What all horse lovers are striving for in their mounts is balance. Anatomical balance starts with a well bred horse whose joints and muscles are as close to an ideal standard as possible. After that, there should be equal tension on muscles when the horse is standing still. When muscles have equal tone, the amount of wear and tear on muscles and joints (which are moved by muscle) will be the result.
Horses (and people too!) are born with a set number of muscle fibers. You can’t add to the number, though you can increase the size of the muscles through exercise. However, you can lose muscle fibers through atrophy, and they can never be regained. Muscle fibers can be lost through injury, but also through non use. That is why it is so important not to rely on prolonged rest to resolve lameness issues. Every body needs rest, and every injury needs some time to heal. After that, activity is essential to prevent becoming permanently disabled.