"The very essence of the horse is found in the way it moves.
When you purchase a horse whether it's for competition or for pleasure, you buy motion. You look for other attributes like conformation, manners, bone, intelligence and so on but most of all you look for a good mover - one that can be trained in your favorite discipline or can give you a pleasurable ride.
The actual value of a horse is in what the horse can do and how well he can do it - the day a fifty thousand dollar horse can't move like one, it isn't one.
The moving parts of the body were created and intended to move freely, easily and completely. When it loses its ability to do this there will be a problem - mainly refusals, to being off, to downright lameness.
The reasons for this can be orthopaedic, systemic or neurological needing veterinarian expertise. Hoof problems can be orthopaedic and require farrier knowledge as well.
These problems can also be caused by a bit of knotted muscle tissue no bigger than the end of your thumb causing pain and pressure interfering with proper motion. In these instances the muscle problem is a spasm. A spasm is a group of fibers being held in contraction in a rigid knot unable to release. Muscles can be the entire cause of a problem or a reflection of a deeper cause. Any problem of a serious nature should have vet or farrier expertise.
While there is no way a massage can do damage, it is important to eliminate a more serious cause first. The right treatment works because it has to work. The wrong treatment does not work because it cannot work. However, once other causes have been ruled out, there is no more sensible, accurate and effective work than deep massage.
The key to successful massage is accuracy."
From "Beating Muscle Injuries for Horses" by Jack Meagher