A sudden soreness and weakness in one arm may be related to over-fatigue, cervical spondylosis, nerve injury and other factors. 1. Overexertion: if you often do strenuous exercise, or often do strenuous housework, it may cause local lactic acid buildup, resulting in arm weakness and soreness, and the symptoms will gradually decrease after rest. 2. Cervical spondylosis: when the cervical spondylosis is more serious and compresses the local nerves, it may cause radiating pain, resulting in arm weakness and pain, and the symptoms may also radiate to the fingers. 3. Nerve injury: for example, brachial plexus nerve injury, generally associated with pulling and fracture, when the injury is serious, resulting in sensory impairment, causing motor disorders, there will be arm soreness, numbness and other symptoms. An arm suddenly sore and weak, in addition to the above factors, but also to consider is frozen shoulder, myofasciitis and other factors. Prolonged malnutrition, calcium deficiency, etc., may also occur with similar symptoms. If the symptoms persist, it is recommended to go to the hospital in time and standardize the treatment under the guidance of the doctor.