There are hundreds of muscles in a body (whether horse, human, or dog, etc) and any of them can develop trigger points, or contracted bands of muscle fibers. When muscles are contracted in this way, they are not available for use due to pain, restricted motion, and distorted posture. While the degree of pain the body is in can vary greatly, most trigger points will be tender and painful when pressed.
A healthy muscle is free. It does not have knots or tight bands of muscle fibers. It is not tender when pressed. A healthy muscle feels soft (but not mushy) and elastic.
Trigger points can cause a loss of coordination and balance, since the affected muscles are not free to use. As we all know, lack of use causes weakness and atrophy. If the trigger points are not released, any attempt to strengthen the muscle will cause the surrounding muscles to pick up the slack, further weakening the muscle with the trigger point. It is a vicious cycle.
Trigger point therapy is a wonderful and quick treatment option if done in time. If a trigger point is left untreated for too, long muscle fibers can become damaged in a way that can not be repaired.