If you were to only do one exercise, this would be my pick. In fact, a few years ago I wrote an article for the Bangor Daily News about it, and you don’t even have to go to the gym to do it. You can do this exercise in the comfort of your living room, your kitchen, or even at work.
It’s sitting down and then standing up, repeatedly.
Here’s a video that shows how to do it. It’s not hard, trust me. But if you don’t keep doing it, it gets more difficult and eventually you might even need some help to get it done.
Gym rats will recognize the exercise as the basis of a “box squat.” If you want more information, check out this article.