A lot of people who get hamstring tightness and pain while running are not able to hold a stable trunk and pelvic position while running. I agree with Kaitlin and having your positions checked while you are running. If you are constantly running with your lower back hyperextended, it will give you the feeling of hamstring tightness. That rope between your hamstrings is probably your sciatic nerve and I would avoid it with the lacrosse ball.