To many of you, running is a unique, complete and irreplaceable sport: better, it's sport itself. To many others, running isn't enough. Once you've overcome the overpowering initial passion, it's good to alternate running with other sport activity. This is how cross training was born.
Cross training helps to boost motivation, but also to increase body mass and those specific muscle which running do not stimulates, to favor recovery and - to cap it all- to complete your overall fitness condition.