Determining the Muscle Voluntary Activation Characteristics in Children: A Methodological Challenge. Commentary on “Child-Adult Differences in Muscle Activation-A Review”

Abstract : In a previously published review article, Dotan et al (10), proposed the hypothesis of a differential motor-unit (MU) activation in children to account for many observed child– adult differences (ie, maximal strength, rate of force development, fatigue resistance, metabolic profile, and responses to training). Specifically, these authors pointed to Type II MU utilization as " being the compromised portion of children's muscle function. " This review was previously commented on by Bassa et al (3) and O'Brien et al (25). In this comment, we propose to contribute to the ongoing debate on this topic. In particular, we will approach the question of MU recruitment from a method-ological point of view to sum up our current knowledge and to identify the methodological steps that must be done to move forward on this topic. Trying to infer MU utilization and recruitment from noninvasive investigation methods in children is challenging. Indeed, owing to obvious ethical considerations, the methodological approaches available to study MU recruitment and utilization noninvasively in children offer limited insights. Until now, the pediatric exercise physiologists have mainly used (1) the rate of force development, (2) surface electromyography (EMG), and (3) the twitch interpolation technique. (1) Some authors have focused on the time course of explosive maximal voluntary contractions to compare MU recruitment in children and adults. Specifically , these studies have examined the rate of rise in contractile force at the onset of contraction, ie, the rate of force development (RFD) exerted within the early phase of rising muscle force (30–200 ms). Among the physiological factors that can affect RFD are muscle fiber type and myosin heavy chain composition (16), muscle cross-sectional area (2), maximal muscle strength (26), viscoelastic properties of the muscle–tendon complex (6,33), and neural drive to the muscle (1,12). If the confounding effects of muscle cross-sectional area and muscle strength can be discarded using appropriate normalization procedures , the relative contributions of viscoelastic and neural properties to the differences in RFD between children and adults are difficult to determine. Nevertheless , Waugh et al (32) recently demonstrated that tendon stiffness and the rate of EMG increase (ie, muscle activation rate) accounted for 35% and 30% of the variability of RFD in children, respectively. The relative contributions of tendon stiffness and rate of EMG increase may nonetheless vary during the course of force rise. Indeed, the influence of muscle activation rate appeared more important than tendon stiffness in the early stages of development of force in children, whereas the opposite was true for adults (32). This is consistent with the recent suggestions of Dotan et al (9). Nevertheless, Waugh et al (32) also demonstrated that when tendon stiffness and rate of EMG increase were combined, they explained only 58% of RFD variability, suggesting that other (unknown) factors are involved. Thus, RFD cannot accurately and fully reflect the extent of MU utilization and recruitment. (2) The use of surface EMG to account for muscle activation is also debatable. Surface EMG quantifies muscle activity but does not determine the proportion of MU unrecruited or driven submaximally by the central nervous system (ie, muscle inactivation). As mentioned earlier, the ability of prepubertal children to maximally drive their MU is debated (3,5,10,20,25). If it holds true that children are not able to maximally recruit their MU, and especially the fastest (ie, Type II MU), peak EMG measured during voluntary contractions may represent a lower proportion of the maximal activation in children than in adults. In turn, this would bias the normalization of EMG activity used to determine the rate of muscle activation during explosive actions. Specifically, this could lead to an overestimation of the MU recruitment in Official Journal of NASPEM and the European Group of PWP COMMENTARY
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Article dans une revue
Pediatric Exercise Science, Human Kinetics, 2014, 26 (3), pp.365 - 368. 〈10.1123/pes.2013-0204〉
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Vincent Martin, Sebastien Ratel. Determining the Muscle Voluntary Activation Characteristics in Children: A Methodological Challenge. Commentary on “Child-Adult Differences in Muscle Activation-A Review”. Pediatric Exercise Science, Human Kinetics, 2014, 26 (3), pp.365 - 368. 〈10.1123/pes.2013-0204〉. 〈hal-01659903〉



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