# Maturity-related acute effects of resistance exercise on muscle architecture and tissue organization of the resting and maximally isometrically contracted gastrocnemius medialis in girls

## TL;DR

This study investigates how resistance exercise affects muscle structure in girls of different maturity levels, finding that maturity influences muscle architecture but not the acute response to exercise.

## Contribution

The study introduces a detailed analysis of maturity-related differences in muscle architecture and tissue organization following resistance exercise in girls.

## Key findings

- Acute resistance exercise increases muscle thickness and alters tissue organization in girls regardless of maturity level.
- Post-pubertal girls have thicker muscles and different pennation angles compared to pre-pubertal girls.
- Biological maturation significantly influences baseline muscle architecture and tissue organization.

## Abstract

This study aimed to examine maturity-related differences in the acute response of resistance exercise on the muscle architecture and tissue organization of the gastrocnemius medialis (GM).

Thirty-three healthy, physically active girls (11 pre-pubertal, 10 pubertal, 12 post-pubertal) conducted isometric maximum voluntary contractions (iMVC) before (pre), immediately after (post), and 15 min after (post_15) a resistance exercise (i.e., single leg isokinetic calf raises). Ultrasound scans were captured at pre, post, and post_15 during iMVCs as well as during rest. Muscle architecture measurements included muscle thickness (MT), superior and inferior pennation angles (PA), and fascicle length (FL). Spatial frequency analysis (SFA: peak spatial frequency radius [PSFR], peak −6 dB width [P6], quality factor [Q6], normalized peak value of amplitude spectrum [Amax], power within peak [PWP], peak power percent [PPP]) assessed muscle tissue organization.

Main effect of time reveals a significant acute increase in MT, Amax, PWP, and PPP from pre- to post-exercise, alongside a decrease in PSFR. However, analysis indicates no significant interaction between time and maturity group (p > 0.05). Concerning the main effects of the maturity group, the analysis shows significant differences: post-pubertal girls exhibit thicker MT, greater superior/inferior PA, and higher P6, but lower Q6, Amax, and PWP compared to pre-pubertal girls.

The acute response suggests muscle swelling with increased fiber spacing and additional scattering material within the muscle, with no variation among girls of different maturity levels. Differences in muscle architecture and tissue organization between maturity groups highlight the role of biological maturation in muscular system.