Musculoskeletal injuries are a common cause of lost training days and fatalities in racehorses. Most bone injuries arise from the gradual build-up of microcracks through training and racing. Training programs that progressively expose horses to higher speeds can promote bone adaptation. However, overtraining may increase the risk of bone injury by causing excessive damage.
To better understand the dynamic biological processes underlying subchondral bone (SCB) injury, we developed a mathematical model that incorporates SCB adaptation, microdamage accumulation, and repair. The model is calibrated to experimental observations of bone volume fraction and time to fracture in racehorses.
We employed a sensitivity analysis using the partial rank correlation coefficient (PRCC) to assess how various biological parameters affect SCB adaptation and damage. Joint load was identified as a key factor driving bone formation (PRCC = 0.8336). For bone damage, joint load (PRCC = 0.9920) and cycles (strides) per day (PRCC = 0.6725) were key contributors.
We simulated the response of SCB to a typical training program for Thoroughbred racehorses in Victoria, Australia. After simulating over four preparations (776 days), the bone volume fraction increases from 0.86 to 0.93 during training over the final preparation. However, training also causes bone damage to increase from 0.4555 to 0.8932, where a value of 1 indicates bone failure. Notably, 77.2% of the damage incurred results from gallops at speeds above 13.8 m/s. Conversely, rest causes a net resorption of bone, resulting in the repair of damage from 0.8876 to 0.5495.
Our findings indicate that: (i) the speed of training is a key contributor to bone damage in racehorses; and (ii) rest from training is essential for repairing damage accumulated. Therefore, to reduce the risk of bone injury, we suggest limiting the volumes of high-speed training and increasing both the frequency and duration of rests from training.