Elbow flexion and extension strength relative to body or muscle size in children

Purpose: The purpose of this study was to examine gender- and age-related differences in elbow flexion and extension strength in children, when linear size measurements and actual measurements of muscle size were used as explanatory variables in a multilevel model. Methods: Thirty-seven children participated in a 3-yr longitudinal study (18 boys and 19 girls). The average age +/- SD at the first test occasion was 13.0 +/- 0.3 yr. Stature, arm length, isokinetic concentric and isometric elbow extension and flexion torques, and MRI-determined elbow flexor and extensor muscle cross-sectional areas (CSAs) were assessed annually. Multilevel modeling was used to describe the relationship between the measured torques and body/muscle size variables, incorporating age, age by gender, and gender as additional explanatory variables. Results: When muscle CSA was included in the static and dynamic torque multilevel models, gender differences in strength were nonsignificant. In contrast, use of stature or arm length alone, suggested gender differences in strength that could not be explained by differences in body size. All torque measures were best explained by inclusion of muscle CSA and a linear dimension in the models. Age also explained additional variance in torque, but the influence of age was action and muscle specific. Conclusion: 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.