When Bob Kaehler tried out for the U.S. Olympic Rowing Team in 2004, he wanted just once more to feel the elation of flying across the water at 32 feet per second, nine human bodies and a boat fused into a perfect expression of power, balance and timing.

He'd made the team in 1992, 1996 and 2000, but he knew this time he was up against long odds. He had a family, a business, not quite enough time and a 39-year-old body. In his favor were experience, technical skill and a thing called "boat-moving ability."

Porter Collins, left, Bob Kaehler, Douglas Burden and Fred Honebein train for the 1996 Olympics in Chula Vista, Calif. Kaehler made the team three times, but he couldn't do it a fourth time in 2004. (By Joel Rogers) Graphic Fighting to Keep Fit Fitness levels in elite oarsmen decreased significantly every 10 years after Olympic competition even though they continued exercising three to four times a week. Their efforts to remain fit may have slowed the physiological changes that come with age but did not exempt them from the steady downhill course. Aging Well Pulling Against Time My Time: At Any Age, You Need Somebody to Love On This Video Set, Seniors Run the Show Resuming the Job Hunt: Bracing for Job Re-entry Knee Osteoarthritis: Exercise Won't Make It Worse M E D I A S C A N: Brain Power Your Co-Workers Like McSteamy? We can help you find the right work environment with competitive benefits. Nursing, Allied Health: Get a New Job In Depth Success Stories Gain inspiration from Lean Plate Club readers who have lost weight ... and kept it off. AIDS/HIV Twenty-five years after the first diagnosis of the first case of AIDS, the world is waking up to the epidemic. Focus on Bird Flu Track the latest developments in the spread of the deadly H5N1 influenza virus. » MORE SPECIAL REPORTS Save & Share Article What's This? Digg Google del.icio.us Yahoo! Reddit Facebook

He didn't make it.

As he looks back, he says there were lots of reasons. His body was just one of them, and perhaps not even the biggest one. But things were different.

"It is hard to say where my physiology really was. It was not where it needed to be. It probably would never have been where it was in 1996," Kaehler, who is 42, said last week. "When you are older, you need to get back in the game sooner. It is doable. But I would have probably needed 18 months, not six months or eight months."

Athletic performance declines with age -- it's the one other thing that's inevitable besides death and taxes. But how does that happen? What is it that slips? And why is it that, sooner or later, when you try to roll the rock of physical conditioning up the hill, you can't get it as high as you once could?

Those are questions all world-class athletes ask when they unwillingly clean out their lockers for the last time. For others -- those who retire before they have to -- it comes later. For Mike Teti, the 50-year-old head coach of the U.S. men's rowing team and a three-time Olympian, it came all at once, on a day in 2000 he still remembers clearly.

The team had a rare day off, and Teti took his newspaper to a coffee shop in Princeton, N.J., to relax. He opened it and couldn't read the print. He went to the boathouse to work out on a power-measuring machine called an ergometer. He had the worst scores he'd ever seen. Getting dressed to go home, he noticed his pants were tight.

"For me, everything happened at once. Almost overnight. And you say, 'Oh my God, I'm over the hill.' "

Kaehler and Teti have long since come to terms with the fact they will never again be the athletes they were. Kaehler, who lives in Holland, Pa., is a physical therapist and runs a coaching business on the side called RedLine Maximum Fitness. ("As in redlining an engine," he says tellingly). Teti exercises to stay fit ("and to eat") and helps bring other rowers to the sweet spot where body, mind and opportunity can win races.

To understand why the decline of athletic performance is inevitable with aging -- and why it is partially reversible at any age -- requires a little knowledge of exercise physiology. (Don't worry, it's worth it.)

Sports that combine strength and endurance -- rowing is perhaps the best example -- are enterprises that in many ways come down to one basic task: finding a way to deliver the most oxygen to muscles as fast as possible.