Lie on your back with your legs straight up, perpendicular to the floor.
Bring your elbows out to your sides and your arms to your chest, making a fist with each hand.
Keeping your foot flat and your core engaged, slowly lower your left leg toward the mat, pausing a few inches before your heel would touch the floor. Make sure your lower back stays anchored to the mat.
Press your elbows into the mat and squeeze your core as you raise the leg back to starting position.
Repeat with your right leg, and continue alternating sides for 40 seconds.