As I watched the 800 meter finals I again took notice of how rigid the torso of the winner, Rudisha was. When I compared it with Nick Simmons there was a large difference. I should say that these are both great runners and I was rooting for Simmons but when looking at his for I always feel like there is a tightness to him, he runs in a less fluid manner than Rusisha. In the final turn of the race the runners gave it all they got and started pumping their arms HARD. This got me to thinking. If you are lacking in shoulder extention and internal rotation, might that inhibit you keeping a stable midline on your arm back swing? If you lack that flexibility your shoulder may very well bring your torso with you and break that tightness we strive for. Here is a mobility WOD from Kelly Starrett to work on your internal rotation, He relates it to the dip but I think it can improve your sprinting/running as well. First video is explanation and second video it the stretch. Check it don't wreck it.