What is the role of the rhomboid in equitation?

The semi-circular ventral serrate rises from the lower parts of ribs one to eight, and from the lower neck. On the other end it merges with the big cartilage of the upper shoulder blade. When it contracts thorax, lower neck and withers rise. Does at that point the rhomboid release to make room for the withers? 

It looks like it rotates several degrees and, if at all, lengthens a bit. Question. Muscles in other words release, i.e. are inactive, lengthening and/or shorten while in action. Or are there other function in muscle tissue, such as expanding and holding?

The rhomboid attaches to the shoulder cartilages as well, however, coming across and down from the upper line. It is attached to the upper line from C2 to T8 and merges with the long ligament (?). It is a deep postural muscle, which in the human communicates between the upright and the level legs of the cross that delineates the human body. It would not be surprising to find out, that in the horse it participates in rearing. 

In horses as in humans the rhomboid is innervated and traversed by dermatome C8, which in the horse (my hypothesis) runs from the height of the withers to the heels of the front legs. In the human C8 covers the ring fingers, which the rider may place right on top of the withers, i.e. C8. 

Further questions.

When human shoulders close, is the rhomboid released, or does it in fact effect their closing? After all human upright and the horse's level position calls for a difference in their gravitational response. Which other muscle can do the job? 

How/where in the horse does the rhomboid connect to upper neck muscles and the head. How/where do rhomboid and ventral serrate connect to the muscles of the trunk and the hindquarters. Are they part of a fascia train? Is Thomas Myer's diagonal line a useful model?

How much work do the muscles of the sling contribute individually to holding the forehand off the ground. How much do they contribute to movement. What is their quality? Are they tonic or phasic? Who developed and who uses these terms today?