If you've had a knee injury or surgery, step-ups will probably feature at some point in your rehabilitation.
Step-ups are an excellent exercise, which have the added bonus that they can be performed at home by anyone with a set of stairs of even a single step.
Indeed, so effective are step-ups that they are not just used in rehab - they are also used by sportsmen as diverse as bodybuilders and runners in order to strengthen the muscles around the knee.
Performing this exercise is easy. Using a step approximately 12" (30 cm) high, step up with your right leg, using the muscles of the leg on the step to lift your weight onto the step - don't push off the floor with the opposite leg.
Pause with both legs on the step, then step down under control. You can choose to either alternate which foot you step with, or perform a set with one leg, and then a set with the other.
Perform three sets of ten to begin with, and if this proves to be too easy, you can either increase the number of repetitions (up to 15 or 20), or add some light hand weights.