The Front Split Stretch is by far one of the best stretches for the muscles at the back of the thigh. This stretch greatly aids your general flexibility, and is also a good option for you if you experience any posture-related back pain. If you are a runner, get this into your regular routine as this stretch will decrease your chances of getting injured. When performing the stretch, see if you can position yourself to feel the stretch right in the muscles located at the back of the thigh and not behind your knee. A good tip for this is poking your bottom backwards whilst you are in position.