• Objective: To determine the correlation of trunk muscle strength with age in children and the effect of gender on changes in trunk muscle strength. • Material and Methods: Strength of the back extensors and back flexors was evaluated in 137 healthy boys and 109 healthy girls who were 5 to 18 years old. Anthropométrie determinations of height and weight were done in all study subjects. Maximal back muscle strength was measured with an isometric dynamometer (BID-2000). Standardized positioning techniques were used to allow comparison with the follow-up evaluations. • Results: Regression analysis revealed significant increases in trunk strength with increasing age; the strength of the boys began diverging from that of the girls at age 9 to 10 years. Back extensor strength also increased with increasing height and weight, and expected differences were noted between boys and girls. Strength levels ranged from 80 to 774 newtons (18 to 174 Ib). Mean back extensor strength in 2-year age intervals ranged from 153 newtons (34.4 Ib) in girls 5 to 7 years old to 504 newtons (113.3 Ib) in boys 16 to 18 years of age. (Each newton is equal to 0.2248 Ib.) • Conclusion: Results of this study demonstrated that age, height, and weight are all important predictors of trunk strength in children, and the effects of these factors are modulated by gender.
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