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.