By NASPEM member, Dr Adam Baxter-Jones
Of course not. However, a lot of parent, coaches and some professionals working with children think they are. In sports, size matters throughout childhood, as physical size often translates into physical superiority and athletic dominance, suggesting that greater size can trump or neutralize greater athletic skill. If, as it is suggested, the physiques of youth athletes need to mirror the physiques of the successful adult athlete, then selection of such traits at a relatively young age are likely preferable. The big mistake here is thinking children’s sport is a miniature version of adult sports. As Karen Pfeiffer rightly acknowledges in another blog on the NASPEM website, our esteemed and revered predecessors, Per Olof Astrand, Oded Bar-Or, Bob Malina, Tom Rowland, Neil Armstrong, and others noted on numerous occasions that children are NOT miniature adults.
One of the most iconic paintings of the early 14th century is Duccio’s “Madonna and child” on which the Christ child sits on his mother’s lap reaching out his hand to his mother’s face. Typical of this period, and future illustrations of Christ with his mother, the Christ child is drawn as a miniature adult, but this is inaccurate. Infants do not have the same proportionality as their mothers. At birth, a child head is half its adult’s size, its trunk a third, its arms a quarter and its legs a 5th. At six months of age the head represent 25% of an infant’s total body length, its trunk over 44% and its legs 31%. By adulthood, the head represents 12%, the trunk 50% and the legs 38% of total body length.
Growth is also not linear. If you think of growth in height as a train journey and each age representing a train station, the distance between stations is non-linear representing different speeds and distances traveled between stations. Height for distance curves of growth show 4 distinct phases; rapid growth in infancy (up to 4 years of age), steady growth in childhood, rapid growth in adolescence (12 to 16 years) and slow growth as adulthood approaches (16 to 18 years) highlighting the fact that most of growth occurs during infancy and childhood.
However, it is not just proportionally that changes with growth, stance also changes. At 18 months the child is bow legged and by 3 years of age they have knock knees, their legs do not become straight until 6 years of age. Thermoregulation also changes and is associated with change in surface area from 0.95 m2 at eight years of age to 1.80 m2 at 20 years of age. The surface area to weight ratio changes from 380 cm2/kg at 8 years to 280 cm2/kg at 20 and explains in part why children lose body heat more readily than adults. Caloric utilization also changes, during infancy basal utilization represents 50%, activity 30% and growth 20%, during puberty these utilizations change to 35%, 25% and 40% respectively and then in adulthood to 55% basil and 45% activity.
All of these changes with growth explain why children and adults respond differently to aerobic and anerobic exercise. Cardiovascular function is different, children have smaller lung and heart capacities resulting in lower stroke volume which is compensated in part by higher maximum heat rates and greater arterial-venous oxygen differences. They also rely more on fat metabolism because of a lack of muscle glycogen and glycolytic enzymes. Their anaerobic capacity increase with age becoming fully developed by 20 years, probably due to lack of muscle mass.
Taken together all these aspects of a child’s physical and physiological development explain why Duccio was wrong in his depiction of a mother and child and explain why children are Not miniature adults.
Adam D.G. Baxter-Jones, PhD, is the Interim Associate Provost, Health, at the University of Saskatchewan. Dr. Baxter-Jones studies growth and maturation in children as they pertain to exercise, physical activity, body composition and health. He directs the Saskatchewan Growth and Development Study (SGDS) and the Pediatric Bone Mineral Accrual Study (PBMAS).