The penis becomes shorter after running is due to the decrease of blood engorgement of the penile spongy body, there will be a shrinkage of the penis, which is generally a normal phenomenon. The size of the penis is mainly related to the degree of congestion of the corpus cavernosum of the penis, in the external stimulation or when there is a sexual impulse, the corpus cavernosum of the penis will be congested with blood, so that the penis becomes bigger and harder. When exercising, there is no congestion of blood in the penile spongiosum, and the blood of the body will also be dispersed to the muscle tissues of the body, and the penis will appear smaller than usual. Men in adulthood, the penis will not develop twice, regardless of whether the penis is large or small, as long as the health of the foreskin, inflammation and other diseases, generally do not need to worry about, daily good hygiene habits, do not masturbate excessively, underwear and so on, to be loose and comfortable. If there is a prepuce or other inflammation, it is recommended to consult a doctor in time to clarify the cause of the disease and give appropriate treatment.