The optimum surface for equestrian sports has yet to be determined but has been described as needing to minimise concussion through energy absorption, whilst still returning suitable power to aid performance (FEI Equine Surfaces White Paper).
There are four key papers that outline the impact of surface effects on equine biomechanics during locomotion, namely:
Racing Surfaces White Paper (2012) –This white paper has been written by the Racing Surfaces Testing Laboratory, USA and is a key document outlining the current understanding of racing surfaces.
Racing Surfaces White Paper – This white paper has been drafted as a collection of published scientific papers and data. It is considered a work in progress and will be updated as new scientific studies and track data become available.
FEI Equine Surfaces White Paper – This is the biggest international collaboration of its kind, bringing together the latest data and published scientific papers on arena and turf surfaces, and the effects these have on horses in training and in competition.
Equestrian Surfaces – A Guide – This document has been produced by the Swedish Equestrian Federation’s reference group for riding surfaces to share current scientific data that applies to equestrian surfaces, in combination with practical know-how from experienced arena specialists.
You can view these by clicking the titles of the papers but the THN have outlined the key messages on surface characteristics and hoof/ ground interaction below.
Graph 1: A method of standardization for arena surfaces: To enable comparative research, standard terminology is needed which must then be linked to terminology that is already in place within the industry. This diagram depicts common rider terms that can be described by functional properties. The normal line describes a pooled mean from data collected worldwide, the red mean line shows how far the arena tested differs from that mean and then the variability is shown by the blue bars and error bars. With additional knowledge the risks associated with each term will become more explicit. (Equine Surfaces White Paper).
Surfaces have 5 functional characteristics:-
Firmness – The shock experienced by the horse when the hoof contacts the surface.
Impact firmness relates to the hardness of the very top surface and the initial stiffness during primary impact.
Cushioning – How much a surface is supportive compared to how much it gives when riding on it.
Cushioning relates to how the whole of the surface reacts to the forces produced by the horse during locomotion.
This encompasses the amount of force reduction or damping and the stiffness of the surface during the support phase of the stride (specifically max load at the mid stance) and will be influenced by the amount of elastic compared to the amount of plastic deformation.
Responsiveness – How active or springy the surface feels to the rider.
Responsiveness relates to the natural frequency or tuning of a surface.
If a surface responds well to the locomotion of the horse it will feel springy or active, as the timing of the way it moves helps to return energy to the horse.
Responsiveness is influenced by the stiffness of the surface, so is closely related to cushioning.
Grip – How much the horse’s foot slides during landing, turning and pushing off.
Grip relates to the interaction between the horse hoof surface interface as well as the interaction between the materials that interlock and hold the surface together.
Uniformity – How regular the surface feels when the horse moves across it.
Uniformity relates to how much the functional properties and characteristics of a surface (points 1-4) change across the whole of an arena/ track.
A surface can be even, so it looks level, but as you ride across it the impact firmness, cushioning, responsiveness and grip change.
Video 1: How arena surfaces affect horse biomechanics
(Premier Equestrian, 2016)
Hoof/ ground interaction and limb loading during the stance phase:-
There are two phases of locomotion; the Swing Phase (when the horse’s hoof is in the air) and the Stance Phase (when the hoof is in contact with the ground). At high speed the hoof is only on the ground for a fraction of a second in each stride, but 3 phases occur:
Hoof landing/ impact – the hoof hits the ground and brakes/ slides to a stop.
— Primary impact – concussion – is associated with high decelerations and low forces when the hoof impacts the surface and rapidly decelerates to zero velocity. The magnitudes of deceleration and shock energy during primary impact are extremely sensitive to the speed of the horse, vertical hardness of the surface and the damping capabilities.
— Secondary impact – slide/ stop – is characterised by higher forces and minimal deceleration when the mass of the horse collides with the leg that is planted on the ground. The body tends to push the leg forward, forcing the hoof to slide over the surface before coming to rest.
Support/ mid-stance phase – the whole hoof is in contact with the ground and during over-ground locomotion carries up to 2 times the horse’s bodyweight depending on speed. At midstance the limb will experience peak vertical loads and there is a transition from braking to propulsion so that the centre of mass of the horse begins to accelerate forwards. It is important during this phase that the surface is able to carry the load being applied by the limb vertically, and provides sufficient shear resistance to allow the horse to propel its body forwards. Excessive plastic deformation of the surface would mean the reaction from the ground is smaller, so the horse must apply more muscular force to maintain the same momentum, which may lead to fatigue.
Rollover/ take off/ breakover – propulsion into the next stride, when the hoof leaves the ground starting with the heels, and ‘rolls’ over the toe. Both the vertical and horizontal forces fall towards zero at toe off, so the limb is gradually unloaded. This stage is important in that altering its duration strongly affects the kinematics of the limb during the swing phase, which in turn affects the next stance phase.
Stages of the stance phase showing the differences in acceleration (red) and ground reaction force (blue). When the blue arrow is tilted, it indicates that both vertical and horizontal components of the ground reaction force are present. The arrow shows the direction in which the ground is pushing the horse. (Equine Surfaces White Paper).
Important forces used to measure surface response
Ground reaction forces – is the force exerted by the ground on the horse in contact with it for e.g. a horse standing motionless on the ground exerts a contact force on it (equal to their bodyweight) and at the same time an equal and opposite ground reaction force is exerted by the ground on the horse.
Rebound/ resiliency – is a measure of energy absorbed by the surface that is returned to the horse. If surface rebound is well timed to the locomotion of the horse, the surface will feel springy or active.
Shear stress – is a force that causes layers of a surface to slide upon each other in opposite directions. Horizontal shear characteristics of the surface determines how quickly the foot is brought to a halt during a stride. If there is a partial shear failure of the track material as the hoof is brought to rest, there is slippage of surface particles. Therefore, shear strength of a surface (resistance to shear) should be optimised.
Shear strength – is the resistance of surface material slipping when loaded with the weight of the horse.
Management and use of surfaces
Research has demonstrated that the properties of a surface change from one day to the next according to use and environmental factors therefore, the maintenance of a surface is just as important as the characteristics of the surface itself. Several maintenance strategies are outlined in Equestrian Surfaces – A Guide(Section 11 -pg 104-123).
Substantial research has identified injury risks associated with surface type and going in racehorses.