Nordic Hamstrings Curl
Find a partner to secure your legs or tuck your feet under under a low bar. Brace your abdominals and tighten your glutes, then lower your body slowly down to the ground. Catch yourself with your arms, then push yourself off the ground, while keeping your spine in a straight line. Do not break from the hips or stick your butt in the air on the return to the starting position.
At COR, we call this exercise the "cramper". It puts high stress on the calves and hamstrings, often resulting in a cramp. Read more about the cause of cramps here and if you get a cramp, stand-up, walk around, and loosen up the legs.