Here's an advanced variation of the lunge:
Lunge with your right leg forward and, after you stand in the starting position, bring up your right leg, still bent 90 degrees, over the floor, thigh parallel to the floor. Your back is straight. You can also hold this for three seconds. Now repeat after you lunge with your left leg forward. For a harder exercise, jump up after you thigh is parallel to the floor. Do one set of ten repetitions until you can do more.
For standard lunges, search this blog.
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.