# Children With Fragile X Syndrome Display a Switch Towards Fast Fibres in Their Recruitment Strategy During Gait

## TL;DR

Children with Fragile X Syndrome use fast muscle fibers more during walking, which may explain their fatigue and difficulty with exercise.

## Contribution

The study reveals a distinct fast-twitch fiber recruitment strategy in FXS children during gait, not previously characterized.

## Key findings

- FXSFull and FXSMos children show higher energy in fast-twitch fibers compared to controls.
- Increased Instantaneous Mean Frequency suggests reliance on fast fibers in FXS children.
- This recruitment pattern may explain exercise intolerance and fatigability in FXS.

## Abstract

Fragile X Syndrome (FXS) is a genetic disorder caused by the lack of FMRP, a crucial protein for brain development and function. FMR1 mutations are categorized into premutation and full mutation (FXSFull), with somatic mosaicism (FXSMos) modulating the FXS phenotype. Recent studies identified muscle activity alterations during gait in FXS children. This study aims to explore the relationship between these muscle activity changes and motor fibre recruitment strategies during gait in FXS children.

Fifty‐four FXS children and fourteen healthy controls participated in the study. Gait trials at self‐selected speeds were recorded using four synchronized cameras and a surface electromyography system that captured bilateral activity of Gastrocnemius lateralis, Tibialis anterior, Rectus and Biceps femoris muscles. The continuous wavelet transform, using the ‘bump’ mother wavelet, provided the percentage distribution of signal energy across nine frequency bands (50‐Hz increments within a 450‐ to 10‐Hz spectrum) and the Instantaneous MeaN Frequency (IMNF) time‐frequency distribution.

Results indicated that both FXSFull and FXSMos children exhibit a distinct fibre recruitment strategy compared to controls, with a higher percentage of total energy and elevated IMNF (p < 0.05).

This increased reliance on fast‐twitch fibres may contribute to the observed fatigability and exercise intolerance in FXS children.