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Keeping Up with the Kids

Credit: natasnow/Adobe

Credit: natasnow/Adobe

By Anthony Blazevich & Sebastien Ratel

Children seem to be able to play for hours without tiring. Only now are we beginning to understand the physiological reasons why.

We’ve all seen it before. Children seem to run, jump, hop, throw, kick and bounce for hours on end. It’s impossible for us adults to keep up.

When children play they tend to choose to perform short bouts of activity separated by brief periods of rest, unlike adults who regularly perform long bouts of slow exercise like running or walking long distances. And when children do play, they are able to do it for a very long time.

This ability to continue physical activity for long periods is quite remarkable and largely unexpected. Children have shorter limbs than adults, so they need to take more steps to walk and run, and this should make them tire quicker. Children don’t make best use of their in-built energy return systems, where the work done by muscles is stored in and then released from elastic tendons to propel us or our bats and balls.

So children tend to waste more energy when they do work. Also, they tend to be less skilful, so they adopt less efficient movement patterns that consume energy and should cause greater fatigue.

So why is it that children seem to have boundless energy? In a study led by Prof Sebastien Ratel at the Université Clermont Auvergne in France and published in Frontiers in Physiology (, we tried to find out exactly how “fit” kids were, and why they might be able to perform these high-intensity bouts of exercise, separated by short rest periods, better than us.

We asked young children 8–12 years of age to perform a 30-second all-out cycling effort, and measured both their power output and their energy production during and immediately after the bout. We then compared their results to active, young adults (19–23 years old) who had similar physical activity profiles to the children. We also compared them to a group of highly trained endurance athletes (19–27 years), many of whom had represented France in international running or cycling races.

The results of the experiment was clear. Children not only fatigued less during the 30-second bike sprint, but they fatigued at the same slow rate as the adult endurance athletes. Perhaps even more astounding was that the children recovered equally well, and in some respects faster, than the athletes.

A key question is why kids show these remarkable abilities. The current belief is that the endurance capacity of kids comes from their ability to generate muscle energy from oxygen-dependent “aerobic” pathways better than us. In fact, this ability is comparable to endurance-trained athletes. The generation of energy from these pathways allows for the muscle to continue working without the accumulation of metabolic products that cause fatigue: acidosis, lactate, phosphates and others.

However, adults produce relatively less energy that way, and instead tend to use anaerobic pathways more. These can produce a lot of energy quickly, but tend to result in rapid muscle fatigue.

The measurements of oxygen consumption and lactate produced during exercise and in recovery are consistent with this belief. They also match other data that we collected in separate tests on our subjects, where we found that the children could generate the same amount of cycling power (relative to their own peak power output) using their aerobic systems as the athletes, and much more than the untrained adults.

Researchers have previously shown that muscle aerobic systems kick into gear faster in children than adults, so there’s less need for energy to come from the fatiguing anaerobic system. The aerobic system also helps to recover the muscle quicker by reversing the build-up of fatigue-causing products in the muscles. This benefit seems to come from children having relatively more “slow twitch” muscle fibres, which contain more of the important enzymes needed to generate energy from aerobic pathways.

Put simply, kids are built more like highly trained endurance athletes than most of us.

Our results have several practical implications. For example, they suggest that children should be encouraged to play games and sports in which they perform repeated, short bouts of exercise rather than ones where they continue to move slowly for long periods (or, of course, stay still for long periods). This type of activity should match their physiological strengths well, and the enjoyment that comes from this should increase their motivation to continue to play. In a world where rates of childhood inactivity are increasing, and thus obesity and disease, this may have considerable health benefits.

However, for young, talented and motivated athletes we might take the other tack. Since their aerobic systems already function well, we might be better to focus on improving their sports skills (and the skill of moving efficiently) or their muscular strength or speed. In this way we can improve their weaker elements in order to set them up for a prosperous future.

Clearly, our strategy for the delivery of exercise programs should differ between child prodigies and those who participate for health and enjoyment.

We can also consider other health implications. It seems we lose muscle aerobic capacity in the adolescent years between childhood to adulthood, and we may therefore need to continue to perform aerobic exercise training to maintain it. Metabolic diseases such as diabetes as well as some forms of cancer are becoming more prevalent in adolescents and young adults, yet they are still exceptionally rare in children. Given that the function of the aerobic powerhouses in our muscles, the mitochondria, affects both aerobic capacity and metabolic disease and cancer risk, we can speculate that the loss of muscle aerobic capacity in adolescence and early adulthood might be a key step allowing metabolic diseases to take hold. Future research will hopefully more clearly examine the link between muscle maturation and disease, and also determine whether exercise that helps to maintain our child-like muscles is a potent medicine to prevent disease.

Regardless of future research, at least now we have a very good idea as to why we can’t keep up with our kids in the back yard or park.

Anthony Blazevich is Professor in Biomechanics at Edith Cowan University. Sebastien Ratel is Associate Professor in Physiology at Université Clermont Auvergne, France.