Moving on land: an explanation of pedometer counts in children

The pedometer has become a popular instrument to measure physical activity in humans. The pedometer records physical activity as a simple, raw measure of ambulatory movement: the number of steps taken. Two fundamental questions of the use of the pedometer in pediatric research are: How many steps do children take in a day? How many steps should children accumulate on a daily basis? This raises the question: is 13,000 steps the same amount of physical activity in a 6-year-old and a 12-year-old, given the difference in body size and stride length? Although previous authors have concluded that the number of steps taken is an equivalent amount of physical activity in children regardless of body size, the conclusion has not been explained by a scientific rationale or on an empirical basis depending on the energetics and mechanics of terrestrial locomotion and body size. One approach to explain this question is allometric scaling, which has long been utilized by comparative physiologists to show the relationships between body size and locomotion in animals from shrews to elephants. The aim of this paper was to use allometric equations, examples from comparative mammalian physiology, and results from previous papers that have examined the cost of locomotion in children to provide a better understanding of the interpretation of the pedometer in pediatric research. We show that empirically derived results confirm to allometric equations that consider the relationship between body size and terrestrial locomotion to cover a given distance at a given speed. Thus, the number of steps taken and the energy expended for a particular task is greater in smaller children. However, if the cost of locomotion is related to one step, animals tend to be equally economical, whether large or small. Therefore, the number of steps taken as measured by a pedometer may be interpreted as an equivalent level of physical activity between a 6-year-old and a 12-year-old, if taken at the same speed. However, the locomotor activities in daily life consist of walking and running up- and downhill, carrying loads, and at varying speeds or work intensities, and this exemplifies the limitations of the pedometer. In conclusion, this paper provides a better understanding of the interpretation of pedometer counts and highlights that the findings depend on the expression of physical activity.