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 to determine the importance of intensive, high-speed exercise schedules prior to injury. Seventy-seven horses w hich sustained an FMI while racing and 45 horses which sustained an FM I while race training were successfully matched by race or timed worko ut session with one control horse and included in the analyses. Race a nd timed workout (racing-speed exercise) histories were obtained for t he case and control horses. Two-month cumulative, racing-speed cutoff distances were calculated from the control horse sample by two methods . Median racing-speed exercise frequencies and distances of the contro l horses were used to estimate age-specific (2, 3, 4 and greater than or equal to 5 years), 2-month cumulative, racing-speed distances (Meth od 1). For the second method, the last race or timed workout for each control horse occurring just prior to, or on the date of injury for th e matched case horse was identified. Cumulative racing-speed distances 2 months prior to these exercise events were determined for each cont rol horse and used to estimate median age-specific (2, 3, 4 and greate r than or equal to 5 years), 2-month cumulative racing-speed distances (Method 2). The cumulative cutoff distances estimated from both metho ds were used to classify each matched pair according to the presence o r absence of a 2-month cumulative, racing-speed distance which exceede d the age-appropriate cutoff distance (exercise distance cluster) with in 6 months prior to injury. Mantel-Haenszel matched-pair odds ratios and 95% confidence limits were calculated separately for the racing an d race-training fatal injuries. The relative risk for racing FMI was s ignificantly greater for those horses which ran 2-month, cumulative ra cing and timed workout distances in excess of the cutoff values determ ined with Method 1 (relative risk (RR) = 3.0, 95% confidence interval (CI) = 1.2, 7.6) and Method 2 (RR = 7.2, 95% CI = 2.6, 20.6). The rela tive risk for race-training FMI was significantly greater for those ho rses which ran 2-month, cumulative racing and timed workout distances in excess of the cutoff values determined with Method 2 (RR = 3.4,95% CI = 1.0, 13.2).