Whether you played basketball, tennis, hockey or any sport that requires a constant pounding on the joints where bones connect...you know full well how they degrade over age. I can still hit the ball as hard, I still possess the hand eye stuff, the mind works on the court...it's the knees, the knees, the knees. Golf is a good alternative. Clay or Har-tru courts help for tennis. When you are in your 20's, it's tough to comprehend what people mean when they say "my knees have slowed me down"...I never understood what the problems was. Once you hit 30-35, the constant pounding on the joints has reared its ugly head and you know what the problem is.