You ahve no doubt seen soemone do this simpole move at the gym. Get a step, or up to three, and put one foot on it and the other on the gorund. Keep the knee of the leg on the step(s) slightly bent. Step on and off the step with the other leg. Do for five sets of ten repetitions. Go to the other end of the steps) and switch legs.
The more the steps, the harder the exercise. If you want to make it even harder, grab free weights, and rest one on the moving leg (while holding it).
Disclaimer: None of the above information can be taken as a substitute for advice from a medical professional such as a physician.
My third book, Pocket Guide to Fitness, is available on www.louizapatsis.com, http://www.authorhouse.com, www.bn.com and http://www.amazon.com. If you look up my name on those Web sites, you will find my other books The Boy in a Wheelchair and Life, Work and Play: Poems and Short Stories.