The effects of varying cushion depth on dynamic loading in shallow sand thoroughbred horse dirt racetracks

Surface consistency is an important factor for the safety of thoroughbred racing surfaces. Factors that influence the consistency in dirt tracks include homogeneity of surface material composition, moisture content, and cushion depth. The influence of cushion depth on the dynamic load and accelerations experienced by the horse at a range of moisture levels typical to operating conditions (14%, 16%, and 18% gravimetric for the material tested in this work), and surface maintenance conditions (sealed and harrowed), is addressed. A biomechanical surface tester, designed to simulate the forelimb impact of a galloping thoroughbred horse, was repeatedly dropped on five different surface conditions, each at two cushion depths (100 mm and 150 mm). The difference of 50 mm, a depth range often found within a single track had a statistically significant effect on the peak load and the secondary phase loading rate experienced by a horse, particularly under the outlying moisture content conditions (relatively dry or moisture saturated). The tested material behaved more similarly at the two cushion depths under moisture conditions at which maximum dry density occurred (16%). Peak loads and loading rates were significantly different between the two depths for harrowed, 14% moisture conditions, and sealed, 18% moisture conditions. These cushion depths and surface material moisture levels are within normal operating conditions for thoroughbred race meetings on shallow sand tracks and therefore may influence the development of musculoskeletal disease and the safety of horses and jockeys.