People who remain physically active not only live longer, they also die faster.

That might not sound good, but it is — unless you like to suffer. The phenomenon, called "compressed morbidity," means that when you finally go downhill, you go fast. The period of sickness is shorter than for people who aren't physically active, who are more likely to have long-lasting health issues leading up to their deaths.

"People get into middle age, and their health begins to ratchet down, often due to specific diseases that frequently occur due to poor lifestyle: hypertension, diabetes, obesity and coronary artery disease," said Dr. Mike Joyner, a physician-researcher and expert in exercise physiology at the Mayo Clinic. Being sedentary is a common cause for a lot of these diseases.

"Then what happens is that their physiological function goes down and they get progressively more disabled," Joyner said. "By the time they're in their 60s and 70s, they've become frail and have this long period of reduced functionality before death. Seventy percent of 70-year-olds can't get up off the floor without grabbing onto something."

He explained that the biggest predictors of five-year life expectancy include grip strength, self-selected walking speed and how fast you get out of a chair. Weak and slow equal get your affairs in order.

"How early you start and how many years you keep exercising is probably more important than amount," said Dr. Jim Fries, professor emeritus at Stanford University. As a pioneer in the subject of staying spry, he practices what he preaches. "More exercise is better, but there are diminishing returns. And starting young is better than starting at 50, but starting at 50 is better than nothing."

Joyner, who once ran a lightning-fast marathon (2 hours and 25 minutes), was more optimistic about the later-life start. He explained that a solid effort in middle age could change your physiology to make it akin to a lifetime exerciser.

It's not just aging that exercise delays — but death.

"Exercise adds 16 years in terms of postponing morbidity and nine years postponing mortality," Fries said, referencing a 2012 research article he wrote for Current Gerontology and Geriatrics Research.

Fries is a fan of running because it trains the entire body and keeps organ function high. It's also good for preventing cognitive decline. He is cautious, however, about committing to any specific prescription of exercise in terms of type, time or intensity, saying there isn't enough research to support "fine-grain" recommendations. Instead, he advises, "Start as early as you can and do as much as you can. Also, do something you like."

Don't quit strength training

Joyner is more prescriptive in his exercise recommendations, suggesting a combination of aerobic exercise, like running or cycling, but also advocating exercises that build strength, like lifting weights.

"You've got to do both," he said. "Aerobic is more important in young and middle age, but older people don't need a lot of aerobic capacity to go about their daily tasks. Seniors are more limited by musculoskeletal frailty."

Resistance training improves bone strength and prevents falls, but it also makes it easier for seniors to look after themselves and stay out of the nursing home by being able to carry their own groceries and laundry basket or open their own jars.

"Resistance training is not just for those meathead guys," said Dr. Spencer Nadolsky, a bodybuilding family physician in Suffolk, Va. "My older patients who lift weights do a lot better at being independent."

And don't worry about having to join a health club.

"People are making fitness way too complicated," Joyner said. "I don't have a snowblower for a reason. Mother nature gives you a free workout."