Wikipedia says the following about equine treadmills:
The high-speed treadmill: Offers the option of adjusting speed, distance, slope, and degree of weight-bearing (through weighted saddles) of the horse. Heart rate may be monitored to assess impact of training on the cardiovascular system. Training factors that are especially stressful to a particular injury, such as degree of slope in a horse with a tendon injury, may be adjusted gradually to slowly increase the force placed on the recovering tissue. This allows for individualized rehabilitation. However, horses require more work on the treadmill when compared to work under-saddle to produce the same degree of oxygen consumption, and therefore the treadmill is not a suitable substitute for regular training with a rider when conditioning an animal for performance.
Is this video somewhat hypnotic?
Under Water Treadmill: Aquatic therapy has been shown to improve muscle strength, especially in the core, back, and stifles, and to increase both cardiovascular condition and proprioception in horses. The underwater treadmill is a popular tool for equine rehabilitation, and can offer targeted therapy based on water depth. At lower depths, horses will pick their legs up out of the water to clear it in the flight phase of the stride. In this case, the depth may be adjusted to improve the range of motion of a specific joint, offering the option of customizing treatment to a particular injury. Higher depths can increase pelvic flexion and raising of the back, helping to strengthen muscles that are commonly used by riding horses, conditioning them without the added weight of a rider. High water levels can also reduce body mass, similar to the effects seen with swimming, and may be beneficial for joint injuries or fractures. However, the animal will develop muscular and cardiovascular fitness much faster that they will develop skeletal strength. This may make the horse appear to be better-prepared for strenuous work than it truly is, and early return to work may place the bones, joints, tendons, and ligaments at much greater risk of injury. Therefore, care must be taken to build up bone strength before the animal is placed into regular work under-saddle.