Your physiotherapist may wish you to perform your exercises eccentrically. A muscle is worked eccentrically when it contracts and lengthens simultaneously.
To perform this exercise, stand on the bottom step of your stairs. Place your feet so that the balls of your feet are on the step, with your heels off the step. Balance by holding onto the stair rail.
Push up onto the balls of both feet. From this position take the weight solely onto the leg to be exercised and slowly lower your heel down to below the level of the step.