Stand facing a wall, with the toes of one foot about an inch away from the wall and your hands against the wall for support.
Place all your weight in the heel of your front foot and then gently try and touch your knee towards the wall. If you can easily touch, shift your foot back by another inch and repeat the movement until you find a position in which you cannot keep the weight in your heel any longer.
Move forward slightly so that you can touch again and then repeat the movement from here for said number of reps.
