Attach a resistance band to a door anchor. Place the door anchor at the bottom of a door. Connect the other end of the band to a D-ring on an ankle strap. Do the same for a pair of resistance bands. Lie on your back with hips and knees bent 90° and hands at your temples.
Bend one knee towards your head while straightening the other. Use abdominal muscles to tilt your upper body by bringing your opposite elbow to the raised knee. Continue by alternating your other knee.
The bicycle crunch is one of the most effective ab exercise according to the American Council on Exercise.