Myths In Equine Anatomy
By Jochen Schleese

There is one inherent, critical mistake that seems pervasive in what is being written lately: if your horse seems to be narrowing behind the shoulder area, it is not due to muscle atrophy, but rather to muscle lengthening as the muscle is used.

To use an analogy, when you place your left hand on your right shoulder, your left biceps is big (flexed & contracted). When you stretch out your arm (i.e. lengthen or stretch the muscle) you can see how narrow your muscles now appear. The same concept applies to a horse’s back that is stretched. As your horse becomes more supple, the idea is to have the longissmus dorsi lengthen (which a lot of people mistake for muscle atrophy).

This is simply incorrect. Just like in bodybuilding, understanding muscle definition and biomechanics will explain why certain parts of the body become bulky, while others become more defined and appear slimmer. After working out for a year, a bodybuilder probably needs a new jacket because his upper body has gotten bigger. He’ll also need new pants because his waist has gotten smaller – and not because of atrophy due to pressure from his belt.

As explained to us by veterinarians, atrophy can only occur under severe instances of constant pressure, which will first damage the hair follicles (resulting in hair loss and/or white hair). Just make sure that you aren't exacerbating what you perceive to be the problem rather than addressing the root cause – your saddle probably does need to be adjusted. Remember, there are many things that could contribute to back problems other than the saddle.