Lie flat on your back, with one knee bent and foot flat on the floor. With the other leg straight, pull your toes towards you, push your heel away, and squeeze your thigh muscle to lock the knee straight. Then, tighten your core muscles, breath out and lift the straight leg up until its level with the other thigh. Slowly return to the start position, and just before your leg reaches the floor, begin the exercise again.

Try not to let the spine arch or pelvis twist when you lift the leg. Also ensure the knee stays locked straight throughout the whole movement - do not attempt if you have a quads lag following knee injury or surgery.