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
     • Reduces:
          ­ 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
          ­ Edema
     • Increases:
          ­ 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
          ­ Relaxation

Physical Properties of Water

Hydrodynamics:

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 :

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.

Buoyancy :

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.