The average head of a horse weighs around 20 pounds. One of the muscles that has the task of holding the head up is the rhomboid. The rhomboid covers the withers and travels all the way up the crest of the neck to the poll This powerful muscle not only has the job of lifting the head, but also attaches to the scapula and pulls it forward and up. I usually start body work sessions by checking the rhomboid. Feeling the health of that muscle can give me a good indication of what is going on in the neck and the back.
Since the rhomboid is so strong, it is not easy to change. Any constriction in the neck, especially right in front of the withers, can take many massage sessions to relax. I always show owners/riders/grooms how to continue the work I do on the rhomboid on their own. Balancing the muscles of the neck is crucial to athletic performance: when the rhomboid is very tight you will see the horse swing his head sideways at each stride. By flinging his head and neck away from the shoulder that is working, he avoids feeling the discomfort of a very tight rhomboid.
It will take persistence to free the rhomboid, but the rewards are many: jumping ability will increase. The horse will feel more elastic in dressage. The stride will feel longer and less choppy.