A: Your knees! High impact exercises are very important for giving you bone density, and muscle tone, but you need to be concerned about your knees.
What I do in class to help students:
1) I do leg toning exercises, w/ all of the leg muscles. the reason is, when you are dancing, sometimes certain leg muscles become stronger than others creating muscle imbalances. Muscle imabalances in your legs are rough on your knees because when one gets stronger than the other, it can pull the knee toward the stronger muscle and create all kinds of complications with your knees.
2) I dont do the same leg toning exercise every class. The reason is, the more you do the same exercise the less effective it is. Your body adapts it it... so i vary the exercises and work the muscle groups a bit differently on a monthly basis.
3) I preform at least 2 quadriceps stretches after class. The quads tend to get a bit tight and pull the knees up toward them. Ever since i have been doing this, i have noticed a HUGE difference in my knees.