Stand up straight facing away from a wall, with a chair to your side for support.
Position a large stability ball against the wall and rest it against your calfs just above your ankles.
Move your weight onto one leg.
Tighten your thigh and buttock muscles, pushing your knee back into the ball to lock your leg straight.
Next, bend your knee, moving your hips and body backwards.
Use the chair for support as needed.
Continue until your thigh is close to horizontal.
It should look as though you are sitting up straight in a chair.
Ensure your knee has not travelled forwards at all.
Steadily return to the upright position and repeat.