The sport of mountain biking is one of the best active sports you can do for a number of fitness reasons and depending on what style of mountain biking you partake in, it can be also a great deal of fun as well as keeping you fit at the same time. People are usually unaware of how may muscles you use when riding a mountain bike, and how quickly you can burn calories off riding a bike as it can be an extremely good work out for the body. If you plan on riding cross-country along different styles of trails, riding a bike can be quite a task. So if you riding over a vast distance up and down hills, along slippery single trail it can be very exhausting on the legs and the lower back. So if you are just starting to ride bikes for the first time, please don't just jump on the bike a go for a 20 mile ride as it may do you more harm than good. One thing that is recommended is to do short rides to start with, maybe only a couple of miles which will take no time at all on a bike to get your fitness training up to scratch before hitting the bike ride.