The saddle is a giant problem for the horse. Even if the saddle fits the horse when bought, the horse will change muscle-mass and bodyfat over time. This will make the saddle either too big or too small in a certain moment.
Injuries on muscles from the saddle is very common. If the saddle is too narrow, it will move forward and put pressure on the muscles around the shoulderblades. If too wide, the saddle will fall down on the spinal processes and occasionally hurt those.
Very often the saddle is placed too far up the horse's neck . When the horse lifts its frontleg, the shoulder- blade moves backwards and strikes against the metal part in the saddle.These injuries affect the length of the stride of the front leg.
Also harnesses must fit the horse properly to aviod injuries. Very often the harness put pressure to the spine. Today you can find anatomically designed harnesses on the market.