The deep neck flexors (longus coli, longus captius) are key to having cervical stability. When these are injured or under tension from any type of neck injury, the muscles more superficial tend to be very tight.
Because there is no inherent stability in the neck, pure thoracic rotation is often lacking. This can present as mid back pain without any neck pain, except perhaps the neck feeling tight. Many times patients will come in with annoying mid back pain and often tell me they stretch the neck a lot but it doesn't seem to help.
Standing neck rotation will usually appear normal. But here is how I've been looking at it.
1. Arms straight out like your holding a gun. Turn right with the neck going with the arms. Judge both left and right a few times. Again, the neck follows your hands.
2. Repeat step one, but now the patients is asked to keep looking at you. Essentially, the neck is stable, the thoracic ring is rotating. Judge distance again. Did it change?
If there is an unstable neck, the rotation will usually be dramatically less. As the mid back rotates and the neck is standing still, the cervical spine still has to rotate. Gary Gray considers this as using the arms as a "driver" for neck rotation.
Just another observational tool in your tool box to judge mid back and cervical rotation. Enjoy!