Start by sitting on a bench grabbing it under your legs, lean backwards as far as possible. Lift one leg and extend it out, Lift the other leg and bend your knee towards your chest, switch legs rapidly. Make sure that legs are fully extended at the end of the each move, hold the soles of your feet perpendicular to your fully extended leg to work calf muscles, keep your core tight. Harder variation is to let go of the bench, balancing on your pelvic bone and placing your hands lightly on the sides of your head, twist through your core so the opposite arm comes towards the raised knee. You don’t need to touch elbow to knee, instead focus on moving through your core as you turn your torso. Lower your leg and arm at the same time while bringing up the opposite two limbs to mirror the movement.