The impact of cumulative bone fatigue on musculoskeletal injury risk in racing Thoroughbreds.

Musculoskeletal injuries (MSI) in racehorses are not typically spontaneous events, but develop gradually through cumulative cycles of loading over time. Stride characteristics can now be recorded with wearable technology, providing opportunity to quantify the skeletal impact of galloping over time. We estimated bone fatigue accumulation based on speed (m/s) and number of strides from n = 3168 race starts by n = 405 Thoroughbreds commencing racing in Tasmania, Australia between 2011 and 2016. The percentage bone fatigue accumulated was divided by various time periods (career, active career, preparation, 1-5 start windows) to generate rates (workloads). Cox proportional-hazard models were used to determine effects of workloads on time to MSI (n = 71 first MSI events), presented as Hazard Ratios (HR); 95 % Confidence Intervals. Horses with higher rates of bone fatigue accumulation over a one start period (HR 3.37; 1.01, 11.22; p = 0.048) and over their career (HR 1.80; 1.53, 2.11; p < 0.001) had a greater risk of MSI. For workloads over an intermediate time period (previous four starts), low (<0.25) workloads resulted in restricted mean survival time (RMST) to MSI of 760 days and a 2.89-fold greater risk (p = 0.005), and high workloads (≥0.75) a 405-day RMST and 8.51-fold greater risk compared to medium (0.25-0.75) workloads with a 1027-day RMST. These findings suggest monitoring skeletal fatigue with wearable technology may allow early identification of horses at increased risk of lameness and injury, timely modification of workloads, and inform safer training practices.