Start lying on a mat in a supine position (facing upward) with your knees bent and your feet flat on the mat. Your heels should be about a foot away from your glute muscles.
From this position, lower both legs to the right side of your body. Your right leg should be touching the ground, and your left leg should be stacked on top of the right. Your upper back and shoulders should be flat on the mat.
With your hands behind your head or crossed in front of your chest, engage your core (gently brace your abdominal muscles) and slowly lift your shoulders and back off the ground, performing a crunch.
With control, lower your body back down to the starting position. This counts as one rep.
Repeat for 10 to 30 seconds on each side, then take 30 seconds of rest.