When an injury first heals, massage can be essential for returning the body to its pre-injury state. Massage can also help with an old injury and the compensations and wrong patterns that can develop as a result. Undoing long standing distortions from old injuries will take more sessions of body work than newer injuries.
If left untreated for too long, muscles affected by injury can spread to many other muscles (and the way they move) in the body. When working on a new horse, I often find myself uncovering many layers of dysfunction, as each contracted muscle leads me to another. It can take several sessions to restore balance, and if the muscles have been very tight for a long period of time, it is very likely that the skeleton has been pulled out of alignment. Deep tissue massage and chiropractic done in conjunction with each other will be needed to restore symmetry and full motion.