When your posture is tall and erect, the muscles of your back can easily support the weight of your head and neck — as much as 12 pounds. But when your head juts forward at a 45 degree angle, your neck acts like a fulcrum, like a long lever lifting a heavy object. Now the muscle weight of your head and neck is the equivalent of about 45 pounds. It is not surprising people get stiff necks and shoulder and back pain. This position compresses the neck and can lead to fatigue, headaches, poor concentration, increased muscle tension and even injury to the vertebrae over time.
— source news.sfsu.edu | Jan 4, 2019