If your head is in a forward posture, it can add up to 30 lbs (~13 kg) of abnormal leverage on the cervical spine (neck). In time, this can pull the entire spine out of alignment. Forward head posture may even result in the loss of up to 30% of vital lung capacity. These breath-related effects are primarily due to the loss of the cervical lordosis (the inward curve of the cervical spine), which blocks the action of the inferior hyoid muscle responsible for helping lift the first rib during inhalation.
Rene Cailliet, M.D., January 1996