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How Michael Phelps's body has changed over his five Olympic Games

Michael Phelps's body recovers much differently now than it did even in the London Olympics. SI speaks with an expert to gain greater insight.

At the 2016 Rio Olympics, Michael Phelps will be sixteen years older than the teenager who finished fifth in the 200m butterfly at the Sydney Olympics in 2000. While he won’t be the oldest member of the U.S. swim team (Anthony Ervin is 35), this will be his record fifth Games, and the second time he retires—he already retired once after London 2012. And to understand what difference all those years might make when it comes to his body, Sports Illustrated spoke to Anne Friedlander, an adjunct professor in human biology at Stanford University and former director of the mobility division at the Stanford Center for Longevity.

“His muscles haven’t changed that much, but enough at this level to have an impact,” Friedlander says. “Every little teeny change plays out in the hundredths of seconds that means the difference between someone who’s in first place and someone who’s an also-swam.”​

Phelps’s qualification times for Rio compare closely to those for London four years ago. His 100-meter butterfly was a fraction quicker (51.00 seconds compared to 51.14 seconds), his 200-meter butterfly and 200-meter individual medley both a fraction slower (1:54.84 to 1:53.65, and 1:55.91 to 1:54.84, respectively). But Phelps swam just three events at the 2016 U.S. Olympic Team Trials in Omaha, racing the 100- and 200-meter fly and the 200 individual medley, compared to five at the trials for London four years ago.

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At a press conference six days into the eight-day trials, Phelps explained that his rest and recovery strategy coming into the event hadn’t worked like it used to. “My legs haven’t felt like they normally feel during a taper at all, this whole meet,” he said. That day he had raced three events, the 100-meter fly preliminary and semifinal, and the 200-meter IM final. “I’m going to get a massage, I’m going to go get in a 50-degree ice tank and then I’m going to go home and pass out,” he said.

“For him, the biggest problem is recovery time,” Friedlander says. “When you’re young the response of your body, the inflammatory response, is much stronger and faster.”

Mechanical stress and chemicals produced during exercise can damage muscles and connective tissues. The inflammation and soreness that results is part of the healing process, and how the body remodels muscle to get stronger. This process also includes cells called satellite cells that are located on the outside of muscle fibers. When these stem cells are activated, they multiply and differentiate into muscle cells that are used to rebuild muscle tissue.

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“The satellite cells in muscle as you get older get less active, less responsive,” Friedlander says. “In young muscle there’s a lot of them and they respond really quickly. Over time you get slightly less of them and they’re less responsive.” Part of that change appears to be down to differences in certain chemicals in blood as someone ages. According to a paper by stem cell researchers Margaret Goodell and Thomas Rando, published in Science in December 2015, studies have shown “that stem cells are profoundly influenced by their environment and imply that blood-borne factors may be responsible for at least some of the age-associated declines in stem cell functionality.”

The solution to the slower recovery response, according to Friedlander, is to train smarter, with extra rest. “To do less volume training, more recovery time, maybe more interval-type work,” she says. “And that works well for individual events. The problem is that when you stack events together you lose that recovery time, and you haven’t been training with that volume.”

Moving events closer together means the slower recovery responses in an older athlete will begin to overlap, increasing the level of soreness and inflammation. After winning the 200-meter IM, Phelps had to get back in the water less than 30 minutes later for the 100-meter fly semi, and though he qualified for the final, he recorded just the sixth fastest time. “Tonight was brutal. That hurt. That 100 fly hurt,” Phelps said. “I haven’t done a double this close in a really long time.”

Besides a slower physiological recovery, Phelps also faces psychological pressures that may distract him away from training, signs of his new life after competition. He has a fiancé now, Nicole Johnson, and a three-month-old child, Boomer. The swim school where Phelps first overcame his fear of water is now named after him, and he has a foundation that promotes water safety and healthy living, which he started with a $1 million bonus he got from Speedo after Beijing 2008.

Reduced training volume can begin to cause physical changes within muscles, affecting the ratio of type I (endurance) and type II (power) fibers. “As people age there’s a selective loss of those type II muscle fibers,” Friedlander says. “You lose your power fibers and people lose their upper end strength.” The human body will remodel itself according to the challenges it faces. Lower intensity training, with a reduction in max power workouts, in part to protect connective tissues weakened by age and overuse, will favor type I fibers over type II.

Another change related to aging, according to Friedlander, is that people have fewer mitochondria. These are tiny structures found within cells that produce adenosine triphosphate (ATP), the main source of cellular energy during aerobic exercise. They also are believed to be responsible for removing lactate, a chemical produced during anaerobic exercise that causes weakness and fatigue. Over time, free radicals produced by mitochondria during energy production can lead to damage and mutation of the same structures.

“He might not be able to clear his lactate as readily,” Friedlander says, “he might not be able to repair his mitochondria or generate the energy that he needs.” According to Phelps, his blood lactate concentration after his 200-meter IM final and just before his 100-meter fly semifinal in Omaha was 5 mmol/L, five times what it would be in a person at rest. After the semifinal it had risen to 13 mmol/L.

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However, Friedlander says that overall, “the trajectory of aging is really different if you’re training or not training.” Phelps’s recovery abilities, muscle strength, and energy production will be significantly better than someone of the same age who is not working out; he will effectively be younger than his 31 years in Brazil. The problem is that his competitors are working out too, and so will be younger still.

Not everything related to aging is negative, though. Phelps will likely be a little wiser this time around, with four Games’ worth of experience, and many intervening years, to lean on. He’s faced his share of highs and lows. He’s twice been arrested for DUIs, lost a sponsorship deal with the Kellogg Company after being photographed smoking from a marijuana pipe, and spent time in rehab. He has said that he wasn’t happy at all at London 2012; that he hated swimming then. But a happier, more at ease, Phelps has come back for Rio.

According to the central governor theory proposed by exercise physiologist Tim Noakes in 1997, fatigue is a mental artifact rather than a physical process in muscles. The sensation of fatigue exists both to protect muscles from overexertion and to keep a reserve of effort in place for emergency situations. Perhaps a wiser Phelps can find a better way to gain access to that mental reserve than his competitors. And despite older muscles, maybe he might still have an edge.