# Overuse‐Induced Muscle Disorder: Establishing a Rat Model to Unravel the Role of Fibro‐Adipogenic Progenitor Cells in Intramuscular Fibrosis

## TL;DR

A rat model of overuse-induced muscle disorder was developed to study how excessive exercise causes muscle weakness and fibrosis without visible injury.

## Contribution

A novel rat model using neuromuscular electrical stimulation to study overuse-induced muscle fibrosis without evident muscle injury.

## Key findings

- Excessive isometric exercise caused muscle weakness and fibrogenesis in rats.
- Fibro-adipogenic progenitor activation occurred without visible muscle fiber damage.
- Continuous loading, not injury, may trigger intramuscular fibrosis in overuse disorders.

## Abstract

Overuse‐induced muscle disorders (OIMD) frequently occur in athletes due to excessive and improper use under high physical demand, often leading to muscle pain and weakness. Limited studies have shown intramuscular fibrosis in OIMD, with fibro‐adipogenic progenitors (FAPs), also known as mesenchymal stromal cells (MSCs), playing a crucial role in this fibrosis. This study aimed to develop a rat OIMD model using neuromuscular electrical stimulation‐induced isometric exercise (NMES‐ISO) and to investigate mechanisms by which excessive exercise without adequate rest leads to OIMD. We hypothesize that daily NMES‐ISO would cause muscle weakness and fibrogenesis mediated by FAP activation triggered by muscle fiber damage. Male Wistar rats received daily NMES‐ISO loading on the plantar flexor muscles at a lengthened position. Two weeks of NMES‐ISO led to decreased muscle torque without muscle mass reduction. Masson‐Trichrome stain revealed collagen‐rich fibrogenic lesions in the gastrocnemius muscles, while Evans blue stain detected no muscle fiber damage during the first 5 days. MRI showed increased T2*wi signals correlating with fibrogenic areas. Excessive NMES‐ISO stimulated FAP proliferation. This study established a rat model of OIMD using NMES‐ISO, characterized by muscle weakness and intramuscular fibrogenesis. Contrary to our hypothesis, FAP activation occurred without overt muscle injury, suggesting excessive mechanical loading may directly trigger FAP proliferation and fibrogenesis. It was cautioned that even relatively safe isometric contraction training could lead to FAP activation and intramuscular fibrosis without proper methods and recovery periods.

Our study demonstrated a novel overuse‐induced muscle disorder model in rats using neuromuscular electrical stimulation, enabling investigation of the mechanisms by which continuous loading leads to muscle dysfunction.This model activated mesenchymal stromal cells without evident muscle injury and caused intramuscular fibrosis, challenging the traditional concept that repetitive muscle injury underlies overuse.These findings provide a new perspective on the pathogenesis and treatment of overuse‐induced muscle disorders.

Our study demonstrated a novel overuse‐induced muscle disorder model in rats using neuromuscular electrical stimulation, enabling investigation of the mechanisms by which continuous loading leads to muscle dysfunction.

This model activated mesenchymal stromal cells without evident muscle injury and caused intramuscular fibrosis, challenging the traditional concept that repetitive muscle injury underlies overuse.

These findings provide a new perspective on the pathogenesis and treatment of overuse‐induced muscle disorders.