Case <p>A 9-year-old girl presented with visible, rhythmic fasciculations in the peroneal muscle groove of her right ankle at rest, accompanied by pain and mild movement limitation. Dynamic ultrasonography showed persistent intrasheath subluxation of the peroneus brevis tendon, with fasciculations correlating with tendon dislocation. After excluding motor neuron disease and metabolic disorders, she underwent surgery involving fibular groove deepening and retinaculum repair. At the 6-month follow-up, ultrasound demonstrated tendon stability and no recurrence of fasciculations at rest or during exercise.</p> Conclusion <p>This study identifies peroneus brevis subluxation as a novel potential cause of persistent, visible fasciculations, adding to the traditional view that fasciculations always indicate a neurogenic origin. In this case, anatomical reduction that eliminated abnormal biomechanical stimulation successfully resolved symptoms, supporting the hypothesis that surrounding tissue structure abnormalities can cause focal fasciculation in children. This finding offers a new direction for diagnosing and treating disorders involving sensorimotor integration dysfunction and highlights the important interplay between musculoskeletal structural abnormalities and neurological symptoms in clinical practice.</p>