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 (N_f) summed across each race start: N_f = 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.

Figure 1

Box-plot showing the percentage fatigue life accrued in 25,234 Thoroughbred race starts in Tasmania, Australia when scaled to pre-determined experimental joint loads.

Figure 2

Scatter density plot of race distance to the percentage of fatigue life accrued per race with Box-Tidwell power transformed fitted linear regression line from the final multivariable model (transformed race distance = race distance per 100 m^0.353). Petals of shaded areas (“sunflowers”) represent the number of observations, where the number of observations increase from blue (< 3 observations) to shaded sunflowers of yellow (3 to 503 observations) to orange (504 + observations). Overlapping lines (indicating greater number of overlapping petals) equate to the highest density regions within each sunflower.

Figure 3

Coefficient plot (regression coefficients and associated 95% confidence intervals) showing the proportional effect of track surface type and rating (synthetic vs turf tracks rated from firmest to softest) on the percentage of fatigue life accrued over Thoroughbred race starts in Tasmania, Australia, according to estimated joint loads scaled to a maximum of 90 MPa.

Figure 4

The interaction between weight carried and race class on the percentage of fatigue life accrued over Thoroughbred race starts in Tasmania, Australia in multivariable linear regression modelling, according to estimated joint loads scaled to a maximum of 90 MPa. *HCP/BM = Handicap/Benchmark races.