Rationale:Monomelic amyotrophy (MMA) is a benign motor neuron disease with bilateral muscular atrophy in asymmetry and abnormal in the electromyography (EMG). However, we report a case by the muscle biopsy which shows symptoms of slowly progressive amyotrophy despite having a normal EMG.Patient concerns:A 51-year-old male was diagnosed with a lower limb amyotrophy, insidious at the onset and located in the distal thigh and the proximal crus near the knee, slowly progressive weakness, and wasting of his right gastrocnemius muscle for the last 20 years.Diagnoses:He was diagnosed with MMA by the clinical profile, natural history, examinations, and the biopsy.Interventions:We perform dynamic physical therapy for the patient in this case.Outcomes:The positive effects of dynamic physical therapy in this case with MMA were shown in this report.Lessons:The outcome of physical therapy is satisfactory.