I've owned a fine pair of New Balance 652's for 9 months, playing in them an average of 2 hours a week. They still look like they are in decent condition aside from a fabric tear in the left heel area. Lately though, my knees and legs have been stiff/sore, and I'm wondering if it's due to playing more tennis in these warmer months or whether it's because my sneakers have lost some support over time, or both. So... even though my sneakers look fine, and feel okay, is it possible they need to be replaced? How do you know aside from obvious wear and tear when a shoe is past its prime?