Epidemiological studies of injury among equine athletes have found that show jumping horses have a high prevalence of injury to tendons and ligaments in the lower limb, especially the Suspensory Ligament (SL) and Superficial Digital Flexor Tendon (SDFT). These injuries are the result of repetitive, high tensile loads on the tendons and ligaments resulting in high cyclic tendon and ligament strains. Due to limb anatomy, the magnitude of tendon and ligament strains are highly correlated to joint angles (particularly fetlock angle). However, joint angles are affected by many intrinsic and extrinsic factors that are not well understood. One key extrinsic factor impacting joint angular motion and soft tissue loads and strains is the force transferred to the limb when the hoof interacts with the ground. Studies of racing surfaces have shown that ground reaction forces are greater on dirt than synthetic surfaces. Similarly, more injuries have been observed on dirt surfaces than synthetic surfaces during racing. However, researchers have not studied the effect of surface type (dirt/synthetic) on ground reaction forces of arena surfaces or limb kinematics during jumping, which are predicted to be deterministic factors for soft tissue loading and injury risk of jumping horses. Therefore, the goal of this research program was to evaluate the influence of surface type on arena ground reaction forces and limb kinematics during jumping, and ultimately to quantify soft tissue strains and forces during jumping to directly evaluate soft tissue injury risk.
Motion of the carpal, fetlock, pastern, and coffin joints were captured using kinematic markers and high-speed video for four horses jumping over a 1.1 meter oxer at 12 different arenas (5 dirt, 7 synthetic). Vertical impact properties (vertical impact force, vertical displacement, and vertical deceleration) and shear properties (shear force, adhesion, and coefficient of friction) were also measured synchronously with kinematic motion on each arena surface by using arena surface testing equipment to quantify ground reaction forces. Collected live animal kinematic data were then used as an input to a computer model of the equine forelimb to calculate muscle, tendon, and ligament strains and forces observed during a jump.
Shear properties were not different between dirt and synthetic surface categories. Additionally, vertical impact force and surface stiffness were not statistically different between surface type categories. Rather, vertical displacement (P=0.021) and soil rebound (P=0.005) were the only vertical impact variables affected by surface type. Furthermore, fetlock extension, hoof deceleration, and forces in flexor tendons and ligaments, which all are predicted to increase the risk of injury, were not correlated with surface type. Although arena owners have decided that synthetic surfaces reduce the risk of equine injury compared to dirt based on anecdotal evidence, and some studies have compared 1-2 surfaces of each type and found that surface properties which are predicted to increase injury (such as vertical impact force or fetlock extension) are magnified on dirt surfaces, the findings from this research program suggest that previous studies have underestimated the variation in surface behavior. When this variation is captured by increasing the sample size of tested surfaces, it appears that surface type categories do not sufficiently describe the underlying mechanical properties of arena surfaces, increase fetlock hyperextension during jumping, or increase the strains and forces of flexor tendon and ligament structures during jumping which are predicted to be related to injury. Rather, vertical impact forces and surface stiffness were magnified on surfaces with more compaction (P<0.001). Moreover, jump phase (takeoff/landing) had a significant effect on joint hyperextension (P<0.001) and forces in flexor soft tissue structures (P<0.005), where greater extension and force were observed at landing.
