In October 2021 I was lucky enough to attend the Centaur Biomechanics Equine Sports Seminar Virtual Summit. As always, the lectures were of a very high quality and there was an immense amount to take in. Here I share with you some of my understanding.
Dr Elin Hernlund is a researcher in biomechanics, a clinician with a focus on equine orthopaedics, and a teacher in functional and applied anatomy of the equine locomotor apparatus. Her research interests include better understanding the movement and behavioural changes in horses due to orthopaedic pain, including changes in facial expression and body posture.
We start with a question - Is the surface a risk factor for injury in sports horses? The rider often feels that the surface is relevant in how the horse goes. It turns out there is very little evidence that surfaces are related to the risk of injury. Sports injuries often have a multifactorial background. One of the things that seems to be important in the development of an injury is the cumulative load.
There is a four step sequence of injury prevention research. Initially we must establish the extent of the injury problem, in particular the incidence and the severity. Next we try to establish the aetiology and mechanisms of the injury. We go on to introduce a preventative measure, and then to assess its’ effectiveness by going back to establishing the extent of the injury problem.
Dr Hernlund is involved in research that focuses specifically on the footing and the training affects injury, particularly in show jumpers. A retrospective investigation of risk factors for owner-reported lameness in UK dressage horses indicated that wax-coated and sand with ribber surfaces were associated with less detrimental surface properties than sand, sand and PVC, wood chips or grass. Woodchips were not strongly associated with slipping, and sand with tripping. It’s important to bear in mind that this was owner reported information.
As study of 263 elite show jumping horses in 4 countries looked at identifying risk factors for ‘days lost to training’ in show jumpers. The training and management factors, including the surface type, were recorded. There were 39,028 days at risk, where 6% were days lost (55% due to non-acute and 22% to acute orthopaedic injuries. Variation in training was protective. Too much rest was a risk factor. Limited use of sand surfaces was a risk. Use of sand and wood chip surfaces was protective.
In order to manage surfaces as risk factors for injury, then we must be able to measure the surface. The mechanical properties of the surface need to be studied in relation to injury. Research into ‘going’ in Thoroughbred racing shows that the risk of distal limb fracture increases as the ground becomes more firm. The risk of superficial digital flexor tendon strain injury for racing Thoroughbreds also increases as the firmness of the ground increases. This risk increases more steeply for hurdlers than for steeplechasers.
The surface is what’s called ‘strain rate dependent’, which means that it responds differently depending on the load and the loading rate that it’s being subjected to. This means that as a person walking on the surface, we cannot feel what the horse will feel as he interacts with the surface. Dr Hernlund and her team have developed the Orono Biomechanical Surface Tester (OBST). This machine measures vertical and horizontal motions, and load cells and accelerometers. It attempts to mimic the horse, with the hoof landing speed, hoof load, and hoof load rate, which is important due to viscoelastic properties.
There are 5 functional surface properties that we can measure and that are relevant to the different phases of the stride. These are impact firmness, cushioning, responsiveness, grip, and uniformity.
Impact firmness is particularly important in relation the hoof landing. This is a high acceleration event with a low mass, meaning that the hoof interacts mainly with the top of the surface. Next comes the cushioning, the weight acceptance phase, where the weight is being put on the limb. There is dissipation of energy from the impact into the surface. Here the horse interacts with the surface more deeply, into the top and middle layer.
The responsiveness is how much energy the surface gives back to the horse, and involves the top and the middle layer. A firmer surface is more responsive. The grip of the surface will affect what happens to the hoof as it lands. We are looking for some shear resistance in the top layer of the surface to reduce slip, and for the hoof to push off from. However, high grip surfaces have been shown to lead to changes in loading at the knee and ankle in humans, and there is a fine line for how much grip is optimal.
Uniformity is the spatial variability over the arena. It’s whether the 4 properties we’ve already discussed are spread evenly throughout the arena. For competition and racing surfaces, this is very important. In racing, where there is a crossing, there is sometimes a different surface underneath, which then changes the properties of the surface. The horse would not be able to adapt to this as they can’t see the difference, and so he’s unable to adjust to it, which could be a potential risk of injury.
A study looked at what riders thought of different surfaces. The researchers looked at 25 competition and warm up arenas. They measured the surfaces using the OBST, and did a questionnaire for rider evaluations of functional properties. This led to 749 arena evaluations from 198 riders. There was a relationship between what the riders felt and the OBST measurements, but it wasn’t always clear. It seems that the impact, for example, is easier for the rider to appreciate, but the cushioning not so easy.
What factors influence the characteristics of the surface? One important factor is the grain shape. Sand grains can be rounded, sub rounded, sub angular, and angular. Each of these behaves differently. From experience, the sub rounded grains are the most preferred for equestrian surfaces. Have you ever thought about where the sand comes from? Typically, equestrian surfaces have made use of the waste from that particular country. So for example, in Sweden where there are lots of forests, wood chip has been popular. In England, there are many different geographical areas, which creates different sands. The sand in the UK is often washed by the sea, which creates the sub rounded grains. Another option is crushed stone, which creates sub angular and angular grains. The bigger stones give drainage and a stable base for an arena. The smaller grains of crushed stone, though, can become compacted over time as they lock together. The sand grains create air spaces between them, which the crushed stone grains don’t.
Grains of different sizes are able to compact more, and are less able to absorb moisture, as there is less space between the grains. Similar size rounded grains enable more ‘void’ space between the grains, and therefore can hold more moisture. More highly compacted surfaces give a higher impact, and higher moisture gives higher impact. To reduce variability in the surface we need to control water content and compaction. The key is to choose good materials, and to work mechanically with what you have by good arena maintenance.
The FEI has published a white paper on surfaces, and there is a scientific surface guide that you can download for free on the FEI website.
Thinking about the deeper layers, we know that the horse interacts with the surface at a deeper level than we might expect. Two arenas with the same top layer can behave very differently with an added cushioning layer underneath, such as a mat. During a competition, the compaction from increased use affects the impact firmness and compaction of the surface.
Is there a perfect footing? The main thing we know is that varying the surface is incredibly important. This varies the load on the horse’s legs and body. In competition, it is good to have uniformity in the surface. However, in training, it’s good to apply strategic use of uneven surfaces. The science in training of human athletes supports this strategic use of varied surfaces in training.
For more information, great webinars and a whole heap of relevant research, visit www.centaurbiomechanics.co.uk.
© Sue Palmer, The Horse Physio, 2021
Treating your horse with care, connection, curiosity and compassion