Running is the most elemental sport. The equipment is simple: shoes, socks, shorts, shirt. The activity is natural. We once ran after antelopes on the savannah, and we now run around playgrounds as kids. For the most part, we compete against ourselves. And because it’s so personal, and so elemental, the inevitable decline that comes with age can be wrenching. Aging reduces our performance at everything athletic, but sometimes it’s hard to make out what’s happening. The ball doesn’t seem to go quite as far; the racket or the bat doesn’t swing quite as fast…….