Researchers from the Clermont-Auvergne University found that children seem to run around all day without getting tired because their muscles resist fatigue and recover very quickly in the same way as elite endurance athletes. This is the finding of a recent study by Birat A et al. published in the journal Frontiers in Physiology¹.

The study compared the metabolic and fatigue profiles of 12 healthy boys (8-12 years old) to 12 untrained men (19-23 years old) and 13 well-trained men (19-27 years old). The children and untrained men recruited in the study were those performed recreational physical activities for less than 4 hours per week, whereas the well-trained men were national-level competitive athletes (i.e., long-distance runners, cyclists and triathletes) recruited from local sports clubs¹.

In the study, all three groups were asked to perform cycling tasks and the assessment by aerobic and anaerobic exercise outcomes was done. The participants’ performance outcomes, heart-rate, oxygen level and lactate-removal rates were measured. The fatigue index was calculated as the difference between maximal anaerobic power and minimal power, expressed as percentage of maximal power¹.

The study results were astounding as the metabolic profile of children was comparable to well-trained adult endurance athletes and in some aspects better than them. Compared to the well-trained group, children had similar post-exercise recovery rate of oxygen uptake, faster recovery of post-exercise heart rate and better ability to remove lactate from blood circulation. Thus, from a physiological perspective, children are similar to well-trained adult endurance athletes¹.

On the other hand, in the present study, children outperformed untrained adults in all the measured parameters. The fatigue index assessed after the anaerobic exercise was highest in the untrained adult group (51.8% ± 4.1), and lowest in the children group (35.2% ± 9.6)¹. This could be attributed to the fact that during moderate-to-intense exercise, children rely more on oxidative metabolism than on anaerobic metabolism. Higher relative contribution of aerobic metabolism is also associated with a lesser accumulation of anaerobic metabolism derived by-products (e.g. H⁺ ions, lactate, inorganic phosphate), and thus, leading to less fatigue^1,2.

Children have efficient aerobic processes, however with advancing age, from childhood to early adulthood, there is an increase incapacity and relative contribution of anaerobic energy production².The effects of maturational and growth processes on lower dependence of oxidative energy production during exercise and a lesser efficiency in removing metabolic by-products may lead to greater fatigue and poor recovery ability^1,2. Together with the increase in sedentary behavior, fatigue and poor recovery from exercise may partly contribute to the increasing prevalence of insufficient physical activity.

According to World Health Organization (WHO), physical inactivity is the fourth leading risk factor for global mortality and it is associated with 6% of deaths globally. Moreover, it is estimated to be the major cause of approximately 30% ischemic heart disease, 27% diabetes, and 21-25% breast and colon cancers³.

The understanding of the muscle physiological changes with growth may help in the development of exercise-based strategies for the prevention and treatment of metabolic diseases, which are often associated to physical inactivity². The research also provides insights for the development of better training programs for adolescence and early adulthood to offset the decrement of aerobic metabolism².