Commentaries on Viewpoint: Do oxidative and anaerobic energy production in exercising muscle change throughout growth and maturation? Manifestations of a common underlying cause.

TO THE EDITOR: Ratel et al. (5) presented the view that children’s glycolytic capacity increases with maturation. This clearly manifests itself in the maturational increase of anaerobic power. Conversely, the apparent concurrent decrease in oxidative capacity, as presented in the Viewpoint, is not coupled with a corresponding decrease in aerobic performance capacity. Also, children demonstrate lower (size normalized) muscle contractility (single-contraction force and velocity) (2), which cannot be explained by inferior glycolytic or even phospholytic apparatus. The totality of these performance disparities cannot be explained by the energy kinetics or histochemical evidence of child-adult differences. Rather, those differences can be reconciled by proposing that children are appreciably more limited in their capacity to utilize higherthreshold, glycolytic, type-II motor units. Lower reliance on glycolytic motor units and greater dependence on oxidative ones can explain a whole range of metabolic and performance child-adult differences, notably the following. 1) Children’s seeming oxidative superiority (5), e.g., anaerobic threshold at higher %V̇O2 max. 2) Children’s inferiority in both anaerobic and contractile capacities, and lower peak-lactate levels (1, 2). 3) Children’s lower fatigability (4) and faster recovery from high-intensity exercise (1). 4) Children’s glycolytic/phospholytic enzyme deficiencies (5) (a disuse effect, due to type II motor units being little used). While true maturational changes in muscle oxidative and glycolytic capacities cannot be ruled out, they can explain neither the scope nor the extent of observed child-adult differences. Although direct support for the maturation-dependent motor-unit utilization theory is currently scant (e.g., 3), the hypothesis does account for all the above and other diverse observations.