The Science Behind Why Kids Can Run Around All Day

They’re more like adult endurance athletes than we once thought.

Most of us know children who can run and play for hours and hours, taking only short rests. As a parent or caregiver, it can be exhausting.

For scientists, why this is the case has long been the source of debate — is it due to fitness? Or something else?

Our study published in April looked at performance and recovery of children and adults doing strenuous cycling. It shows children not only out-perform most adults, but can perform as well as highly-trained adult endurance athletes, and then recover even faster afterward.

Tired from parenting? Blame your muscles. Image via The Conversation/Shutterstock.

Children’s muscles are different

Repeated experiments have shown that the muscles of children tend to fatigue more slowly than adults’.

These results seem to fly in the face of what science would predict. For example, children have shorter limbs, so they have to take more steps and should therefore theoretically use more energy.

Children are also less able to make use of tendon energy return systems — that is, they store less energy in their tendons so they can’t reuse this energy to propel themselves during movement.

And children show greater activity in muscles that oppose or control movement, a reflection of the fact that typically they are less skillful and therefore, use more energy.

So, how do their muscles stay fresh?

Aerobic and anaerobic exercise

One possible explanation for the remarkable muscle endurance of children could be their different use of energy pathways.

Anaerobic (“oxygen-independent”) pathways produce large amounts of energy without the need for oxygen but tend to cause rapid fatigue. For example, sprinters rely on anaerobic metabolism to run fast over short distances.

Aerobic (“oxygen-dependent”) pathways tend to produce energy at a slower rate but allow us to work for many hours without muscle shutdown, like in a well-run marathon.

We know from existing research that children seem to be able to get more of their energy from aerobic pathways than adults, minimizing the fatiguing anaerobic contribution. Their aerobic machinery also kicks into gear faster than adults, so they don’t need to rely as much on anaerobic metabolism when exercise first starts.

These benefits are believed to partly result from children having a greater proportion of so-called “slow-twitch” muscle fibers, which have a greater activity of important enzymes that drive the release of energy from aerobic pathways.

Such findings prompted us to speculate that children’s muscles might actually respond to exercise in a similar way to adult endurance athletes’, since they too show these characteristics.

Let’s go cycling

We tested our speculation in a study run by researchers at Université Clermont Auvergne, in France.

Children (average age 10.5 years), young adults (21.2 years) with a similar physical activity level as the children, and age- and height-matched endurance-trained male athletes (21.5 years) were asked to complete two cycling tests on a stationary bicycle.

In the first test, power output was continually increased until exhaustion. In the second test, the subject completed a 30-second all-out cycle sprint. These tests allowed us to measure numerous physiological responses to exercise and assess both the rate of fatigue and then recovery specifically during brief, maximal-intensity exercise.

We found that the children fatigued as much in the all-out cycle as the endurance-trained athletes (about 40% loss of power) and much less than the untrained adults (about 50% loss).

Data also show that the proportion of energy derived from aerobic pathways in the 30-second cycle sprint was similar in the children and athletes, and more than in untrained adults.

These results clearly show that fatigue rates in response to high-intensity exercise may be the same in children as they are in highly-trained adult endurance athletes and that this is associated with an incredible generation of energy from aerobic energy pathways.

But data collected during recovery from the exercise also revealed startling outcomes. The rate at which oxygen use declined after the exercise was the same in children and athletes. The rates at which heart rate returned to normal and lactate (a compound associated with muscle fatigue) cleared from the blood were even faster in the children, and again, much faster than in untrained adults.

These data show that children’s muscles recover rapidly from high-intensity exercise, possibly revealing why children are able to produce repeated exercise efforts when most of us adults continue to feel exhausted.

How children’s muscles work

Such data provide strong hints as to how to optimize exercise and sporting performance in children.

Children might benefit from short, high-intensity exercise bouts to boost anaerobic capacity and a focus on movement skill, muscular strength, and other physical attributes more than in adults.

Adults (and adolescents), on the other hand, may need to place a greater emphasis on improving their muscle aerobic capacity.

There may also be important health implications. Metabolic diseases, including diabetes and many forms of cancer, are increasing in prevalence in adolescents and younger adults but are still rarely seen in children. It might be the case that the loss of muscle aerobic capacity between childhood and early adulthood is a key maturation step that allows metabolic diseases to take hold.

It will be interesting in the future to examine the link between muscle maturation and disease and test whether the maintenance of our childhood muscles through exercise training might be the best medicine to prevent disease.

Either way, at least we now have some idea as to why children are able to play and play and play when we adults need to take a break. Kids are already elite.


"Seventy percent of the Earth is covered with water, now you camp on it!" proudly declares Smithfly on the website for its new camping boat — the Shoal Tent.

Why have we waited so long for camping equipment that actually lets us sleep on the water? Because it's an awful idea, that's why.

"The world is your waterbed," Smithfly says on its site. But the big difference is that no one has ever had to worry about falling asleep and then drowning on their waterbed.

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While it is possible that one could wade into the water, unzip the tent, have a pleasant slumber, and wake up in the morning feeling safe and refreshed, there are countless things that could go terribly wrong.

The tent could float down the river and you wake up in the middle of nowhere.

You could have to go to the bathroom in the middle of the night.

This guy.

It could spring a leak and you could drown while wrapped up in eight feet of heavy nylon.

A strong current could tip the tent-boat over.

There isn't any way to steer the darn thing.

This guy.

Mashable shared a charming video of the tent on Twitter and it was greeted with a chorus of people sharing the many ways one could die while staying the night in the Shoal Tent.

Oh yeah, it's expensive, too.

Even though the general public seems to think the Shoal Tent is a terrible idea, according to the Smithfly's website, it's currently sold out due to "popular demand" and it will be "available in 6-8 weeks." Oh, and did we mention it costs $1,999?

via zoezimmm / Imgur

There are few more perniciously dangerous conspiracy theories being shared online than the idea that vaccines cause autism.

This has led to a decline in Americans vaccinating their children, resulting in a massive increase in measles. This year has already seen over 1,200 cases of measles, a disease that was eradicated in the U.S. nearly 20 years ago.

A 2015 Pew Research study found that 83% of Americans think the measles vaccine is safe, while 9% think it's not. Another 7% are not sure. But when you look at the polls that include parents of minors, the numbers get worse, 13% believe that the measles vaccine is unsafe.

There is zero truth to the idea that vaccines cause autism. In fact, a recent study of over 650,000 children found there was no link whatsoever.

RELATED: A new study of over 650,000 children finds — once again — that vaccines don't cause autism

A great example of the lack of critical thinking shown by anti-vaxxers was a recent exchange on Facebook shared to Imgur by zoezimmm.

A parent named Kenleigh at a school in New Mexico shared a photo of a sign at reads: "Children will not be enrolled unless an immunization record is presented and immunizations are up-to-date."

This angered a Facebook user who went on a senseless tirade against vaccinations.

"That's fine, I'll just homeschool my kids," she wrote. At least they won't have to worry about getting shot up in school or being bullied, or being beat up / raped by the teachers!"

To defend her anti-vaccination argument, she used a factually incorrect claim that Amish people don't vaccinate their children. She also incorrectly claimed that the MMR vaccine is ineffective and used anecdotal evidence from her and her father to claim that vaccinations are unnecessary.

She also argued that "every human in the world is entitled to their own opinion." Which is true, but doesn't mean that wildly incorrect assumptions about health should be tolerated.

She concluded her argument with a point that proves she doesn't care about facts: "It doesn't matter what you say is not going to change my mind."

RELATED: 12 medical professionals shared their most memorable anti-vaxxer stories and you won't stop face-palming

While the anti-vaxxer was incorrect in her points, it must also be pointed out that some of the people who argued with her on Facebook were rude. That should never be tolerated in this type of discourse, but unfortunately, that's the world of social media.

Here's the entire exchange:

via zoezimmm / imgur

via zoezimmm / imgur

via zoezimmm / imgur

via zoezimmm / imgur

via zoezimmm / imgur

via zoezimmm / imgur

via zoezimmm / imgur

via zoezimmm / imgur

via zoezimmm / imgur

via zoezimmm / imgur

The post received a ton of responses on imgur. Here are just a few:

"'In my opinion...' 'I believe...' That's not how facts work."

"You're entitled to your opinion. And everyone else is entitled to call you a dumbass."

"'What I do with my children is no concern to you at all.' Most of the time, true. When your kid might give mine polio, not true."

"If my child can't bring peanut butter, your child shouldn't bring preventable diseases."

It's important to call out people who spread dangerous views, especially how they pertain to health, on social media. But people should do so with respect and civility.


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