Relationship between Thoroughbred workloads in racing and the fatigue life of equine subchondral bone

Abstract

Fatigue life (FL) is the number of cycles of load sustained by a material before failure, and is dependent on the load magnitude. For athletes, ‘cycles’ translates to number of strides, with load proportional to speed. To improve previous investigations estimating workload from distance, we used speed (m/s, x) per stride collected using 5 Hz GPS/800 Hz accelerometer sensors as a proxy for limb load to investigate factors associated with FL in a Thoroughbred race start model over 25,234 race starts, using a combination of mathematical and regression modelling. Fore-limb vertical force (NKg-1) was estimated using a published equation: Vertical force = 2.778 + 2.1376x − 0.0535x2. Joint load (σ) was estimated based on the vertical force, scaled according to the maximum speed and defined experimental loads for the expected variation in load distribution across a joint surface (54-90 MPa). Percentage FL (%FL) was estimated using a published equation for cycles to failure (Nf) summed across each race start: Nf = 10(σ-134.2)/−14.1. Multivariable mixed-effects linear regression models were generated on %FL, adjusting for horse-level clustering, presented as coefficients; 95%CI. Scaled to the highest joint load, individual starts accrued a mean of 9.34%FL (sd. 1.64). Older age (coef. 0.03; 0.002–0.04), longer race-distances (non-linear power transformed), and firmer track surfaces (ref. Heavy 10: Good 3 coef. 2.37; 2.26–2.48) were associated with greater %FL, and males accrued less than females (p < 0.01). Most variables associated with %FL are reported risk factors for injury. Monitoring strides in racehorses may therefore allow identification of horses at risk, enabling early detection of injury.

Associations between Racing Thoroughbred Movement Asymmetries and Racing and Training Direction

“…Although the values determined to be significantly different for the displacement indices measured were small between the clockwise cohort from Hong Kong and the Singapore cohort that trained and raced in a predominantly anticlockwise direction, given the large number of stride cycles that Thoroughbred horses undertake in training, it is feasible that the accumulation of the effect that these small differences have on the total force on the limb over time may still be important to consider [12,[16][17][18]. For values that showed a significant difference, the Hong Kong horses had negative mean values, suggesting a propensity for left asymmetry, i.e., an asymmetry indicating a reduced downward or up-ward movement during the stance phase of the (left) limb that is on the outside of the circle during clockwise training and racing.…”

Anomalous Incidence of Fatal Musculoskeletal Injury in North American 2-Year-Old Thoroughbred Racehorses in the Year 2020

“…It could be the case, for example, that horses experienced lower-than-expected workloads early in the year, followed by some degree of ‘catch-up’ later, closer to beginning racing. If so, then there is the potential that horses did not have the opportunity to accumulate sufficient work-load cycles to generate appropriate bone adaptation early in the season [ 29 , 30 ].…”

“…That the anomalous incidence of fatal musculoskeletal injury only affected 2-year-old horses suggests that a suitable training schedule at the beginning of their career is an important aspect of racehorse safety. It can be reasonably speculated that bone adaptation driven by training workload cycles is an important factor here [ 29 , 30 ]. Further investigation is required to understand if this finding could, say, point towards an optimal training pattern to minimise risk for both new and experienced racehorses.…”

“…An important factor that has been identified relating to appropriate training schedules is bone adaptation. Careful management of racehorse workloads is required to avoid bone microdamage accumulation [ 27 , 28 , 29 , 30 ].…”

High‐level competition exercise and related fatigue are associated with stride and jumping characteristics in eventing horses

“…9,[12][13][14][15] However, these parameters are indicators of the cardiovascular and energy balance of the horse. Other criteria like bone strain, an important cause of musculoskeletal injuries, potentially induced by the number of strides and load intensity per stride, 16 are not considered in this context. Novel accelerometric technology allows accurate assessment of the biomechanical factors of the horse in the field.…”