Lunges are a powerful exercise for a variety of reasons, not the least of which is they hurt. And why do they hurt? Because they involve almost all the muscle fibers in the lower body at the same time, a rare thing in most exercises. What makes them even more awesome is the variety with which you can do them.
My clients and readers might replace the word 'awesome' with something I can't print. I understand, y'all.
There are plenty of modifications for people who need to go easier on their joints and such, but there are also ways to make them harder. One way to do that: Add a jump.
Adding a jump makes almost any exercise more aerobic, but adding it to your lunge will also help build both strength and power in the lower body. Plus, that little jump will help you burn more calories, so you get a lot from that one little exercise. The key is to, first, land softly. You have to concentrate to do this but, when you do, you'll find that your core kicks in and takes some of the impact off the joints, which leads me to the second key: Absorbing the impact with your muscles, rather than your joints.
Of course, you can always skip them altogether and go have a Starbucks. I hear their lattes are pretty good.
Do it right: Begin in a split stance, feet about 3 feet apart with the right leg forward, left leg back. Bend the knees into a lunge, going as low as you can (but not past 90-degree angles in the knees) and and spring up as high as you can. Land softly on the balls of your feet and go back into your lunge, repeating for 1-3 sets of 8-16 reps on each side.