Stand and rest one heel on a low step or box in front of you, leg straight, toes pointed up. Keep a soft bend in your standing leg and hinge forward from your hips, keeping your back long rather than rounding it to reach further. You'll feel this along the back of the extended leg, from behind the knee up toward your glute. Keep your hips square to the front rather than letting the one on the extended-leg side roll open, which can trade hamstring stretch for a twist through your lower back. Lower the height of what you're resting on if your hips can't stay level.