Horse owners and event managers put a lot of contemplation and money into designing the best arena for equine athletic pursuits. Data shows that 48% of elite show jumping horses suffer injuries over a seven-month season. Musculoskeletal injuries account for 77% of days lost to competition, with tendon and ligament injuries accounting for 20% of days lost. Horse injury also has the potential to cause human injury, particularly in equestrian jumping sports.
Due to the high impact of injury and economic loss from musculoskeletal injuries, research has examined forces on the limbs with the objective of developing mitigation strategies. Ground reaction forces (GRF)—the force exerted by the ground on a body in contact with it—set up conditions for injury of the musculoskeletal system. Shear forces describe what happens when a force is applied parallel and along a surface in order to offset a force acting in the opposite direction.
Christina Rohlf, BS, of the University of California, Davis, discussed the influences of GRF and shear forces on equine athletes and how different surfaces might be modified to ameliorate risks at the 2021 AAEP Convention. She explained how landing after a jump incurs several changes in the lower limbs:
- An increase in hoof-to-ground reaction forces amplifies forces on tendons to stabilize the limb.
- An increase in fetlock extension and angle increases tendon length, resulting in strain and risk of tendon injury.
- Low shear forces allow excessive slippage of the hoof, leading to greater fetlock hyperextension with risk of tendon injury.
- Early braking increases stress on the suspensory apparatus.
Shear forces have an effect on performance: A high shear force increases stride length and lessens hoof slide.
Dirt and synthetic arena surfaces vary in their ground reaction forces. Vertical forces are most affected on dirt while synthetic surfaces have higher shear forces.
It is possible to optimize shear mechanical behavior by adjusting cushion depth or moisture content of arena surface material. A deep cushion lessens the vertical ground reaction, and this is adjustable by altering rake height. Moderate moisture content decreases ground reaction forces, which is adjustable by modifying watering frequency.
A study evaluated 12 arena surfaces—five with dirt and seven with synthetic materials, all maintained similarly for cushion and moisture. No wax, rubber or other additives were in the surfaces. Sand, silt and clay were not significantly different between surfaces. Although there was an expectation that surface material could affect shear surface properties, it turns out that this study identified no significant differences of shear ground reaction forces between surfaces.
However, Rohlf noted that the surface type does not describe the surface below dirt or synthetic material. Therefore, adjustment of fiber content can influence shear properties of the footing. A higher fiber content decreases adhesion (stickiness of soil particles to hoof) and increases the coefficient of friction (resistance to movement as hoof slides through the surface). This information can be used to optimize shear behavior of arena surfaces.