Inferior Exercise Economy in Children: Perpetuating a Myth?

Abstract

It is one of the most established, well-documented pieces of dogma in the pediatric exercise physiology literature: compared with adults, children exhibit a greater metabolic requirement to move body mass—an inferior exercise economy—during weight-bearing exercise (1,7). In a typical example provided by Unnithan and Eston, 9–10 year old boys had a 24% greater mass-relative oxygen demand while running on a treadmill at 9.6 km h-1 compared with 18–25 year old men (47 ml kg-1 min-1 versus 38 ml kg-1 min-1; 15). Cross-sectional studies indicate that this economy improves with increasing age. In an early study, Skinner et al. demonstrated a progressive decline in metabolic demand during uphill treadmill walking from 46 ml kg-1 min-1 in a group of 6–8 year old children to 41 ml kg-1 min-1 in a cohort of 12–16 year old youth (13). Longitudinal studies have verified this trend. In the Amsterdam Growth, Health, and Fitness Study, data from 192 subjects showed a mean decrease in VO2 per kg from 37.6 to 30.3 ml kg-1 min-1 between the ages of 13 and 27 years (16). Rowland et al. determined treadmill walking economy at 3.25 mph and 8% grade annually for five years in children beginning at age 9 years (9). Mass-relative oxygen uptake fell progressively over this time span from an initial mean value of 31.0 ml kg-1 min-1 to 26.5 ml kg-1 min-1. The findings have been remarkably consistent: metabolic cost per kg body mass at a given exercise load progressively decreases in a continuum through the growing years. VO2/kg at a given treadmill speed and grade setting can be predicted to decline by an average of approximately 1.0 ml kg-1 min-1 for each year of increasing age until midlate adolescence. From the perspective of developmental exercise physiology, this “wasteful gait” of children has been considered as a disadvantage in their capacity for endurance exercise compared with adults. “The high metabolic cost of walking and running,” it has been suggested, “makes the small child less of an ‘aerobic machine” (1). From a more pragmatic standpoint, the progressive change in exercise economy