Editor-In-Chief: C. Michael Gibson, M.S., M.D. 
The cremasteric reflex is a cutaneous reflex observed in human males.
This reflex is elicited by lightly stroking the superior and medial part of the thigh in a downward direction. The normal response is a contraction of the cremaster muscle that pulls up the scrotum and testis on the side stroked.
More specifically, the reflex utilizes sensory and motor fibers of the L1 spinal nerve. When the inner thigh is stroked, sensory fibers of the ilioinguinal nerve are stimulated. These synapse in the spinal cord and activate the motor fibers of the genital branch of the genitofemoral nerve which causes the cremaster muscle to contract and elevate the testis.
In children, this reflex may be exaggerated, and this can lead to the mistaken diagnosis of undescended testes.
Upper and lower motor neuron disorders, as well as a spine injury of L1-L2, can cause an absence of the cremasteric reflex. It has also been reported to be absent in 100% of cases of testicular torsion, making it a useful sign in this difficult diagnosis.
According to the Straight Dope, it is a classic urban legend that Sumo wrestlers raise their testicles in preparation for training and/or combat. While it may be possible to do this via the pubococcygeal muscle or acquired control over the cremaster muscle, it is not practiced by Sumo wrestlers and Sumo does not allow contact with this region.