The use of underwater treadmill exercise for training and rehabilitation of horses has become increasingly popular in recent
years. Many claims are made as to the usefulness of this form of exercise in horses; although relatively little published
information is available to substantiate these assertions. In humans, underwater treadmill therapy has long been recognized
for benefits in rehabilitation. In horses there is a lack of information on physiological responses, protocols, guidelines
and implementation of this therapy. This paper will review principles of underwater exercise and currently available knowledge
of underwater treadmill exercise in horses.
Claimed Benefits of Underwater Exercise in Horses
Loading on painful or healing structures
Pain in joints with degenerative joint disease
Muscle spasm, joint stiffness
Development of muscle atrophy
Lay-up time if underwater exercise continues when land exercise is restricted
Resistance for muscles during exercise
Tone in hypotonic muscle parts
Muscle mass and strength
Joint range of motion
Soft tissue extensibility
Cardiovascular fitness and endurance
Circulation which promotes healing
Support to limbs – reducing chance of injury
Physical Properties of Water
In order to more fully understand the effects of underwater versus land treadmill exercise some understanding of the physical
properties of water is needed.
Hydrodynamics describes movement through water and depends on the speed and direction of the immersed object. Strength training
results from the increased resistance to movement underwater when compared to air. Resistance to movement in water increases
with an object's surface area and speed, and this can be exploited by increasing the surface area of a limb or body or by
adjusting the speed of movement under water.
Density is the relationship between mass and volume and determines whether an object will sink or float. Lean heavily muscled
animals (such as horses) float less easily than animals with a greater amount of body fat.
When a body is immersed in water it is subject to the opposing forces of gravity and buoyancy and it experiences an upward
thrust equal to the weight of the fluid displaced (Archimedes principle of buoyancy). Buoyancy is one of the major reasons
for the use of aquatic environments in rehabilitation therapy in humans and small animals, as weight bearing is reduced, resulting
in less stress on bone, joints and connective tissue. The amount of weightlessness depends largely on the percentage of the
body that is below the water's surface. In humans it has been calculated that approximately 90% of body weight is supported
when immersed to the neck and this decreases as the water level decreases. When dogs are immersed to the level of the greater
trochanter, 38% of body weight is borne. As water decreases to the level of the lateral femoral condyle, weight bearing is
increased to 85% of body weight and at the lateral malleolus is 91% of body weight. (Tragauer and Levine 2002). The effect
of different water levels on weight bearing in horses is unreported to date. Due to the large amount of body weight in the
proximal versus distal part of the equine body it could be anticipated that minimal reduction occurs until the trunk of the
horse is substantially submersed, as is seen with below ground water treadmills. In addition, the large proximal muscle mass
in horses may cause less reduction in weight bearing, until a larger portion of the body is submerged, when compared to dogs.
It is also possible that a difference exist between breeds with a larger proportion of body fat (such as native ponies) when
compared to more athletic/heavily muscled breeds such as Quarter Horses.