How do surfaces affect movement, performance and joint health? For guest editor Nick Skelton, Dr Elizabeth Barr discusses the latest thinking on the impact of conditions underfoot
Nick says: “Our showjumping horses train and compete on a variety of artificial and grass surfaces. While we can control what they work on at home, we have to take what we find at shows — where the quality of the footing can mean the difference between a win and a disappointing round.
“As riders we get a certain feel for ‘good’ going, but I often wonder about the short- and long-term implications of performing on surfaces that are less than ideal. A lot of riders are confused about what’s best for jumping. I’d love to know if this is an issue for other disciplines, too.”
The footing a horse works on will affect not only how he performs on a given day, but also his chances of staying sound throughout his competitive career. Much has been discussed regarding different surfaces and the implications for injury.
But most veterinary research has been targeted towards the effect surfaces have on injury in thoroughbred racehorses. Transferring this research to sport horses is unlikely to be accurate, given the varying demands placed upon the joints of horses competing in the different disciplines.
Additionally, it’s possible to argue that, even within the sport horse world, different surface characteristics are desirable for different disciplines.
Measuring the impact
So what do we know about how surfaces affect sport horses? To evaluate the effect of a surface, we need a way of measuring gait. Various methods have been used for this, with the two main branches being kinematics and kinetics.
Kinematics studies motion via a system of movement sensors and cameras. Kinetics, on the other hand, measures the forces applied by the hoof on the surface as the horse moves (known as ground reaction forces, or GRFs).
Different kinetic methods are available, the most commonly used being force plates or pressure-measuring pads. These are generally set into the ground and therefore not practical for use in testing of surfaces. Special horseshoes have been developed to measure the loads on a foot on different surfaces.
This technology could be used to evaluate training surfaces, but has limitations in terms of expense and availability of equipment. Also, the complexity and weight of some of these systems may mean that a horse wearing such a shoe will not move normally, thus affecting results.
The ideal surface
It doesn’t take a scientist to work out that a hard surface will “jar” the horse on landing and that a horse will find it hard work to jump out of a very deep surface.
But the reasons behind this are a little more technical. The force patterns that are applied to the foot and limb during every stride are complex. A hard surface will generate a large concussive force on impact as the hoof decelerates rapidly.
On a soft surface, the deceleration occurs over a longer time period, which reduces the peak impact and concussive force. But too soft a cushion — that is, a deep surface — will continue to collapse as the horse tries to push off, either at the start of the stride or, in the case of a jumping horse, on take-off. The result is a loss of forward momentum and speed, with an increase in energy expenditure and more rapid fatigue. This may cause injury.
How quickly the foot is brought to a halt will also depend on the characteristics of the surface. So the ideal surface should provide cushioning and some slide on initial impact.
Once loaded by the weight of the horse, however, this surface should also provide adequate carrying capacity and resistance in order to support the hoof during propulsion. Where a surface absorbs too much energy and does not return energy to the limb in rebound, it may be described as being “dead” by the rider.
There is some data for showjumpers regarding the effect of surface on kinetics of landing.
One paper from a group at the Swedish University of Agricultural Sciences attempted to show differences between turf and a sand arena.
Results were complicated by the fact that effects could not be shown to be associated with the surface only. Other factors had to be taken into account, such as differences in fore, hind, trailing and leading limbs, an effect of fence type and the interaction between studs and the turf.
In comparison to the sparse information regarding arena surfaces and showjumpers, researchers at the Animal Health Trust have performed various studies into arena surface type and lameness in dressage horses.
Perhaps unsurprisingly, it was found that lameness was increased by training on surfaces that were patchy and uneven. Lameness was reduced where surfaces remained uniform. The original study also showed that tripping and losing balance were associated with increased risk of lameness.
The same authors undertook a subsequent study to look at the association between different surfaces and these detrimental surface properties. It was found that wax-coated and sand and rubber surfaces were associated with less tripping and slipping compared with sand, sand and PVC, woodchips or grass.
Of the surfaces, woodchips were most strongly associated with slipping and sand surfaces were most strongly associated with tripping. The study also showed that any arena surface should have a base, with limestone being recommended and crushed concrete being best avoided.
Good arena maintenance was also shown to be essential, especially with the increasing numbers of horses using an arena in any one day. The importance of surface maintenance has been further underlined by studies in thoroughbred racehorses.
Additionally, it has been shown that rough, deformable surfaces result in variable forces and variable load in the hoof, which predisposes to injury. In a recent study performed at the University of Central Lancashire, harrowing and rolling were compared on one particular surface. It was found that small alterations in the surface cushion appeared to be able to produce subtle changes in stride characteristics.
With greater knowledge, enforced time out of competition and potentially career-ending injuries may be reduced and avoided.
Which injury — and why?
Particular causes of lameness have been shown to be more common in particular disciplines.
Eventers, showjumpers and National Hunt racehorses, for example, are at high risk of superficial digital flexor tendon injury. Dressage horses are prone to hindlimb suspensory ligament injury, while Flat racehorses tend to incur carpal (knee) injuries.
Although we know that equine orthopaedic injuries are dependent upon a number of factors, currently we do not know which injuries are associated with a particular surface — or if, indeed, this association does actually exist. It is likely that injury predilection sites occur as a result of the demands of the sport itself.
Whether the different surfaces used in the disciplines have an effect, however, is worthy of further research.
How limbs absorb impact
1. When the hoof impacts the ground, it hits vertically and decelerates rapidly. This shock of impact is dampened predominantly by the soft tissues of the hoof and is assisted by the ground surface, if the surface is soft.
2. Although the hoof has stopped, the body of the horse is still moving forward, essentially colliding with its own stationary leg. As the body pushes the leg forward, this forces the hoof to slide and then stop.
3. When it stops, the hoof pushes into the ground and the ground exerts an equal and opposite force (the GRF). This force is transferred up the leg and dissipated by the horse’s musculoskeletal system.
4. During the next phase of the stride (the support phase), the limb accelerates the body upward. The forces on the limb are at their maximum at mid-stance — the point where the limb is directly over the stationary foot — and after this the forces change from braking to propulsion. Appropriate traction between the shoe and surface is critical at mid-stance.
5. The last phase of the stride is the rollover, at which point the foot is unloaded.