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. www live jasmin co 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.
This review aims to substantiate the notion that child–adult differences in muscle activation constitute an underlying factor that best accounts for a comprehensive array of observed performance and metabolic child–adult differences. While other factors may partially explain some of the observed differences, the review will attempt to establish that differential motor-unit activation can solely accounted for these child-adult differences.
Muscle activation has mostly been examined in relation to maximal isometric strength (25,34,37,47–49,67). Children’s maximal volitional muscular force, contractile velocity and muscular power are notoriously lower than adults’, especially in males (see (18) for review). Although absolute differences are largely attributable to body- and muscle-size differences, they clearly persist after body dimensionality has been taken into account (see 18,53,91 for review). Thus, size differences do not suffice to fully account for the observed strength differences.
Agonist-antagonist muscle cocontraction can potentially explain some strength and power differences between children and adults. Simultaneous activation of antagonist muscles detracts from the externally measured force and power output, attributed to the examined agonist muscles. Thus, higher agonist-antagonist cocontraction could partly explain children’s lower strength and power. Some studies have indeed reported greater coactivation in children (40,47), but others have not (10,57). Notably, age-related cocontraction differences have been mostly observed in submaximal, multijoint, or dynamic contractions. However, in maximal isometric contractions, most studies show minimal or no age-related cocontraction differences (34,37,69). Thus, while cocontraction differences may account for some strength and power differences in dynamic contractions, they cannot explain differences in maximal isometric strength.
Substantially lower type-II fiber composition of children’s muscles could explain many of the observed child-adult functional, metabolic, and other differences. For primarily ethical reasons, muscle-composition data of healthy children are scant. The available data are largely derived from clinical biopsies of children with various diseases, or from cadavers. Several studies suggest similar muscle-fiber composition in children and adults (14,21,92), but others support as much as 10% or higher type-I muscle-fiber composition in prepubertal children (54,63). On the other hand, contractile characteristics such as contraction time and half-relaxation time, generally regarded as reflecting muscle composition, have been shown to be similar across age groups in numerous studies (13,26,27,65,72).
Despite their methodologically-limited and conflicting nature, the available data do not allow dismissal of possible, functionally-significant child-adult differences in muscle composition. These compositional differences could fully or partly account for most of the observed functional differences, as outlined in Table 1 , but (as will be shown below) cannot explain all observed child-adult differences. Although numerous differences can be explained by multiple factors, it is the purpose of this review to present the body of evidence that supports a single underlying factor which singly, or in conjunction with other factors, explains all observed differences.