This may kill you. It almost did me. This goes to show that, no matter how strong your muscles are, if you vary something and use different muscle fibers and angles, you will get a good workout.
A trainer who does not blog, but to whom I will give credit to as soon as he emails me, did this in his boot camp routine. Kneel on a mat. Have a folded towel before you. Place your palms on it, shoulder-width apart. Rest on your shins and balls of your feet. With a straight head, neck and back, go forward slowly, utilizing your abdominal muscles, and come back to starting position. Do at least one set of ten repetitions, and work up to as many sets as you can do in the coming weeks. If you do this even three times a week, it's great. Advanced Variation: Support yourself on the balls of your feet (and toes), and not your knees and shins.
Now lie on you back. Place your feet on the folded towel. Keep your back straight, with about an inch between your lower back and the mat. Keep the rest of your body relaxed. Bring the towel close to your glutes with your feet only while youlift your hips off the ground as far as you can. Ouch! This really works your hamstrings, especially close to your knees, and your glutes. Do at least one set of ten repetitions, and work up to as many sets as you can do in the coming weeks. If you do this even three times a week, it's great.
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.