Chances are your hamstrings are tight. So tight that they are limiting your progress or causing you pain. Weighted stretching – using a small load to assist in the stretch – has been around for a long time. It is the safest and most effective way to safely increase range of motion in adults.
New range of motion is weak range of motion. The addition of a small weight with your stretch will ensure that your new flexibility is accompanied by the strength to back it up.