Do you mean pre or post run stretching? I'm a believer that the latter isn't especially useful and that the former, at best, also does little, but can often increase the risk of injury. I don't run much these days but I used to go a fair bit. Things improved lots when I stopped stretching beforehand. Now I just do what I do before deadlifts/squats which is bodyweight squats, cossack lunges and leg swings followed by warm up sets (in the case of running that'd be brisk walking for a a couple of minutes, old man jogging for a couple and then running).