Head banging is a vigourous and often violent rhythmic movement of the head and upper body. It is associated with hard rock and various types of heavy metal, with recognisably different styles: like the classic up-down (movingly depicted in the  "Bohemian Rhapsody" scene in Wayne's World), the circular swing (think Slipnot or Jason Newstead), the full body (the up-down taken to extreme amplitudes), or the cheeky side-to-side.

However there are there is emerging evidence that head banging, while making you more “metal,” has associated health risks. Jason Newsted, with a trademark circular swinging style head banging, left Metallica in 2001 citing “physical damage” as one of his reasons for leaving.The guitarist from Evanescence, Terry Balsamo, had a stroke in 2005 which his doctors attributed to head banging. There have also been cases of people receiving whiplash from headbanging such as Craig Jones of Slipknot and it is not unusual for people to report headaches and bloody noses after head banging.

Declan Patton and Andrew McIntosh, of the School of Risk and Safety Sciences, University of New South Wales in Sydney, decided to measure the public health risks posed by head banging and to suggest some possible solutions. Their study was published in the  British Medical Journal in December 2008.

Patton and McIntosh went to several hard rock and heavy metal concerts to find the most common style of head banging used by rock fans. Theses concerts included Motörhead, Mötley Crüe, Skid Row, The Hell City Glamours, L.A. Guns, Ozzy Osbourne,Winger, Ratt, Whitesnake, and W.A.S.P. Most people engaging in head banging chose to perform the up-down style. This is not surprising give the up down style is the most appropriate in the confined space of a heavy metal audience, where there is simply not the room to perform the circular or full body styles.

Using the results of their observations they constructed a biomechanical model of head banging, assuming basic up and down sinusoidal head motion. This equation allowed them to predict the risk of head or neck injury given the tempo of the music and the amplitude (range of motion) of the head banging. The application of biomechanical methods identified a definite risk of mild traumatic brain injury from head banging, the risk increases with tempo and range of motion. It should be noted that the risk is self limiting to a degree, as you would probably pass out before you did yourself real damage.

Reference: Head and neck injury risks in heavy metal: head bangers stuck between rock and a hard bass.     Declan Patton, Andrew McIntosh. BMJ  2008;337