Back in the glory days of the Ringling Brother's Circus, one of the highlights featured the contortionist Paul Brachard and his amazing "teeth sitting" act. Brachard would do a handstand in front of a teeth bit, then slowly do a backbend until he was literally sitting on his head. Finally, he would clamp his teeth on the bit and slowly raise his hands off the ground. Incredibly, Brachard was able to support the weight of his bizarrely twisted body with only his teeth.
Most people assume that contortionists are able to bend their bodies in extraordinary shapes because they are double jointed. According to mainstream medicine, however, double-joints do not exist. Rather, contortionists are highly flexible in several key joints, and especially in the spine.
Several internal and external factors affect human flexibility, including internal resistance within a joint, the elasticity of muscle tissue, the elasticity of ligaments and tendons, age, gender, and practice. Generally, young people are more flexible than older people, and women are more flexible than men. With practice, however, anyone can become more flexible.
This does not mean that anyone can become a professional contortionist--most are born with unusually flexible spines and other joints. Although often derided as natural freaks, contortionists train constantly to perfect their craft. Starting as early as age five, a contortionist further loosens her joints and connective tissues until they allow her body to twist into extraordinary shapes.