A Time-Efficient and Reproducible Rat Model of Myofascial Pain Syndrome-Like Phenotypes Induced by Repeated...

Existing animal models of myofascial pain syndrome (MPS) target different pathological mechanisms and often rely on invasive or intensive interventions to induce inflammatory or neurophysiological responses. However, models specifically addressing peripheral myofascial structural remodeling, particularly fibrosis-associated phenotypes, are still limited. Therefore, a simplified and reproducible model focusing on myofascial remodeling-related dysfunction is needed.

Fifty adult male Sprague-Dawley rats were randomly assigned to control, sham, one-, two-, or three-session groups (n = 10/group). Intervention groups received combined mechanical impact (right gastrocnemius, 2.5 g, 12 m/s) followed by downhill treadmill running (-16°, 16 m/min, 60 min) every five days, for one, two, or three cycles. Behavioral and functional outcomes, including muscle nodule incidence, mechanical pain threshold, ankle mobility, and balance beam performance, were assessed weekly for four weeks. Ultrasonography, muscle stiffness, and histology evaluated structural changes.

Control and sham groups showed no alterations. All intervention groups developed transient nodules and reduced pain thresholds at week 1. One-session rats recovered by week 4, two-session rats partially recovered, while three-session rats exhibited persistent nodules, mechanical hyperalgesia, increased stiffness, and long-term functional impairment. Ultrasonography and histology in the three-session group revealed localized structural abnormalities, fiber disorganization, and collagen deposition.

Three sessions of mechanical impact combined with eccentric exercise at five-day intervals produced a reproducible rat model of repetitive muscle injury-induced MPS-like features. The model consistently induced peripheral myofascial structural remodeling, mechanical hyperalgesia, increased muscle stiffness, and functional deficits for at least four weeks. This time-efficient protocol may provide a useful platform for mechanistic studies and preclinical evaluation of therapies targeting peripheral fibrotic and mechanical mechanisms associated with myofascial pain. However, it primarily reproduces peripheral myofascial alterations rather than the full clinical and pathophysiological spectrum of MPS.