The left arm is smaller than the right arm muscles for a number of reasons, mainly seen in the right arm muscle exercise with the left arm, but also seen in the left arm atrophy and left arm related neuropathy, etc., need to further improve the examination, in order to clarify the cause of the disease. 1. Uneven exertion: Under normal circumstances (except for left-handed people), the activities of the right upper limb are more than that of the left upper limb, and the right arm exerts more than the left arm in labor or sports, which in the long run leads to the muscles of the left arm being smaller than that of the right arm. 2. Wasted atrophy: it is mostly seen in patients with serious injury of the left upper limb such as fracture and muscle tear due to trauma, and patients who are given prolonged external fixation treatment. The synthesis of muscle protein is weakened and the muscle contraction force is decreased, so the muscles of the left arm are smaller than those of the right arm, and most of them are accompanied by a feeling of weakness. 3. Neuropathy: mainly due to cerebral infarction, neurogenic cervical spondylosis and other neuropathy caused by the pain and numbness of the left upper extremity, which in turn causes the reduction of the activity of the left upper extremity, muscle atrophy, so the left arm muscle is smaller than the right arm muscle. It is recommended that patients seek medical examination in time in order to clarify the cause of the disease and be given appropriate treatment measures by professional physicians.