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Eagles backup quarterback Nick Foles and New England Patriots quarterback Tom Brady meet during a interview before Super Bowl LII last year.

Eagles backup quarterback Nick Foles and New England Patriots quarterback Tom Brady meet during a interview before Super Bowl LII last year.

Nick Foles is less agile than Carson Wentz, and his arm is not as strong - two reasons football pundits cite for why the beloved backup quarterback will not be on the Eagles' roster next season.

A third difference between the two does not take an expert to appreciate. At age 30, Foles is four years older.

True, Tom Brady is more than a decade older than Foles, and he just scored his sixth Super Bowl win with the New England Patriots. Then there is tennis great Martina Navratilova, who won the 2006 U.S. Open mixed doubles title a few weeks before turning 50. And what of Ed Whitlock, who, at age 73, ran a marathon in 2:54:49?

Notwithstanding those elite performances, experts in human physiology say that certain physical traits start to slide even before age 30.

An athlete can compensate for some of the decline with training, expertise, and, in a team sport, support from teammates, said Angela D. Smith, past president of the American College of Sports Medicine and an honorary professor at Thomas Jefferson University. Think of the veteran baseball slugger who is good at guessing what type of pitch is coming. Or the aging quarterback who gets an extra split-second to throw the ball on every play because he is protected by skilled offensive linemen.

But ultimately, the race against time is one we all are going to lose. The body becomes less adept at converting oxygen into energy. It needs more time to recover from injury. Muscle mass is harder to maintain. Eventually the brain - the quarterback of our biological functions - does not crackle with the same youthful quickness.

"Something happens to people in their 30s," said Michael Joyner, a physiologist and anesthesiologist at the Mayo Clinic. "If they train really hard, it will be their later 30s."

Determining exactly when each physical trait starts to go is tricky.

The type of study needed to nail down the answers would be long and expensive. Scientists would have to follow a group of athletes for decades, comparing individuals' performances later in life with those from when they were younger. Training and diet would have to be kept constant.

Instead, some researchers have studied age-related physical decline with a snapshot approach, measuring a specific trait across a large group of younger and older people at one moment in time. Others have analyzed record times in running or swimming races for different age groups, reasoning that those are a good indicator of the best possible performance at each age.

While each approach has its limitations, researchers have a good idea of how age affects these key aspects of athletic performance:

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In a 2006 study, psychologists asked more than 7,000 adults of all ages to press computer keys immediately upon seeing a signal on a small display screen. In one experiment, repeated 20 times for each person, participants were told simply to press the zero key as soon as that number flashed on the screen. In another experiment, they had to make a choice - pressing 1, 2, 3, or 4 to match whichever of those numbers appeared on the display.

"Simple" reaction time - just pressing the zero when that was the only option - stayed fairly constant up until age 50, on average.

But four-choice reaction time began to decline much earlier. The average 30-year-old was two hundredths of a second slower than the average 20-year-old, reported the researchers from the University of Glasgow and the University of Edinburgh. And by age 40, average four-choice reaction time had slowed an additional three-hundredths of a second. That is a meaningful decline in pro sports, where it could spell the difference between say, a quarterback getting rid of the ball or getting sacked.

The faster decay in "choice" reaction time makes sense, said Jayatri Das, chief bioscientist at the Franklin Institute, where visitors can test their reaction time in the SportsZone exhibit. The speed at which a person can choose from among the four numbers would depend on how fast they can process that extra step in the brain's prefrontal cortex, and processing speed declines with age, she said. This is thought to be due partly to a decline in myelin - the insulation-like sheaths that coat our nerve cells.

Simple reaction time, on the other hand, requires little "cognitive load," the Scottish researchers wrote. Age-related decline in this task does not become apparent until much later.

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Several factors determine an athlete's ability to keep chugging along. The good news is that one of them, exercise economy, stays fairly constant well into middle age, said Douglas R. Seals, a professor of integrative physiology at the University of Colorado Boulder. Loosely related to efficiency, it is defined as the rate at which a person consumes oxygen when exercising below peak level.

A bigger factor is what hardcore workout types know as VO2 max: the maximum amount of oxygen the body can deliver to the muscles and use as energy. Age is less forgiving in that respect, with peak performance starting to slide by the mid-30s.

At rest, an average man in his 20s uses less than 300 milliliters of oxygen per minute, said the Mayo Clinic's Joyner - less than the amount of fluid in a can of soda. During heavy exercise, he can crank that up by a factor of 10, reaching 3 liters per minute.

"Elite athletes can get to six liters per minute," Joyner said. "The rowers can get to seven or eight."

But by our mid-30s, try as we might, humans cannot squeeze as much energy from each lungful of air. Oxygen delivery is the job of the heart, and peak performance starts to decline in early adulthood, said Seals.

That is the result of an age-related double whammy, Seals said. The heart becomes stiffer with age, meaning it takes in less blood with each beat, and also it beats more slowly. A good rule of thumb: to determine a person's maximum heart rate, subtract age from the number 220.

Finally, when that blood gets to the arms and legs, the muscle cells of older people generally have fewer mitochondria - the tiny rod-shaped structures that play a key role in converting oxygen and nutrients into energy.

In 2005, data from the Baltimore Longitudinal Study of Aging revealed that maximum capacity for using oxygen declines by 5 to 6 percent during a person's 30s and another 10 to 11 percent during the 40s. It declines even faster every decade after that.

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With sustained exercise, people can maintain a good deal of muscle mass well into middle age. Yet at some point before then, the size of "fast-twitch" muscle fibers starts to decline, along with explosive power and sprinting speed, said John P. McCarthy, an exercise physiology researcher at the University of Alabama at Birmingham.

But researchers agree more study is needed here.

Despite the decline in fast-twitch fiber size relative to the "slow-twitch" variety, endurance runners seem to slow more with age than their short-distance counterparts.

That finding may be misleading, as it comes from studies of record-setting performances in masters-level events. Whereas millions of older runners compete each year in 5k and 10k races, sprint competitions are relatively scarce, so there are not many points of comparison.

"There just aren't as many opportunities to compete," the Mayo Clinic's Joyner said.

The few aging sprinters who do compete are likely to be outliers on the spectrum of human ability, further skewing the results.

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In sports that depend heavily on technique, skilled athletes can maintain their moves for decades, said Smith, the past president of the sports medicine society.

In figure skating, for example, former Olympian Brian Boitano, now 55, regularly posts clips of his advanced jumps on Instagram.

Smith herself competed as a masters-level figure skater until two years ago, at age 63, often beating competitors who were decades younger.

But when older athletes fall, they can be more prone to injury and need more time to heal. Smith decided it was time to quit, citing her difficulty in recovering from a broken arm at 62.

"Everything just took a lot longer, and was a lot harder," she said. "And to go at the speed I enjoy, doing the kinds of things I like to do, it wasn't safe."

Could Foles compete for anywhere near that long? What about Wentz, who lost time with a back injury last season and a knee injury before that? Or the seemingly ageless Brady? Quarterbacks are now safer from injury due to changes in the rules, and orthopedic surgery has advanced dramatically in the past few decades. Injuries that might have meant spending months in a cast are now addressed with minimally invasive techniques.

Some athletes also have tried various performance enhancers, legal and illegal.

Still, the odds are against extending one's competitive life for long.

The oldest quarterback to start an NFL game was the Atlanta Falcons' Steve DeBerg, at age 44, just three years older than Brady is now.

"Steve DeBerg is 44 years old and trying to play in a sport where old guys are swallowed up like raw meat in a zoo," the New York Times wrote at the time, in 1998. "That creaking noise you heard yesterday was DeBerg's knees when he walked onto the field."

The end result was not pretty, with the aging quarterback completing nine out of 20 passes for 117 yards and one interception. He also gave up a fumble that was returned for a touchdown, and the Falcons lost, 28-3.

The scoreboard said the game was won by the New York Jets. But the real victor may have been Father Time.

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