At 31 years old, old man Michael Phelps became the oldest man to win a gold medal in an individual Olympics swimming event, giving those of us who are only slightly less old hope that our bodies are not yet decrepit.

After watching Phelps return to glory in the 200 meter butterfly earlier this week, I wondered how he continues to compete at such a high level. Phelps is unparalleled in the history of swimming, but the average age of the US Olympic swimming team is just 23.2 years old. How has Phelps (and fellow old man Ryan Lochte, who is now 32) maintained his skills so far into his career? And is it even remotely possible that Phelps could race four years from now in Tokyo?

There’s a treasure trove of scientific studies that have looked at how swimmers age, and Phelps’s success at 31—while unlikely to become the norm for Olympic-class athletes—actually follows a trend scientists have been seeing over the past 20 years.

A 2014 study published in SpringerPlus found that between 1992 and 2012, the age of Olympic finalists increased in nearly every single swimming event. Usually the effect was relatively small—an increase from 22 years old to 23 years old, for instance. But in some cases, it was larger: The average age of the women’s 50 meter freestyle competitors went from 21 years old in 1992 to 27 years old in 2012. Standard deviations from the mean age increased as well, meaning we’re seeing a wider age range in Olympic swimmers.

Why haven’t we seen gold medalists in their early or even late 30s before now? Mullen has a theory: Swimmers simply burn out by the time they hit their late 20s.

No one is arguing that Phelps is a better swimmer now than he was in 2008 in Beijing, when he won a record eight gold medals. His times show that Phelps has indeed slowed down a little over the years: His split in the 4x200m freestyle relay was 1:43.31 in 2008, 1:44.05 in 2012, and was just 1:45.26 this year in Rio. Team USA won gold each time.

In the 4x100m freestyle relay, he swam a 47.51 in 2008, a 47.15 in 2012 (a silver medal effort), and a 47.51 this year. In the 200 meter individual butterfly, Phelps swam a 1:52.03, “choked away” the lead to Chad LeClos with a 1:53.01 in 2012, but won the gold this year with a 1:53.36, a time that’s slower than his 2012 loss.

Judging a swimmer’s overall fitness and skill using relay events (Team USA had essentially won the 200 meter freestyle relay by the time Phelps entered the pool this year) and a couple individual events isn’t really ideal, and there are of course all sorts of variables we can’t account for. Phelps swam multiple races in one day earlier this week, and there are “pool effects” that can affect the times of all swimmers (early word is that Rio has a relatively fast pool).

The most definitive thing we can say now—before advanced statisticians and analytical scientists take over after the Olympics are over—is that Phelps is still very, very good at swimming. But he is definitively a different swimmer now than he was 4, 8, or 12 years ago.

At the Athens Olympics in 2004, Phelps was primarily an endurance swimmer: “He was still physically maturing, so his strength and speed were not fully developed. This is why he was mostly a 200 meter and longer swimmer,” John Mullen, founder of, told me.

In Beijing in 2008, Phelps destroyed the world—he was at his peak physically and training-wise, which is why he went undefeated there and set a number of world records. In London in 2012, Phelps won four golds, two silvers, and actually finished fourth in one event—a down year for him.

“This was apparently a point when his training was at its lowest,” Mullen said. “Therefore, he was not in high enough aerobic shape, resulting in poor finishes in the longer events such as the 400 meter individual medley and 200 meter butterfly.”

Overall swimming performance peaks at around 24 for men and 22 for women, according to a study published in the European Journal of Sport Science, but interestingly there doesn’t necessarily seem to be an obvious physiological explanation for this. Studies have shown that arm strength doesn’t really start to decline until men and women hit about 40 years old and, notably, swimmers don’t experience the sorts of impact injuries you might see in land sports.

"The longer a swimmer competes, the more likely they can acquire skills and improve their biomechanics. Biomechanics are the main determinant of swimming success, making this so important."

Even if there were to be a mild decline in power throughout a swimmer’s 30s, “power” isn’t the most important thing when it comes to swimming very fast. A 1982 study found that out-of-pool performance on things like bench and leg presses is less correlated with in-pool performance among elite swimmers. A 1992 study showed that the greatest predictor of male sprint swimming speed is “stroke index,” which is a biomechanical measure of a swimmer’s technique.

This is why a study in Experimental Aging Research showed that, as athletes age, “performances in running, swimming, and walking were reasonably well maintained,” while cyclists, triathletes, and athletes involved in jumping events seem to drop off a cliff after they leave their peak physical years. A few different longitudinal studies have suggested that after the late 20s, swimming performance drops off by something like .6 percent annually, but that effect is less pronounced in elite swimmers.

“Strength and power and maintained or improved throughout the mid to late twenties, maybe even early thirties,” Mullen said. “The longer a swimmer competes, the more likely they can acquire skills and improve their biomechanics. Biomechanics are the main determinant of swimming success, making this so important.”

We’re likely seeing a Phelps who has rededicated himself to his craft after a less-than-perfect showing in London.

But why haven’t we seen gold medalists in their early or even late 30s before now? Mullen has a theory: Swimmers simply burn out by the time they hit their late 20s. Swimming performance has been very closely tied to a high training volume, which both allows the best swimmers to maintain near-perfect biomechanics and, more importantly, better lung and cardiovascular capacity.

Phelps notoriously spends upwards of six hours a day in the pool; anything less than that and his performance would likely fall off. It seems at least plausible that Phelps was feeling burnt out in the leadup to the 2012 Olympics, considering that he retired immediately following them.

“It is difficult to maintain the high volume of training throughout a swimming career,” Mullen said. “The high volume of swimming makes burnout high and decreases the ability to maintain a high training volume. This results in a lower aerobic capacity.”

Phelps has said multiple times that Rio will be his last Olympics, but in a news conference earlier this week he noted that it was his “potential” last Olympics, and left open a slim possibility that he’d return in Tokyo. Lochte said that if the 2024 Olympics are hosted in Los Angeles, he wouldn’t be surprised to see Phelps return as he’s pushing 40. Whether he’s able to do it might come down to whether his heart’s truly in it—not whether or not his body can hold up.