‘*’ – Assuming the possibility that the type-I ber composition of prepubertal children is higher than that of adults by as much as 10–15%.
‘**’ – Differences in coactivation may account for part of the observed strength child-adult difference in multijoint and dynamic movements, but apparently, not in single-joint, isometric contractions.
Hypothesis
The first to propose such a qualitative difference between children and adults was the late Erling Asmussen, in 1955 (6). Having observed that the increase in strength during childhood and adolescence is more than can be expected from the increase in body size, Asmussen proposed that children do not activate, or use their muscles to the extent typical of adults (5,6). Subsequently, in view of more supporting evidence, this view has been fostered by others (4,13,18,34,35,37,48,72).
The proposal, by Asmussen and subsequent researchers, implies that children’s maximal neuromuscular activation is generally lower. That is, children recruit a smaller percentage of their total motor-unit pool. While we endorse Asmussen’s idea, we propose to modify it with a more specific hypothesis. Namely, that the child-adult muscle functional gap is due to children’s inability to recruit, or fully use, higher-threshold (type II) motor units to the extent typical of adults. Thus, we specifically point to type-II motor-unit utilization as being the compromised portion of children’s muscle function.
Note: “Recruitment” is a dichotomic term, denoting whether motor units are neuromotorically accessed. “Utilization”, on the other hand, denotes the extent or intensity of use of recruited motor-unit, which largely depends on their firing rates.
The Evidence
Unequivocal neuromotor evidence would be needed to conclusively validate the differential motor-unit activation hypothesis. Unequivocal, direct validation would require sufficiently large samples of individual motor-units to be qualitatively monitored for activation, as well as for relative torque and timing patterns. No such evidence, or appropriate technique, currently exists. Presently, therefore, it is necessary to rely on indirect evidence to support the differential muscle activation hypothesis. Table 1 provides an overview of the evidence discussed below.
Volitional vs. Nonvolitional Force Production
The interpolated-twitch technique is frequently used to estimate the degree of motor-unit activation during volitional effort, in relation to total potential activation. During maximal contraction, an electrical stimulus is superimposed onto a muscle or its motor nerve, and the evoked interpolated twitch torque is measured. The difference (or ratio) between volitional and evoked nonvolitional maximal force is an index of the degree of muscle activation (or percentage of the motor-unit pool recruited) during a given volitional contraction. Although the technique is not free of controversy (28,84,90) and love roulette its accuracy might be limited, it has produced seemingly valid findings.
In comparative studies, the interpolated-twitch technique has generally shown lower muscle activation in children (13,18,47,72), although the difference was not always statistically significant (50,87). Belanger & McComas (13) were first to apply the technique to children and suggested that children’s motor-unit activation was lower (
99%). Close scrutiny of this frequently cited study reveals that boys defined as prepubertal were 6–13 yrs old (thus, not necessarily all prepubertal), and that the percentage child-adult difference in motor-unit activation was not statistically significant (p = .20). Examining just the 6–10-yr-old subgroup revealed volitional muscle activation that was much lower than the adults’ (p = .06). Indeed, the authors concluded that “younger children were less able to activate plantar flexor motoneurons than older ones and adolescents” ((13) p.566). Subsequent studies, using different indices of volitional vs. nonvolitional force production, have also demonstrated reduced motor-unit activation in children (18,47,72). Grosset et al. (47), using plantar flexion, demonstrated motor-unit activation deficits progressively decreasing with age in 7–11-yr-old children and in adults. Others demonstrated a similar trend, although the differences did not always reach statistical significance (50,87). A partial explanation for the inconsistent findings may be that the level of muscle activation is muscle-group dependent. For example, Blimkie (18) showed lower motor-unit activation in boys (78%) than in men (95%) in the knee extensors, but not in the elbow flexors.
