Steroids?

then increase the bodys threshold, preformance, muscle mass etc to be honest most sports use them deep down but they will not come out and tell you this because the sporting body and anti doping body says you can take under a certain limit then your not a cheat and if you take over a certain limit then you are classed as a cheat. cycling which is far less now and very rare to find dope cheaters, atheltics is most common now, football. can effect your body in many different ways some is yet to discover over time but mostly changes in physical appearance such as break out of spots or rashes etc and mentally your personality and mind will change and you could have split personalities or be aggressive and most common is a change in testosterone. type steroids into wikipedia it will give better info :D