The British Medical Journal investigates the health risks from head-banging and recommends protective gear and "adult-oriented rock":
Main outcome measures: Head Injury Criterion and Neck Injury Criterion were derived for head banging styles and both popular heavy metal songs and easy listening music controls.
Results: An average head banging song has a tempo of about 146 beats per minute, which is predicted to cause mild head injury when the range of motion is greater than 75°. At higher tempos and greater ranges of motion there is a risk of neck injury.
Conclusion: To minimise the risk of head and neck injury, head bangers should decrease their range of head and neck motion, head bang to slower tempo songs by replacing heavy metal with adult oriented rock, only head bang to every second beat, or use personal protective equipment.
(Image: Headbanging without the face, a Creative Commons Attribution (2.0) image from jessebikman's photostream)