Lie supine (on your back), with your hips and knees flexed at 90-degree angles as if you are sitting on a chair.
Position your arms out to the sides for stability.
Execution
Keeping your shoulders in contact with the floor, slowly lower your legs to one side until you feel a mild stretch in your lower back.
Reverse the motion to raise your legs back to the starting position and lower them to the opposite side.
Repeat.
Exhale as your legs rise, and inhale as they descend.
Comments and tips
To avoid injuring your back, keep the movement slow and controlled. Do not allow momentum to build up.
Keep your hips and knees flexed at 90-degree angles.
The lying bent-knee oblique twist can be performed with your calves resting on an exercise ball, in which case the ball will roll and guide your legs down in a smooth circular motion.
Make the lying bent-knee oblique twist easier by keeping your feet on the floor.
Make the exercise more difficult by holding a stability ball between your bent legs (see second video). You can also just straighten your legs (lying oblique twist; see third video).
