Muscle strength and activation were compared in boys and men during maximal voluntary elbow flexion and extension contractions. Peak torque, peak rate of torque development (d/dmax), rate of muscle activation, and electromechanical delay (EMD) were measured in 15 boys (aged 9.7 +/- 1.6 years) and 16 men (aged 22.1 +/- 2.8 years). During flexion, peak torque was significantly lower in boys than in men (19.5 +/- 5.8 vs. 68.5 +/- 11.0 Nm, respectively; p < 0.05), even when controlling for upper-arm cross-sectional area (CSA), and peak electromyography activity. Boys also exhibited a lower normalized d/dmax (7.2 +/- 1.7 vs. 9.5 +/- 1.6 (Nm.s-1).(Nm-1), respectively; p < 0.05) and a significantly longer EMD (75.5 +/- 28.4 vs. 47.6 +/- 17.5 ms, respectively). The pattern was similar for extension, except that group differences in peak torque were no longer significant when normalized for CSA. These results suggest that children may be less able to recruit or fully utilize their higher-threshold motor units, resulting in lower dimensionally normalized maximal torque and rate of torque development.

Ten prepubertal girls and 15 young women were tested for maximal torque, peak rate of torque development, electro-mechanical delay (EMD), and time to peak rate of torque development during isometric elbow flexion. Absolute peak torque (17.0 ± 7.7 vs. 40.5 ± 8.3 Nm) and peak rate of torque development (105.9 ± 58.6 vs. 297.2 ± 113.0 Nm·s −1 ) were lower in the girls (p < .05). Normalized to muscle cross sectional area, torque was similar (8.27 ± 2.74 vs. 8.44 ± 1.65 Nm·cm −2 ), as was peak rate of torque development, normalized to peak torque (6.21 ± 1.94 vs. 7.30 ± 2.26 Nm·s −1 /Nm). Both, time to peak rate of torque development (123.8 ± 36.0 vs. 110.5 ± 52.6 ms) and EMD (73.2 ± 28.6 vs. 51.9 ± 25.6 ms), were longer in the girls, although EMD's difference only approached statistical significance (p = .06). Age-related isometric strength differences in females appear to be mainly muscle-size dependent. However, the time to peak torque and EMD findings suggest differential motor-unit activation which may functionally manifest itself in fast dynamic contractions.Numerous studies have demonstrated that children's maximal muscle strength is lower than that of adults (5,18,42). In males, muscle strength is consistently lower in boys even when normalized for body size (5,42 CIHR Author ManuscriptCIHR Author Manuscript CIHR Author Manuscript some studies report a lower body size-normalized strength in girls compared with women (29), others do not (5,28,31,38,43).Several factors, besides body size may contribute to age-related differences in maximal strength, including differences in muscle activation, fiber composition, and agonistantagonist cocontraction. The scant available literature on muscle fiber composition suggests that the latter is similar in children and adults (12). This is supported by the similar contractile characteristics (contraction time and half-relaxation time) during electrically evoked twitch contraction in children and adults (4,8,9,22,35,39). Although few studies have suggested a lower percentage of type II muscle fibers in boys compared with men (15,34), the difference is too small to account for the age-related muscle strength differences observed between boys and men. No comparison of muscle fiber-type composition has been reported between girls and women.The rate or degree of muscle activation has been suggested to be lower in boys compared with men, based on indirect evidence and varying techniques (1,2,4,5,23,25,33,39,45). However, all these comparisons were made with heterogeneous or male-only samples. There are no comparisons of muscle activation indices or examination of their roles in girl-woman strength differences.Differences in the rate of force development are likely manifestations of the nature of muscle activation or its composition. It too appears to be lower in children than in adults, whether measured during maximal voluntary (1,21), or electrically stimulated twitch contractions (4,22), but has been demonstrated only in males. No age comparisons have been published fo...