Child—Adult Differences in Muscle Activation — A Review

Abstract

Children differ from adults in many muscular performance attributes such as size-normalized strength and power, endurance, fatigability and the recovery from exhaustive exercise, to name just a few. Metabolic attributes, such as glycolytic capacity, substrate utilization, and VO2 kinetics also differ markedly between children and adults. Various factors, such as dimensionality, intramuscular synchronization, agonist-antagonist coactivation, level of volitional activation, or muscle composition, can explain some, but not all of the observed differences. It is hypothesized that, compared with adults, children are substantially less capable of recruiting or fully employing their higher-threshold, type-II motor units. The review presents and evaluates the wealth of information and possible alternative factors in explaining the observations. Although conclusive evidence is still lacking, only this hypothesis of differential motor-unit activation in children and adults, appears capable of accounting for all observed child-adult differences, whether on its own or in conjunction with other factors.

Discussion: “The kinetics of blood lactate in boys during and following a single and repeated all-out sprints of cycling are different than in men” — Do children indeed release and remove lactate faster than adults?

“…As we have previously suggested, we also believe that children recruit their type-II motor units to a lesser degree compared with men (e.g., Dotan et al 2012). Certainly, these attributes could both be true.…”

The electromyographic threshold in boys and men

“…According to the size principle (Henneman et al 1965), the lower level of voluntary recruitment implies that children activate their higher threshold, fast-twitch, type-II MUs to a lesser extent than do adults. The lesser activation of type-II MUs and greater reliance on slow-twitch, type-I MUs may, in turn, explain many performance, metabolic, and neuro-motor childadult differences beyond just isometric muscle strength (Dotan et al 2012). Direct supportive evidence for this differential MU activation hypothesis is lacking due to technical or ethical constraints associated with invasive techniques.…”

The effect of heavy resistance exercise on repeated sprint performance in youth athletes

“…These characteristics can be directly influenced by the age of the participant as children possess less voluntary muscle speed, strength, and power, even when corrected for age or maturation state (Van Praagh & Dore, 2002). Likewise, children and adolescents possess less quantity of type II muscle fibres in the vastus lateralis muscle compared to adults (Lexell, Sjöström, Nordlund, & Taylor, 1992;Sjöström, Lexell, & Downham, 1992) and have a reduced ability to utilise these higher-threshold motor units (Cohen et al, 2010;Dotan et al, 2012) which are more responsive to heavy resistance exercise (Hamada, Sale, MacDougall, & Tarnopolsky, 2000;Howarth & Kravitz, 2008). The combination of these factors may influence the ability of adolescent participants to derive benefits from heavy resistance exercise.…”