Use of only linear dimensions rather than muscle CSA to account for differences in size may have clouded our understanding of strength development in children.

The aims of this study were to examine age and sex differences in elbow extensor and flexor anatomical muscle cross-sectional area (mCSA) measured by magnetic resonance imaging (MRI) and the location of maximal mCSA as a percentage of the distance from the distal to the proximal end of the humerus. Ninety-five individuals spread across the age groups 9 - 10 years, 16 - 17 years and ?21 years participated in the study. Muscle cross-sectional area derived from the manual MRI tracing proved to be highly reliable in terms of limits of agreement (-2.5 to 1.5 cm(2)) and the intraclass correlation coefficient (ICC = 0.998). A sex-by-age group analysis of variance revealed significant effects (P < 0.01) of sex, group and a sex-by-group interaction, the latter reflecting a greater increase in males than females of upper arm mCSA from childhood to adulthood. Extensor mCSA was more proximal (55 +/- 6%) than that of the flexors (28 +/- 6%). A significant effect (P < 0.01) of group was found for location of maximal extensor mCSA, reflecting its more distal position with increasing age. Measurements of muscle size should be made at the individually determined position of maximal mCSA if interpreting data collected during growth and maturation, especially if the muscle group of interest is the elbow extensors and if different age groups are being monitored.

The aim of this study was to examine elbow fl exion torque, muscle cross-sectional area (CSA), and leverage in boys and girls. Thirty-eight prepubertal children (9.6 ± 0.3 years) volunteered to participate. All performed isometric fl exion actions at 10°, 50°, and 90° of elbow fl exion. Magnetic resonance imaging was used to assess elbow fl exor (EF) muscle CSA and brachialis moment arm lengths. No signifi cant gender differences were observed for any of the variables studied. EF CSA was directly proportional to isometric torque at 50° and 90°. CSA explained between 47% and 57% of torque variance. Moment arm estimates explained 19% of the variance in isometric torque at 90°. These baseline data contribute to our understanding of factors infl uencing strength variation during childhood.Upper-extremity strength differences between boys and girls are of particular interest, given that by adulthood, males outperform females in practically all upper-body sport events involving a strength component (10). Examination of the factors contributing to strength variation will aid our understanding of differences in upper-body performance, but initial baseline data on prepubertal children are required in order to put observations during growth and maturation into context.The majority of studies have focused on the ability of intrinsic muscle properties to explain strength variation (4); relatively few have examined extrinsic factors, which include the role of anatomy and biomechanics (5). However, unless muscle force is measured directly at the tendon or at the point of application of a load, leverage (an extrinsic factor) will infl uence measurements of strength. When strength is expressed as joint torque, it represents the product of all the muscle forces and their moment arms contributing to a specifi ed joint action. Given that muscle force is also infl uenced by moment arm length via its effect on musculotendinous length changes and velocity, it is important to understand the role of variation in moment arm lengths, across a joint range of motion and between individuals, in torque development (15).

The aim of this study was to examine age and gender differences in isokinetic eccentric (ECC) elbow flexion (EF) and extension (EE) torques in children. Thirty-seven children participated in a 3-year longitudinal study. The average age +/- SD at the first test occasion was 13.0 +/- 0.3 years. Isokinetic ECC EF and EE torques at 0.52 rad . s (-1), MRI determined muscle cross-sectional areas (CSAs), stature, and arm length were determined annually. Concentric (CON) EF and EE torques for the same children reported previously, enabled functional torque ratios (ECC antagonist/CON agonist) to be calculated [ ]. In contrast to isokinetic EF torques, ECC EE torques were not significantly higher than CON EE torques (across test occasions and for boys and girls). Functional torque ratios did not significantly differ with age or between boys and girls. Multilevel modelling was used to examine age and gender effects once differences in body/muscle size had been considered. ECC EF torque was relatively greater in girls following adjustment for size. These data contribute to the understanding of differences in upper body strength performance and injury propensity during growth and maturation, suggesting CON and ECC torque development are muscle and gender specific.

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