The rhomboid muscle is very important for several reasons:
The shoulder blade hangs from the withers by the rhomboid, making it responsible for the swing of the shoulder.
The rhomboid goes from the withers all the way up the neck to the poll, making it responsible for the ability of the neck to stretch.
If a saddle is placed too far forward it will put pressure on the rhomboid, interfering with the muscles’ function. If the saddle is too narrow, it will pinch the rhomboid muscle, eventually causing it to atrophy. If a saddle is too wide, it will sit directly on the withers, eventually causing lameness. If you see a hollow looking spot behind the scapula, there is most likely muscle atrophy.
Where the rhomboid attaches to the withers it is very thick and strong. When there is a problem with the rhomboid, you may notice tightness in the shoulders and a loss of coordination in jumping. Massage to the stress points in the rhomboid is extremely effective, and improvement in the horse will be immediately apparent.