The Trapezius Muscle
The scapula bone in the shoulder and the connected bones in the horse’s front legs are not attached to the rest of the skeleton by bone. Instead, they are attached to the body by a number of tendons and muscles. The first muscle I will write about is the trapezius muscle.
The trapezius is a paper thin superficial muscle that can be divided in to two parts - the trapezius cervicis in the neck, and the trapezius thoracis over the withers.
The trapezius muscle originates at the nuchal ligament at the top of the neck at C2, and inserts in to the withers at T3. It also inserts along the spine of scapula - the ridge of the shoulder blade that you can feel and sometimes see. The cervical portion of the trapezius moves the scapula forward, while thoracic portion draws the scapula backward.
The thoracic part of the trapezius sits under the bar of the saddle tree and therefore can become easily restricted by a poor fitting saddle or one not placed in the correct position on the horse’s back. When the trapezius muscle is restricted, the horse physically cannot provide a full range of movement through the shoulders and front limbs, therefore there would be a loss of performance which could eventually lead to lameness and other issues down the line if not addressed.
A sign of a horse with issues in the trapezius muscle would be a visible ‘dip’ just below the withers behind the shoulder. When palpating I would expect the area to feel fibrous, tight and dehydrated with a possible spasm over the area. The horse may also show some objections to the area being touched as it will be sore!
As this area is quite bony with the presence of the thoracic dorsal spinous processes in the wither and the proximal edge of the scapula, special care must be taken when treating by applying manual massage techniques to stretch the fascia and muscle fibres and encourage circulation and collagen production in to the area.