Rachel C. Murray, Vicki A. Walker, Carolyne A. Tranquille
Surfaces and Injury
Rachel C. Murray, Vicki A. Walker, Carolyne A. Tranquille
Horses performing in many equestrian disciplines increasingly are being trained on artificial arena surfaces, and, although information is limited, there is some evidence that riding on these different arena surfaces influences horses’ movement patterns. In addition, it appears that arena type, maintenance, and use may have implications for injury. For these reasons, it is important to understand how the horse interacts with the surface because different surface types may be better suited for different disciplines. Poor arena surfaces can increase the risk for injury and compromise the performance of the horse; however, there are many factors involved in creating a safe environment.
Interaction between the Horse and Arena Surface
The interaction between the horse and the surface determines the loads applied to the horse’s limbs and therefore its joints and soft tissues. Consequently, the risk for injury involving these structures is also influenced by this interaction. The initial interaction between the horse and the arena occurs between the hoof and the top layer of surface. It begins with two overlapping impact phases: primary and secondary. The primary phase is the period of initial contact and loading of the hoof and the surface. Here, the hoof rapidly decelerates upon contact with the surface, but the forces applied are relatively low because little of the horse’s mass is involved in the collision with the surface. The secondary impact phase occurs when the mass of the horse is progressively applied to the inclined limb, resulting in a gradual increase in forces until their peak at midstance. During both phases, the forces applied to the limb are determined by the resistance of the surface to impact (hardness) and to shear (resistance to hoof penetration), with greater impact and shear resistance resulting in a more stable surface but greater limb loading. After midstance, the limb becomes propulsive, so the horse pushes back against the surface and enters the swing phase of the stride. Consequently, at this phase, higher impact and shear resistance will support and aid the horse during propulsion.
Arena Surface Composition
The surface components are largely responsible for the functional characteristics of the arena and therefore the risk for injury. However, the presence, or absence, of a base and also its composition can have dramatic effects on how the superficial layers of the surface behave, with absence of a base being related to negative surface properties and increased risk for injury. The size of the arena is also important, with smaller arenas being associated with higher injury risk than larger arenas. This is likely because, in a small arena, horses more frequently turn and repeatedly use and compact each unit of surface.
Sand, whether used alone or in combination with other materials, is the most commonly used surface. Its properties depend on several important factors, including particle size and shape, moisture content, and bulk density. Careful selection and manipulation of these properties can result in more desirable surface characteristics. Sand that is dry, made up of coarse round particles, or has low bulk density is highly mobile and can result in an unstable surface that is likely to cause the horse to trip or lose its balance. Sand particle adherence, and therefore stability of the surface, can be improved by using fine angular sand as well as by increasing moisture content, with an optimum moisture level suggested to be 8% to 17%. The source of the sand is also important because this dictates its hardness and durability. A quartz-based sand is harder and more durable than a soft sand such as calcite.
Adding components to the sand such as rubber, fiber, or polyvinyl chloride or coating the sand in petroleum jelly or wax can limit the evaporation of water, facilitating maintenance of bulk density and improving stability. However, the size and mixture of additional components can affect the surface properties. Epidemiologic data suggest that rubber chunks are preferable to rubber strips and that uniformity of the surface can be maintained more easily if the rubber is used as a top layer rather than being mixed in with the sand. Although wax coating can effectively help to stabilize the surface, the amount of wax added should be suitable for the range of temperatures occurring at the arena surface. Because change in temperature affects the physical properties of wax, it also affects the behavior of the arena surface. For an arena location that experiences wide variations in temperature, it can be challenging to find a wax that provides consistent arena surface properties for the entire temperature range. A surface that changes dramatically or varies with weather conditions is undesirable. Surfaces that change with different weather or ambient conditions, including becoming dry or patchy in hot conditions or deep and boggy in wet conditions, are associated with increased risk for injury to the horse.
Wood shavings, wood chips, and sawdust are also used as arena surfaces and, as is true with sand, the stability of the surface is affected by the size of the components. Wood-based surfaces often have relatively lower surface hardness measures than other surface types, suggesting that they result in relatively smaller peak loading of the limbs. However, this is often linked to greater surface mobility; it has been reported that a woodchip surface is 12 times more likely to cause the horse to slip and lose its balance than any other surface type.
As a result of their physical properties, some surfaces are more suited to certain equine disciplines than are others; for example, in dressage horses, sand is associated with tripping and woodchips with slipping. However, training the horse on a variety of surfaces, including those on which it will compete, is essential for the proprioceptive development of the horse. In dressage horses, sand-based surfaces increase the risk for injury, but this tendency decreases the more often the horse works on the surface. This is likely a manifestation of adaptation to the surface with increased use. Based on this adaptation, it is important to consider time for adaptation to a new surface before introducing peak loading; when training a horse on a new surface, it should not be worked too strenuously until it becomes accustomed to the different surface characteristics.
Discipline-Specific Risk Factors for Lameness Related to Arenas and Artificial Surfaces
Training, warm-up, and competition surfaces play an important role in the musculoskeletal health of the jumping horse, which may have to perform on grass and artificial surfaces. A soft, deep surface requires more effort from the horse and is potentially associated with earlier fatigue of the muscles, tendons, and ligaments. A hard surface can be associated with greater impact damage, potentially increasing the risk for bone, joint, and laminar foot-related injuries.
Upon the horse’s landing after a jump, it is important that the hoof can slide through the surface to a small degree to avoid sudden braking forces on the limb. However, horses tend to be more confident on a stable surface on which there is adequate grip for navigating sharp turns and creating the forces required at takeoff. This may also be influenced by the presence or absence of studs in the shoes. Comparison of hoof landing and braking characteristics of elite show jumping horses performing on grass with studs and in sand arenas without studs have demonstrated that the timing of the peak hoof braking occurs later on the sand surface than on grass. Sand has a looser top layer that allows the hoof to slide easily on impact, whereas on grass, the studs resulted in early braking. Further investigation is needed to assess the penetration of the heels into the surface during limb loading because this appeared to be more pronounced on grass than on sand.
In the past, most competitions took place on grass during the summer months. At present, with competitions taking place year-round in both indoor and outdoor arenas, the frequency of competitions held on artificial surfaces is increasing. The nature of the surface of outdoor arenas depends on the weather conditions and can vary widely from one area of the arena to the other, and also from the start to the end of a given class. The surface of an indoor arena is less affected by weather conditions but may become irregular or compacted in areas of particularly high use, in addition to changing during a given competition or class.
A recent international study investigating surfaces used for training and competition of elite show jumping horses revealed that training and competing on grass was a risk factor for days lost to training. This could be related to the variability in grass and soil at different venues, under different weather conditions, or at different stages within a competition, which would make it more difficult for horses to be adapted to this variable surface. Training and competing on sand with wood sawdust was a protective factor for days lost, which could be related to the lower impact exerted on the limbs by wood surfaces.
Dressage horses predominantly train and compete on artificial surfaces. The repetitive nature of dressage training is thought to play a large role in the incidence and location of injuries commonly seen in these animals. A recent epidemiologic study highlighted several surface-related risk factors for injury, including aspects of arena construction, location, surface composition, maintenance, and use (Box 28-1).