What are the underlying differences between contortionists and others - at the molecular or genetic level?

The answer is not known.

There was an early view in the medical profession, from Beighton, Grahame and others, that contortion arises from collagen disease. That is, they suggest that contortion is a variant of the Joint Hypermobility Syndromes such as Marfan Syndrome and Ehlers-Danlos Syndrome. These arise from mutations in the genes coding for collagen and related molecules.

Others suggest that some aspects of this view are wrong. Patients with collagen diseases have weak collagen, and tend to suffer from a large number of disabilities, with fragile tissues, often including ruptured blood vessels, heart and intestinal problems, frequent dislocation of the joints, and joint pain. In addition, those studied by Beighton, Grahame and associates initially presented at the clinic with pathological symptoms (joint dislocations, joint pain, tissue damage) so the conclusions may be based on a sample already selected for pathology.

In contrast, contortionists tend to be very strong and fit, although they are also very stretchy. Their collagen is strong, not fragile. While laxity may mean that contortionists tend to dislocate their joints relatively easily, with training they are able to strengthen and protect those joints so that in a contortion act they can perform incredible feats of strength (such as a one-hand handstand, holding an inverted position for a long time, or holding the weight of the lower body over the head in an extreme backbend, requiring great strength in the lower spine). Performing contortionists do not in general suffer the problems of tissue fragility seen in Ehlers-Danlos and similar Syndromes.

Therefore it might seem that Ehlers-Danlos Syndrome and contortion ability are quite separate phenomena, presumably with different molecular bases. However, while we do not know the molecular bases of many of these diseases, or of ability in contortion, there is an emerging view that all hypermobility syndromes, benign or not, may be related, and may be part of the same spectrum of phenomena, though with different outcomes in different individuals (reference 1). Moreover, some performing contortionists go on to develop clear signs of Ehlers-Danlos Syndrome later in life (in Ehlers-Danlos Syndrome, hypermobility tends to dominate early in life, while other symptoms tend to dominate later in life). This suggests that in such individuals extreme flexibility and Ehlers-Danlos syndrome are related.

One hypothesis is that flexibility with strong collagen, such as can be found in contortionists, could be related to the degree of crimp in collagen. Collagen gives flexible tissues strength, and under a microscope it has a wavy structure (called “crimp”). When collagen is stretched, the waviness gets straightened out, so the collagen stretches easily at first. Once the waviness has got fully stretched out, the stiffness of the collagen increases markedly. It is likely that variations in the degree of crimp, and how the collagen molecules are packed together, are able to affect the stretchiness of collagen while keeping it strong.

The crimp appears to be derived from the basic molecular structure of collagen and how the collagen molecules are packed (see references 2-4) – variation in the related genes may be responsible for conferring the ability. A PDF file of a basic talk on some of these issues can be found here (the talk as presented included movies - not part of this PDF - including those of the vertebrae moving; it also did not include the more recent views described in reference 1).

There is evidence that the ability can be inherited. People in families with flexible family members often refer to which family members have or have not inherited “the bendy gene”. Early training is also important, but should not start too early because young and vulnerable bodies need to be protected, and strength, body awareness and control have to be trained along with flexibility. One trainer of young people has suggested that 25% of the population have the ability if training is started early.

An MRI study of contortionists by Peoples et al (2008) showed that contortionists could stay remarkably healthy and appear relatively youthful at least up to the age of 49 (the oldest measured in that study): “Given the degree of stress placed upon the spine by these elite athletes there was a surprisingly limited amount of pathological change present within their spinal column. This no doubt reflects their rigorous and dedicated training routines.” (5). These subjects were all Mongolian, and may have been exposed to relatively severe (and hence possibly damaging) traditional training in their early lives.

As performers, contortionists may of course suffer from overuse injuries, just as do other performers, including acrobats and ballet dancers, rather than specifically because of contortion. Anyone doing 3 shows a day, 7 days a week (as can happen in many circuses), is likely to get overuse injuries.

References:

Overview of relation between different Joint Hypermobility Syndromes:

1. Castori M, Colombi M. (2015) Generalized joint hypermobility, joint hypermobility syndrome and Ehlers-Danlos syndrome, hypermobility type. Am. J. Med. Gen. C 169C: 1-5.

The crimp in collagen, and the factors that affect it:

2. Miller KS, Connizzo BK, Feeney E, Soslowsky LJ. (2012) Characterizing local collagen fiber re-alignment and crimp behaviour throughout mechanical testing in a mature mouse supraspinatus tendon model. J. Biomech., 45: 2061-2065.

4. Franchi M, Raspanti M, Dell'Orbo C, Quaranta M, De Pasquale V, Ottani V, Ruggeri A (2008) Different crimp patterns in collagen fibrils relate to the subfibrillar arrangement. Connect Tissue Res. 49: 85-91. 

An MRI study of contortionists: