Spinal Load and Mechanics

A standard adult head weighs approximately 10 to 12 pounds. In a neutral position, where the ears are aligned directly over the shoulders, the spine supports only this actual weight. However, as the head drifts forward:

Weight Multiplier: For every one inch of forward translation, the relative weight of the head on the neck increases by an additional 10 pounds.

Cumulative Force: If the head sits just three inches forward, the forces acting on the neck and upper back can reach approximately 42 pounds.

Muscle Strain: To prevent the head from slumping further, muscles such as the cervical erector spinae and upper trapezius must work significantly harder, leading to fatigue, tension, and pain.

(http://www.autoadmit.com/thread.php?thread_id=5811188&forum_id=2/en-en/#49515506)