Bruising on the legs for no reason, that is, bleeding under the skin and mucous membranes, is considered to be caused by physiological factors, platelet abnormality, abnormal coagulation function, trauma and other factors. 1. Physiological factors: in the summer when the weather is hot, the permeability of blood vessels is higher, especially for women compared to men, their blood vessels are more brittle, even if the ordinary bump can also appear subcutaneous bruising phenomenon. 2. Platelet abnormality: such as the common thrombocytopenia, can lead to skin and mucous membrane bleeding, manifested by the sudden occurrence of extensive and serious skin and mucous membrane bruising, and even large petechiae and hematomas, mainly in the lower limbs. Common diseases include leukemia, aplastic anemia, thrombocytopenic purpura and so on. 3. Abnormal coagulation function: when the coagulation function in the body is abnormal, it can also cause bleeding under the skin mucous membrane, thus leading to the appearance of skin ecchymosis, which is mainly divided into hereditary and secondary coagulation abnormality, in which hereditary coagulation abnormality is mostly seen in hemophilia. 4. Trauma: When some patients are performing strenuous exercise, the external force touches or collides with the soft tissues of the legs, thus leading to traumatic bruises. This condition can also lead to bruising under the skin of the legs, and is accompanied by pain and swelling and other uncomfortable symptoms. Once the leg bruises appear for no reason, we should pay attention to the hospital in time to improve the examination, to clarify the specific cause of the disease after targeted treatment.