Cumulative racing-speed exercise distance cluster as a risk factor for fatal musculoskeletal injury in Thoroughbred racehorses in California

Thoroughbred racehorses which suffered a fatal musculoskeletal injury (FMI) while racing or race training at a California racetrack during 9 months of 1991 were studied. 77 horses which sustained an FMI while racing and 45 horses which sustained an FMI while race training were matched by race or timed workout session with a control horse. Race and timed workout (racing-speed exercise) histories were obtained for all horses. Two-month cumulative, racing-speed cutoff distances were calculated from the control horse sample by 2 methods. For the first method, median racing-speed exercise frequencies and distances were used to estimate age-specific (2, 3, 4 and ≥5 years), 2-month cumulative, racing-speed distances. For the second method, the last race or timed workout for each control horse occurring just before, or on the date of injury for the matched case horse was identified. Cumulative racing-speed distances 2 months before these exercise events were determined and used to estimate median age-specific, 2-month cumulative racing-speed distances. Matched pairs were classified according to the presence or absence of a 2-month cumulative, racing-speed distance which exceeded the age-appropriate cutoff distance (exercise distance cluster) within 6 months before injury. Mantel-Haenszel matched-pair odds ratios and 95% confidence limits were calculated. The relative risk for racing FMI was significantly greater for those horses which ran 2-month, cumulative racing and timed workout distances in excess of the cutoff values determined with methods 1 and 2. The relative risk for race-training FMI was significantly greater for those horses which ran 2-month, cumulative racing and timed workout distances in excess of the cutoff values determined with Method 2.