I recently found an old tire and got out the sledge hammer and beat it up. I was not prepared for what happened. First, my entire torso, shoulders and arms were tired. Second, this was a hell of a conditioning exercise.
I first began by doing a couple of sets of 15 from each side. You can also do this for time or see how many strikes you can get in a certain amount of time. Highly recommended as a good way to get out of the weight room. It won't replace squatting, but it is certainly more fun than leg extensions.