Itchy skin on the hind legs after running may be caused by sweat stimulation, cholinergic urticaria and other reasons. 1. Sweat stimulation: if the patient runs for too long, it will lead to a lot of sweating, and due to the large amount of sweat stimulation and prolonged friction with the clothes, it may lead to localized skin inflammatory reaction of the hind legs, which will lead to itchy symptoms. 2. Cholinergic urticaria: patients with this kind of disease may prompt the release of acetylcholine from the nerve endings after running, which may cause skin allergic reaction and thus itching symptoms. To summarize, if patients have severe or persistent itchy skin on the back legs, it is recommended to go to the hospital in time to clarify the cause of the disease and target treatment.