# Isometric muscle strength profile of lower limbs for children and adolescents between 7 and 15 years of age

## TL;DR

This study examines how lower limb muscle strength develops in Chilean children and adolescents aged 7 to 15, finding that boys outperform girls by age 15.

## Contribution

The study identifies key developmental stages and sex-based differences in isometric lower limb strength among Chilean children.

## Key findings

- Lower limb isometric strength increases from age 10, with boys outperforming girls by age 15.
- Hip flexors, dorsiflexors, hip extensors, and knee extensors explain most of the variance in total lower limb strength.
- Sex-based differences in strength first appear between ages 9 and 12.

## Abstract

Muscle strength is a key determinant of health-related physical fitness and has become a significant biological predictor of overall health and lifespan. This study aimed to characterize the development of isometric strength in the lower limbs of Chilean schoolchildren and adolescents aged 7 to 15 years. Specifically, it sought to: (a) determine when sex-based differences in lower limb strength first appear; (b) identify key developmental stages where significant gains in muscle strength occur in boys and girls; and (c) analyze the contribution of individual muscle groups to total lower limb strength while accounting for sex and age differences.

This cross-sectional study evaluated the maximum isometric strength of seven lower limb muscle groups in a sample of 302 Chilean children, divided into nine age groups at one-year intervals. Hand-held dynamometry was used for isometric strength assessment. Data analysis included a two-factor analysis of variance (ANOVA) for maximum isometric strength and a stepwise multiple linear regression analysis to the entire sample.

Multiple comparisons showed significant differences between the ages of 9 and 12; and sex. The narrowest age range in the progression of maximum isometric strength were: 9–11 years for knee flexors in females (p = 0.0201) and 9–12 years for males (p = 0.0008). Hip flexors, dorsiflexors, hip extensors and knee extensors explained the highest percentage of variance (R2 = 0.897, p < 0.0001) in the total lower limb strength.

Our findings show that lower limb isometric strength in Chilean schoolchildren and adolescents increases from age 10, with boys outperforming girls by age 15. Hip flexors mainly explain total strength, offering a useful reference to detect within and between subject strength deficits.