated a consistently lower incidence rate on the uphill track geometries across both training facilities. CONCLUSIONS: Uphill woodchip tracks are associated with a lower incidence of both forelimb and hindquarter training fractures compared to flat woodchip tracks. These distinct anatomical shifts in fracture occurrence highlight the biomechanical influence of track incline on cumulative microstructural damage.