The Pectoral muscles
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To finish our discussion of the muscles that help hold the front leg onto the horse which are affected by the saddle, we need to talk about the pectoral muscles. These are four muscles that originate (start) on the sternum and insert (attach at the far end) on the humerus or scapula, but only one is directly impacted by the cinch.
There are a number of different names given to the four muscles, but they are always classified the same way. Two are superficial, and two are deeper. The cranial (toward the head) superficial pectoral muscle (g in these illustrations) makes up most of the "chest muscle". It starts on the very front of the sternum and inserts on a lump on the outside of the humerus as well as the fascia of the arm. It acts to adduct (pull toward the center of the body) the front leg and to move the front leg forward.
The second superficial muscle (g') is sometimes called the transverse pectoral muscle. It starts on the sternum but only cranial to where the 6th rib attaches, so it is still forward of the cinch. It also attaches to the humerus and acts primarly to pull the front leg in towards the body. If you pet your horse between his front legs, this is the muscle directly underneath the skin.
The there are also two deep pectoral muscles. The cranial one, also commonly called the subclavius, is h' in the above picture. It starts on the front half of the sternum (also ahead of the cinch) as well as the cartilages of the first four ribs. It goes forward and up and actually attaches on the upper part of the front of the scapula. It is a very important muscle in drawing the forelimb back relative to the body (if no weight is on the leg) or moving the body forward and over a weighted leg. It also pulls the front leg in towards the body.
The caudal deep pectoral muscle, often called the ascending pectoral muscle, is the largest of the four and is the only one that is impacted by the cinch. It starts not only on the sternum (all the way to its caudal end), but also on the outside of the abdominal muscles and the cartilages of the fourth to ninth ribs. It goes forward inside the leg to the front of the humerus and attaches to its top end. It, like the subclavius, is really important in pulling the leg back relative to the body or moving the body forward relative to the leg, depending if there is weight on the leg or not. It also acts to pull the leg in towards the body.
But as you can see, regardless if the cinch lies close to the front leg or farther back, it will put pressure on this muscle. Too much pressure from the cinch, either because it is too tight or because is too narrow and therefore has poor distribution of pressure, will inhibit this muscle's activity in moving the horse forward. It may also cause decreased blood flow, damage and pain in this muscle, all of which will affect the horse's movement. So although the saddle tree doesn't affect this pectoral muscle directly, if the tree doesn't fit the horse well, the saddle will roll and move. If the saddle rolls and moves, the rider often tightens the cinch more. (The cinch is also commonly overtightened if too much padding is used, but that's another story.) So a poorly fitting saddle can cause problems with a muscle on the underside of the horse too.