A warm up is used to get your body ready for a running session, and to try to reduce the risk of injury. Warm ups can differ according to your level of running and whether it is prior to training or an event. If you are going to complete a training run, then all you need to do is set off gently allowing your muscles to warm up in the early part of the run. For the more experienced runner, before training sessions and races you should complete a short jog then a few light stretches and some 100m strides at a quicker pace. You should practice these strides in a training situation don't try them out for the first time before a race.