Many lesions can appear as muscle weakness, which is also the most common symptom of many patients with muscle weakness. It can also be roughly presumed from the clinical manifestations alone whether the patient’s lesion is located in the muscle or the nerve. The muscle weakness is caused by the muscle lesion itself: it is mostly symmetrical weakness of the extremities, proximal end is more common, the patient can show difficulty in going upstairs, difficulty in standing up from squatting, difficulty in lifting the upper extremities, not accompanied by numbness of the extremities, muscle atrophy and other manifestations can appear. If it appears in children or adolescents, it is mostly seen in myotonic dystrophy. In young and middle-aged people, this type of manifestation is often seen in myotonic dystrophy, metabolic myopathy and inflammatory myopathy. In the elderly, these manifestations are often seen in inflammatory myopathies or tumor-associated myopathies. Myasthenia gravis due to peripheral neuropathy: it can occur symmetrically or asymmetrically, with the distal limbs being more prominent, and more often accompanied by sensory abnormalities in the limbs, suggesting lesions of both motor and sensory nerves. The most common cause is vasculitis, and more common is polyneuropathy, of which the most common is peripheral neuropathy due to diabetes mellitus, and acute Guillain-Barre syndrome, among other manifestations. Myasthenia gravis due to neuromuscular junction lesions: often appear symmetrically, if the eyelids droop, the weakness of the limbs, the biggest difference with the first two causes there is a clear phenomenon of morning light and evening heavy, can improve after rest, electromyography repetitive frequency electrical stimulation will show obvious abnormalities, the most common is myasthenia gravis and paraneoplastic-associated myasthenia gravis syndrome.