Stretching the hip flexors to help you run faster has been well-established. Greater hip joint flexibility allows you to have a longer stride and a greater range of motion over which to drive the knee forward from a behind-the-back thigh position. As a result, this key action can increase your running speed significantly.
However, if you watch players doing a hip flexor stretch, you will see many variants, some effective, some not very effective. For example, some players advocate that you should assume a semi-lunge stance and bend your back leg’s knee. However, the more you bend the knee as you go into the squat, the less of a stretch there is on the hip flexors!
Also recommended is that you tilt the pelvis forward to flatten the lower back and stretch the front of the hip. However, tilting the pelvis forward does not flatten the back, it creates an arch in the back. Tilting the pelvis forward, however, is good as this occurs in the running stride! But it also gives more slack to the hip flexors so that you have to go in to a more extreme position to get an effective stretch. This requires that you do the classic long lunge.
If you do the full classic lunge, not the typical split squat that many players call a lunge, you take a long stride and keep the rear leg straight but relaxed as you go into the lunge position. If you keep the trunk erect it produces a strong stretch of the hip flexors. The classic lunge, as once used in the sport of weightlifting, is also great for balance (especially with a weight held overhead) and almost duplicates the airborne position when sprinting. Thus, this stretch has a very close relationship to improving your running speed. For more details, see Explosive Running or Kinesiology of Exercise for exercise execution.