The endocrine system is responsible for producing and releasing hormones into the body which in turn regulate MANY of your body's daily processes. The most useful in sports would likely be anything involved with muscle growth (testosterone) and Adrenaline. Adrenaline activates the sympathetic nervous system, which prepares you for very rapid energy use - i.e. easier to run for a long period of time, heightened awareness... basically lots of adrenaline = incredible hulk.
