Rene Cailliet, an American born physician of French ancestry, was one of the pioneers in physical rehabilitation and is well known for his books on musculoskeletal medicine. He says:
You can realign your entire body by moving your head … your head held in a forward position can pull your entire body out of line.
He goes on to explain that the vital lung capacity is reduced as much as 30% with forward head posture.
Forward head posture also causes compression in the upper neck joints (which causes pain and irritation). In order to prevent your forward head from falling toward your chest, your muscles sustain continual contraction, which causes nerve entrapment and artery compression. It isn’t hard to see that there exists the potential for neurological and vascular complications as a result of forward head posture.
Further Resources: Forward Head Posture: Cause & Cure