Most people are unaware that a large percentage of their upper body muscles are connected to a large part of their lower body muscles. That is why there are many variations of sprints and agility drills. Many different people do sprints and many people also perform agility drills. This page will show you many different types of stretches and exercises for your lower body and upper body. Many people who perform sprints and agility drills for the lower body will also do some squats and deadlifts. The squat will add flexibility to the lower leg while the deadlift will help strengthen the hip flexors.
I recommend doing your squat as many times as you can. If you are going to do a lot of squats, then you might as well start with the basic squat. I also recommend that you use a good pair of high heels so you can keep your leg straight during the squat. Squat is one of the easiest exercises for the lower body, so don't worry too much about getting good form. I use two versions of this exercise - the traditional version and a new version with a ball on the floor. Here is a link to the basic squat.