The Optimal Surface for Training and Competing
Hilary M. Clayton, BVMS, PhD, MRCVS
Mary Anne McPhail Equine Performance Center
Michigan State University
East Lansing, MI 48824
USA
The surface a horse works on affects both performance and soundness.
The ideal surface varies with factors such as the sport, climate,
and location (indoors or outdoors). No single surface is ideal
for all sports and in all climates, and even when a good surface
has been established, maintenance is an on-going concern.
Interaction between Hoof and Surface
The two most important aspects of the mechanical interaction between
the surface and the horse’s limbs are the shock loading of
the limbs immediately after hoof contact and rotation of the hoof
into the surface. The surface properties that affect these processes
are impact resistance (peak deceleration of a moving body that
impacts the surface) and resistance to shear.
Impact Loading
The hoof is moving forward and downward at the instant of ground
contact, after which it is decelerated in both vertical and horizontal
directions. The fore hoof usually has a higher vertical velocity
but a lower horizontal velocity than the hind hoof at the instant
of ground contact (Back et al. 1995), which may explain why the
forelimb is the site of the majority of concussive injuries.
Deceleration of the hoof initiates a shock wave that travels up
the horse’s limb and is progressively attenuated by flexion
of the joints and deformation of the soft tissues. The shock wave
is characterized by high amplitude and a rapid oscillations that
are particularly damaging to bones and joints (Radin et al. 1982).
Consequences of impact loading during locomotion include fatigue
fractures and bone sclerosis, which precede complete fractures
in racing Thoroughbreds, and degenerative joint disease, which
is the most common reason for premature retirement of sport horses.
Repetitive impact loading over a period of several years is a primary
factor in the eventual development of degenerative joint disease.
The early stages of this condition are silent and, by the time
lameness becomes apparent, permanent damage is present. Therefore,
trainers should be diligent in their efforts to reduce impact shock
throughout the horse’s career by working on good surfaces
and taking care of hoof balance and shoeing.
Weight-bearing Phase
The weight-bearing phase occupies the period from the end of the
impact phase until breakover begins. During this phase the vertical
force increases steadily peaking around midstance, after which
there is a period of unloading. The vertical ground reaction force
increases as the limb accepts the body weight. The longitudinal
force retards the horse’s forward motion during the loading
phase and provides forward propulsion during unloading. Transverse
forces produce turning and lateral movements. In order to apply
effective longitudinal and transverse shear forces, the surface
must be sufficiently stable and there must be a high enough shear
resistance to prevent excessive sliding.
As the limb is loaded, the elastic structures that run down the
back of the cannon region are stretched, storing elastic energy,
which is released later in the stride. These structures include
the deep (DDFT) and superficial (SDFT) digital flexor tendons and
the suspensory ligament (SL). Storage and release of elastic energy
reduces the expenditure of metabolic energy during locomotion.
A surface that is ‘tuned’ to the athlete’s limb
maximizes energy savings. In human runners, tuned tracks reduced
race times significantly. Surfaces tuned to a horse’s limb
have not been developed, but it has been estimated that they would
be much firmer than those for human runners (Pratt 1981). Surfaces
that are too soft absorb too much energy during the loading phase;
surfaces that are too hard return energy at an inappropriate time
in the stride and this increases limb loading.
Breakover
Breakover begins when the heels leave the ground and start to rotate
around the toe of the hoof, which is still in contact with the
ground. Breakover is initiated by tension in the distal check ligament
(DCL) acting through the DDFT, combined with tension in the navicular
ligaments.
On a hard surface, the hoof remains flat on the ground until heel
off, and this is associated with high tensile forces in the DCL-DDFT.
On a softer surface the toe rotates into the surface prior to heel
off, which reduces tension in the DCL-DDFT, and reduces pressure
in the navicular region. A good surface allows the toe to penetrate
during push off is beneficial, especially in horses with caudal
heel pain, but then offers sufficient resistance to support the
hoof during push off.
Properties of the Surface
Arenas and tracks are constructed with a base and a cushion. Construction
of the base will not be discussed here, but information on this
topic has been published elsewhere (e.g. Malmgren 1999, USDF 2000).
The cushion interacts directly with the horse’s limb. Important
properties of the cushion material are impact resistance, shear
resistance and frictional properties between the surface and the
horse’s hoof or shoe.
Impact Resistance
The characteristic of footing that affect the hoof-ground interaction
during the loading phase is the impact resistance, which is an
indicator of the hardness of the surface. It determines the rate
of deceleration during impact and the amount of concussion on the
limb. Ideally, the surface should deform somewhat during impact
to absorb some of the impact energy and reduce the potentially
damaging effects of the impact shock wave.
The density of the surface has a large effect on impact shock.
Hard surfaces, such as concrete or blacktop, have high impact resistance,
whereas surfaces containing organic materials, such as wood shavings,
and a small particle size distribution are associated with lower
force and frequency of impact shock (Barrey et al. 1991).
A surface with high impact resistance absorbs little energy and
generates considerable concussion. Working regularly on a hard
surface compromises long-term soundness by predisposing to bone
and joint problems, such as degenerative joint disease. Although
hard surfaces are detrimental to the long term health of the musculoskeletal
system (Cheney et al. 1973), race times are highly correlated with
track hardness (Zebarth and Shear 1985).
A soft, deep surface with a low impact resistance reduces concussion
on the leg. However, a surface that is too soft absorbs so much
energy that elastic energy storage and release is reduced. The
effect is like running on pillows. As a consequence of the loss
of elastic rebound, the horse’s muscles have to work harder
than usual to provide sufficient propulsion, which leads to a premature
onset of fatigue. Vaulting horses, for example, showed about 50%
increase in heart rates when cantering on deep sand compared with
a more resilient surface (Sloet et al.) indicating that the horses
had to work a lot harder in deep sand.
On a deep surface horses are likely to tire more easily, especially
if they have not been conditioned on this type of surface. If the
signs of fatigue are ignored there is a danger of soft tissue strain.
On the other hand, if used in moderation, deep surfaces may have
a conditioning (strength training) effect.
Shear Resistance
Shear resistance measures the force required to cause relative
motion between particles of the surface material. It determines
the resistance to penetration by the toe of the hoof as the horse
pushes off against the ground. Ideally, the toe should penetrate
the surface, but there should be sufficient resistance to give
security during the push off. On a very hard surface, shear resistance
is high and the toe is unable to dig in. This is associated with
greater forces in the navicular region, and is one of the reasons
why horses with navicular syndrome do not perform well on a hard
surface. When shear resistance is low, the toe penetrates the surface
easily but there is no support for the hoof during the push off.
Instead the surface material continues to yield and the horses’ muscles
must work harder to produce the necessary propulsion and turning
forces, leading to rapid onset of fatigue. Deep, dry sand is typical
of a surface that has low shear resistance, but watering the sand
increases the shear resistance due to surface tension of the water
holding the sand particles together. The horse’s response
is to avoid generating large shear forces by taking short, choppy
strides.
The coefficient of friction must also be considered, especially
when horses move over a hard surface. Its value depends on the
materials comprising both the surface and the hoof or shoe. For
example, the coefficient of friction is higher between hoof horn
and cement than between steel and cement. Therefore, a shod horse
is more likely to slip on cement than one that is barefoot. The
material properties of the shoe are likely to affect the incidence
of impact-associated injuries in horses that work on the roads
(Pardoe et al. 2001).
Footing Materials
Sand
Sand is traditionally used as a surface material in arenas. In
fact, the word arena is derived from the Latin word meaning sand,
though in ancient Roman arenas, sand was spread on the floor to
absorb the blood of injured gladiators and animals rather than
as a footing material. Sand is still one of the most frequently
used surfaces due to its easy availability and cost effectiveness.
Sand consists of tiny pieces of rocks or minerals that are larger
than silt or clay but smaller than gravel. Impact resistance of
sand depends on its bulk density, moisture content and particle
size. Both impact resistance and shear resistance increase with
bulk density, the value of which gets higher when the material
is more compacted. One of the benefits of harrowing is that it
breaks up compaction and introduces air into the surface, thus
decreasing its bulk density. Compared with soil, sand has higher
impact resistance, combined with lower shear resistance. Deep,
dry sand tends to give way as the hoof pushes off resulting in
loss of traction, but increasing the moisture content causes particle
adherence due to surface tension, so wet sand is more stable and
less tiring to work on than dry sand (think of running on the beach).
The optimum water content ranges from 7-14% depending on the particle
size distribution (Barrey et al. 1991). Sand has little elasticity;
less than 2% of the impact energy is returned to the limb. Both
the SL and the DCL are subjected to lower strain on sand than pavement
(Riemersma et al. 1996).
Sand varies widely in its physical properties and some types of
sand make much better footing than others. Relevant considerations
include the size, shape and hardness of the grains. Particle size
affects dustiness, compaction and water retention. Ideally, arena
sand should be predominantly medium coarse (0.25-0.5 mm) and coarse
(0.5-1.0 mm) grains. Sand with a lot of fine particles is dusty
when dry and compacts when wet. Angular grains are better than
round ones because the sand is more stable and requires less maintenance.
Round grains roll over each other more easily giving a less stable
footing, especially when deep. Hard (granite) sand is more durable,
whereas soft (calcite) sand tends to break down and turn to dust
relatively quickly. Hardness can be tested by placing a few grains
on a hard surface and compressing them with a spoon. If the grains
are easily crushed, the sand is soft.
Dirt
Dirt surfaces consist of a soil material comprising a mixture of
sand, silt and clay. In general, dirt is finer grained than sand,
which affects its performance as a riding surface. Compared with
sand, dirt does not drain as freely and it holds moisture longer.
Consequently, dirt does not need to be watered as frequently as
sand to prevent dustiness. Impact resistance of dirt decreases
with moisture content, so a dirt surface tends to get harder as
it dries out (in contrast to sand). Dirt surfaces are compressed
and compacted by horse traffic and, due to its high compactability,
more tractor power is required to harrow dirt than sand.
Wood Products
In some areas, wood products are readily available at a reasonable
price. Wood products used on arenas and tracks include bark, wood
chips and shavings. Wood products are often mixed with sand to
give it more resilience, but sometimes they are used alone. Generally,
wood surfaces offer a good cushion, but they can become slippery
if the footing is too deep. Another benefit of wood products is
that they help to hold moisture in the surface, which reduces the
frequency of watering.
Shavings break down relatively quickly and are more suitable for
arenas that don’t have a lot of traffic. Shavings also blow
away when used outdoors. A layer of woodchips beneath the overlying
layers of gravel and sand was responsible for a decrease in hardness
of a Standardbred race track due to the shock absorbency of the
woodchips (Drevemo and Hjertén, 1991).
Rubber Products
Most of the rubber products marketed for use in equine arenas are
from recycled tyres that have had the fibrous and metallic materials
removed. Rubber adds cushiness and resilience to the surface, but
it may allow sand to dry out more rapidly. Most of the rubber products
that are currently marketed do not break down with use, though
it is wise to confirm this before purchasing a specific product.
It is recommended that rubber be used as an additive in sand arenas
rather than using it as alone or as a layer on top of sand.
Turf
Turf can be a very good footing for horses, because the rooting
system of the grass stabilizes the soil in which the grass grows
resulting in a 3-fold increase in shear resistance compared with
soil alone (Zebarth and Sheard 1985). However, the impact resistance
of turf is approximately double that of the same soil without turf.
Elasticity of turf is low (though higher than sand or soil); 2-4%
of the impact energy is returned to the horse’s limb (Zebarth
and Sheard 1985). The main drawback is the difficulty of maintaining
turf in perfect condition.
Under ideal soil moisture conditions, turf has an intermediate
shear resistance, allowing the toe to penetrate the surface as
the hoof rotates, but providing sufficient resistance as the hoof
pushes off. Impact resistance of turf increases with an increase
in bulk density (more compaction) of the soil or with a decrease
in moisture content. Impact resistance is about 40% higher when
soil dries out compared with the same turf when the soil is moist.
Although a high moisture content lowers the impact resistance,
too much moisture allows slipping. The length of the grass or the
presence of thatch does not affect impact resistance (Zebarth and
Shear 1985).
Well-maintained turf provides excellent footing, but deterioration
in surface characteristics under conditions of drought or excess
rainfall is a problem. Good maintenance requires provision for
both irrigation and drainage.
The physical properties and durability of turf can be improved
by working crumb rubber and organic materials into the underlying
soil. The rubber and organics aerate the soil, resulting in deeper
root density, and the growth of greener, healthier turf. The rubber
also reduces soil compaction, resulting in lower impact resistance
and improved drainage. This product appears to function well, particularly
for jumping arenas, eventing courses and polo fields.
Common Problems
Some of the common problems with footing include dust, compaction,
slipperiness, shifting of the material and breakdown of the components.
Dust
Dust is unpleasant for both horses and rider, especially those
with respiratory sensitivities. Dust is a consequence of having
fine particles, usually very fine sand and clay, in the surface
material. These small particles become airborne when disturbed
by the action of the hooves or even the wind.
Addition of bonding agents, such as water and polymers, reduce
dust. The frequency of watering is decreased by addition of a hygroscopic
agent, such as calcium chloride or the less corrosive magnesium
chloride, that retains water.
Compaction
A surface becomes hard when the material is compacted. Surfaces
with a high content of clay are particularly susceptible to compaction,
especially when the surface gets wet and then dries. Harrowing
the surface loosens the material and introduces air, which makes
it more fluffy.
Amendments, such as rubber, wood chips, leather or fibres, are
often added to reduce compaction and give more resilience to a
surface. The presence of fibres or shredded materials has the added
benefit of stabilizing the surface.
Breakdown
Some materials are more prone to disintegrate than others, and
the more traffic there is in the arena, the less time it takes
for breakdown to occur. For example, wood products break down over
time and soft sand disintegrates leading to dust formation. The
best prevention is to select surface materials that are not prone
to break down, especially for arenas that are used by large numbers
of horses. When the footing does break down it may be possible
to control the effects for a while using bonding agents, but eventually
the surface will need to be replaced.
Shifting
Surface materials that shift when horses work on them soon develop
a ‘track’ around the outside of the arena and other
frequently-travelled paths. The fault usually lies with the footing
material; round sand shifts a lot more than an angulated sand.
The remedy is to harrow the surface frequently paying special attention
to high traffic areas and, perhaps, to add a suitable amendment,
such as a fibrous material, as a stabilizer.
Sport Specific Requirements
The requirements for stability versus sliding, and security versus
resilience vary in different sports. Consequently, it is particularly
difficult to provide footing in a multi-use arena. Dressage horses
need resilience in order to move expressively, combined with sufficient
stability to move confidently, especially in the extensions. Jumpers
apply high shear forces, so stability is the over-riding concern.
Reining, cutting and rodeo sports generally favour deeper footing,
and reiners like a surface that maximizes sliding. Racehorses achieve
faster times on a firm surface, and this is characteristic of harness
racing tracks, though long-term soundness may be compromised. Thoroughbred
tracks tend to be a little softer, which reduces the risk of injury,
but if the surface is too deep it becomes insecure. Events that
are typically run on turf are likely to encounter problems due
to heavy wear, for example the take off and landing areas of fences.
These areas may be reinforced by coarse sand or gravel. However,
round gravel tends to roll, making it insecure, whereas angled
gravel is more likely to cause abrasions. The addition of rubber
chips to the soil is useful for stabilizing turf and reducing wear.
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