Humans can’t really be double-jointed, though some of us are owners of surprisingly flexible joints. And that can have some surprising effects, says Jason G Goldman.

No doubt you know someone (or more likely, knew someone as a child) who boasted they were double-jointed. These braggarts would then bend their thumbs all the way back to meet their forearms. But for all their bluster, these schoolyard show-offs were not actual medical freaks. For humans at least, there is no such thing as being double-jointed. Those bendy-bodied boasters? They're just impressively flexible.

If you're born with looser ligaments, you'll be able to move it more - Anatomist Michael Habib

Physicians and researchers call it joint hypermobility or joint laxity, and it simply means that someone is able to move their joints farther than most people can. Most of us can bend our thumbs backwards a few degrees, but some can bend it farther. All of us can bend at the waist, but some the joints of the spine allow some people to bend over so far they can place their hands flat on the floor. Everyone can spread his or her legs, but only some people have joints in the pelvis mobile enough to do the splits.

And to qualify as "hypermobility", that added flexibility must arise without any practice or stretching. Some athletes and dancers, for example, can eventually train their bodies to be more and more flexible, but the sort of hypermobility with which we concern ourselves right now is something you’re born with.

Understanding how a joint can be naturally more or less flexible requires a quick lesson in human anatomy. There are two things that limit the motion of a joint: the contours of the bones and cartilage, or the ligaments. "You can have a physical hard stop – it can't go any further because something hits something – or you can have a ligament holding it in place," says USC anatomist and vertebrate palaeontologist Michael Habib.

Hooks and grooves

When someone is able to pull their thumb back to their arm, that's usually because their ligaments have afforded them a little extra motion. "If you're born with looser ligaments, you'll be able to move it more," says Habib.

Some double-jointed individuals, on the other hand, owe their flexibility to their bones. A common form of hypermobility involves the elbow. Some people can bend this joint in the wrong direction so that their forearm swings beyond 180 degrees. There's a knob of bone that forms the pointy part on the back of the elbow called the olecranon. "It's got this little hook on it. Actually, a big hook," explains Habib. That hook fits into a small groove on the back of the humerus, the upper arm bone. And when you're extending your arm, that hook slides into that little groove. When it hits the end of the groove, you can't bend your elbow any further. "If your olecranon is small, or your groove is deep, you'll be able to go past 180 degrees," says Habib. "So individuals who can hyperextend their elbows can do that simply because their bones are a little bit differently shaped."

A 2004 study of adult female twins in the UK supported earlier claims of a genetic basis for hypermobility: pairs of identical twins were nearly twice as likely to both have hypermobility than were fraternal twins. As long ago as the 1930s and 1940s, researchers observed that hypermobility seemed to run in families.

And research has also revealed that hypermobility diminishes with age. Children on average are more flexible than their parents and grandparents. Women have more joint flexibility than men, though this may in part be because they tend to be smaller. There is also evidence that people of African, Asian, and Middle Eastern descent may also, on average, be more hypermobile than those of European descent.

 If a joint is really flexible, it's not going to be as stable - Michael Habib

For most hyperflexible folks, their abilities pose no harm and don't hurt. But others can be diagnosed with "benign joint hypermobility syndrome" (BJHS), which can prove painful despite its name. In the Journal of the American Osteopathic Association, Michael R Simpson reports that between 4% and 13% of people have BJHS, usually in the thumbs, pinkies, elbows, knees, or vertebrae. 

Dancers’ dilemma

There is one group of hypermobile people who have been the focus of much scientific inquiry: dancers. That's because even in the most benign cases, hypermobility is often associated with a lack of stability. "If a joint is really flexible, it's not going to be as stable, and you end up having to use more of your muscle power to stabilise that joint," says Habib. As a result, hypermobile people end up having to use their energy on stabilising themselves rather than on doing the actions they intend, like lifting (for elbows) or even standing straight (for knees or the spine).

In a 2012 paper, physical therapist Mark C Scheper of Amsterdam's University of Applied Sciences explicitly asked whether generalised joint hypermobility was "a sign of talent or vulnerability" for professional dancers. "For aesthetic reasons, increased flexibility is often promoted and is viewed as an essential integral part of professional dance education," he writes, but it can also lead to more pain and fatigue than in non-hypermobile dancers. And that, in turn, can lead a dancer to develop psychiatric symptoms related to depression and anxiety.

 Hypermobile dancers were less physically fit than women from the health school

For his study, Scheper and his colleagues turned to female dancers from the Amsterdam School of Arts Academy, and compared them to women from the nearby Amsterdam School of Health Professions. He found that generalised joint hypermobility was associated with "lower muscle strength, lower submaximal exercise capacity, and decreased functional walking distance" for patients from both schools. However, joint hypermobility was present in a far larger proportion of dancers than health professionals.

‘Physical challenges’

Hypermobile dancers reported higher levels of fatigue than hypermobile health professionals. The researchers suspect that could reflect the more strenuous nature of a dance education in general, but it is telling that even with all their training, hypermobile dancers were less physically fit than women from the health school. In addition, professional-level dance requires not just flexibility, but also extreme control and precision. Those with hyperflexible joints may become more exhausted trying to maintain that precision.

Joint hypermobility may indeed be more a liability than an asset for professional dancers 

All dancers were more anxious than were the health professionals, but hypermobile dancers had the most anxiety of all. "It could be," writes Scheper, that this subset of dancers "[has] to make additional efforts to reach the requirements of professional dance education but also have additional physical challenges to maintain their skill level."

In light of their findings, Scheper and his colleagues suggest that joint hypermobility may indeed be more a liability than an asset for professional dancers.

But we should avoid generalisations, say sports scientist Howard Bird and Elaine Foley. Armed with more sophisticated knowledge and understanding of human anatomy, Bird and Foley point out that dance itself is not a single intransigent monolith. "The requirements of ballet differ considerably from the requirements of contemporary dance," they write, and even within contemporary styles, the techniques of Martha Graham and Merce Cunningham, for example, are also wildly disparate.

Indeed, evaluating the impacts of extreme mobility on dance requires something more (ahem) flexible than a modest survey of joints.

Correction: This article has been edited to reflect the difference between generalised hypermobility and benign joint hypermobility syndrome

Share this story on Facebook, Google+ or Twitter.