Two fossae at the back of the waist is normal in most cases and is a normal physiological phenomenon. It may also be due to lumbar muscle strain, lumbar spine old fracture, soft tissue scarring adhesion and other factors, can also lead to the back of the waist has two fossa. The two fossae behind the waist in medicine is called “McKessling”, commonly known as “waist nest”, located in the buttocks on the sacrum and lumbar vertebrae on both sides of the joint, mostly by the iliac posterior superior spine after the lateral bony protuberance caused by. In some people, the tendon above the bony prominence is more closely integrated with the skin, so when standing upright, the skin may be depressed due to the pulling of the tendon, resulting in two symmetrical fossae. In patients with lumbar muscle strain, the elasticity of the lumbar muscles decreases, and the muscles pull on the skin when standing upright. Old fractures of the lumbar vertebrae cause bony deformities, creating asymmetrical lumbar fossae. Soft tissue scar adhesions because of the pull of the scar also form an asymmetrical lumbar fossa. Most cases of two fossae at the back of the waist are normal physiologic phenomena and do not require much concern. If the patient suddenly develops waist fossa or waist fossa asymmetry without fitness or obvious changes in body shape, the patient should consult the doctor for a clear diagnosis to avoid delaying the condition.