A sharp dip in front of the withers is one of the most common things I see in horses. When the muscles of the neck and withers (the rhomboids and trapezius) are tight, you will see this dip. It will be impossible for the neck to naturally arch and reach for the bit. A vicious cycle can begin here: The rider wants the horse on the bit and strongly encourages with the hands to get the horse on the bit. Or gadgets, like draw reins, stretchers, or gogues are brought out. You can try to force the horse into a frame, but when the muscles of the top line are stressed, all the efforts will be counterproductive.
A horse that is ewe necked and braced can not be comfortable being pulled into an “on the bit” frame. The effects of tight rhomboid and trapezius muscles will also extend into the shoulders, making free action impossible. The horse will not be truly forward. Also, a horse that can not reach with a lovely arch onto the bit cannot lift its back. The small area in front of the withers can affect a very large portion of the body of the horse, which is one of the reasons I usually start the massage there. When the withers, neck, and shoulders are tight, the horse has no recourse but to compensate by over stressing the hind end or plowing heavily on the forehand.
I always show owners and riders how to work on the rhomboids and trapezius muscles. It is not difficult. The horses usually love it. And it is not something I can fix in one session. It will take patience and persistence to release the deep and strong rhomboid that has become overly tight. I have one client, a thoroughbred off the track, who took almost three years to finally have a beautiful and supple arch from his withers to his poll. He has moved up the levels and is performing beautifully and happily.
Example of tight rhomboids/trapezius muscles