What I have had problems with are my feet. When you think about it, it makes a lot of sense. Most studies report a force of 3x body weight on the feet during a run. That means for this 125 pound runner, my feet absorb 375 pounds with every foot strike. So it's no wonder that my feet sometimes beg for mercy! Actually, it's amazing that they let me run any sort of distance.
Since we ask so much of our feet, it's important that we take good care of them. That includes proper footwear. I've got some experience here, having had more than my share of issues with my feet over the past couple of years!