Lie on your back on the floor / mat in a bent-knee position with feet placed firmly on the floor, arms outstretched to your sides in a "T" position; palms facing upward. Engage your abdominal muscles to stabilize your spine (maintain the small natural curve in your low back). Pull your shoulders back and down without arching your spine. Try to hold this posture throughout the exercise.
Gently exhale, deepen the abdominals and slowly lift one foot off the floor. Keep the knee bent at 90-degrees. Do not straighten the knee. Continue moving until the thigh reaches or comes close to a position where it aligns vertically to the floor. DO NOT go beyond this position. Hold this position for 5 - 10 seconds. With control, slowly return your leg to the floor, completing 2 - 4 repetitions and repeating with the opposite leg.
The goal of this exercise is to control the position of your low back and pelvis during leg movements. Movement of the thigh past vertical will flatten your back, thus changing the position of your low back. If necessary, place one hand or a rolled up towel under the small of your back to monitor any change in the position of your low back during the movement.
Reference: ACE Fitness Programs