Most of us know children who can run and play for hours and hours, taking only short rests.
As a parent or carer, 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?
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 shut down, 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, minimising 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 fibres, which have a greater activity of important enzymes that drive 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 to 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, and possibly reveal why children are able to produce repeated exercise efforts when most of us adults continue to feel exhausted.