It's very important to do legs swings BEFORE you run. This quick and simple exercise serves two purposes:

1. It loosens up the hip and surrounding soft-tissue, warming up the hamstring and glutes. This preventative measure facilitates proper functioning mechanics which in turn helps to eliminate overuse injuries. One example of this would be if your hamstrings/glutes were tight from a previous day's workout causing a shortened or imbalanced extension (typically, the leg coming in) on one or both sides of your stride, which overtime, could result with medial-side knee pain.  Pre-run leg swings preventatively help to facilitate a more proper functioning extension of the leg once running.

2. Practicing leg swings consistently over time, increases greater mobility and flexibility, which is crucial to running your best and staying healthy. 

The video below demonstrates how to do this very smart (and quick) exercise. I recommend doing 10-15 swings slowly increasing your range of motion and intensity.