There's no doubt about it. American runners, from 5K runners to marathoners, are slower than ever before. And it's not because of the reasons you might think.

A pair of Danish researchers recently took a deep dive into the race results at the four most popular race distances in the U.S. — 5K, 10K, half-marathon and marathon — for the years 1996 to 2016. Jens Jakob Andersen, a former elite runner and statistician with the Copenhagen Business School, and Ivanka Andreeva Nikolova, who holds a Ph.D. in mathematics, recently published their findings on the popular running website, RunRepeat.com. They looked at data from 34,680,750 results from 28,732 different races, making this the largest study ever conducted on American racers.

Andersen and Nikolova found that in every popular race distance from the 5K to the marathon, American runners are getting slower. In 1996, the average time for an American marathoner to finish the 26.2-mile race was four hours and fifteen minutes. In 2016, that average time crept up to 4:40.

Marathon Finish Times graph Finishing times for American marathoners having been steadily increasing from 1996 to 2016. (Photo: used with permission from RunRepeat.com)

Similar data was found for 5Ks, 10Ks and half-marathons. Elite racers seem to be the exception to this new norm. In 1996, the American marathon records were 2:10:04 for men and 2:21:21 for women. Today, those records stand at 2:04:58 for men (set by Ryan Hall in 2011) and 2:19:36 for women (set by Deena Kastor in 2006).

So what's the reason for the slowdown for average runners? Andersen and Nikolova couldn't say for sure, but one thing they could do was discount what many might think are the most common reasons that runners are slowing down, such as:

More women are racing than ever before

2017 Boston Marathon At the 2017 Boston Marathon, the holy grail of marathons, women comprised 46 percent of the field. (Photo: Byelikova Oksana/Shutterstock)

Yes, the number of female racers increased over the 20-year period for the study. In 1980, women made up only 10 percent of the field at marathons in the U.S. In 2016, that percentage jumped to 44 percent. And yes, women runners are around 10 percent slower on average than their male peers. But when Andersen and Nikolova analyzed the data by gender, they found that the increase in the number of female runners has done less to bring down the average times than the decrease in the speed of the men.

Slow runners are getting slower

The idea here is that maybe it's not all runners that are getting slower but just that those runners who tend to linger in the back of the race pack are slower than ever, and that's bringing down overall average times. To test this theory, Andersen and Nikolova compared average time for finishers in the 100th, 1,000th, 2,000th and 5,000th places at each race. They found that runners were slower at every level with women slowing down by around 9.87 percent throughout the study period and men slowing down by 9.94 percent.

More people are walking these races

As running events have become more inclusive, some experts have speculated that an increase in walkers in what used to be running-only events has caused a decrease in the overall average time. The RunRepeat study debunked this theory, too. The researchers found no distinct increase in the proportion of participants finishing with times comparable or slower than the average walking pace (19.5 minutes per mile).

So why so slow? On this, the study's authors are less clear. They point to a steady decline in the overall health of Americans as a potential culprit, citing that increases in rates for diabetes, obesity and hypertension coincide with the decline in running times. Of course, to truly flesh out this theory, Andersen and Nikolova would need to know the health histories of all 35 million runners whose races times were included in their analysis.

Another theory, presented by Amby Burfoot (the 1968 winner of the Boston Marathon and a longtime contributor and editor to Runner's World magazine):

Running events are less focused on times

Jenn Savedge with characters from Paws race The author, pausing to pose with some puppies while running for charity at the 2014 Disney Marathon. (Photo: Jenn Savedge)

If you've been to a running event lately, you were probably given more information about the race's T-shirts, medals and other bling than you were about the course or times to beat. That's because race directors have realized that the vast majority of participants want a running event that is more like a festival than an actual foot race. You'll see booths offering food, swag and merchandise, stages set up with entertainment, opportunities for runners to raise money for their favorite charity while they run and side events such as kid runs or family challenges. The emphasis is on participation rather than performance.

For sure, themed running events have brought more runners into the sport. And they have made running events of all distances less intimidating to the average runner. The party-like atmosphere of today's running events (Disney has a series of races on both the east and west coasts in which runners can stop roughly every half mile or so to take their photo with a famous character) means that there is less incentive for runners to throw down a gut-wrenching performance and more incentive to just go out and enjoy the fun of the running lifestyle.

And if that's what's causing the slowdown, it's not necessarily a bad thing. American runners may be slower. But they're running to enjoy a healthy lifestyle and a community of other runners, and that's better than knocking some time off a clock.