Acne mechanica is one rarely heard of.  It has been recently studied at the University of Pennsylvania School of Medicine.  Based on their estimates and studies, they say that at least 15% of acne cases treated fall into this particular category.  It involves physical irritations and is also called stress acne.  It is common among those who are already prone to acne problems anyway. 

They find that some contributing factors include bras, turtlenecks, and tight fitting clothes in general.  It can be caused among athletes by a chin strap, head gear, football pads, tight shirts, or even in the backs of buses, the chairs specifically.  If it is limited to a certain area of the body, chances are you might want to check your clothes and other accessories to see if they could be contributing. 

It can also be found in truck drivers as they often have greater contact with their seat cushions for extended periods of time.  Skin generally feels rough and bumpy and then becomes inflamed with pimples.  It is generally respondent to treatments such as salicylic acid, benzoyl peroxide, tea tree oil, the basic treatments that are used for other types essentially speaking.  Just make sure to do it regularly for prevention and treatment.