Last reviewed February 27, 2013
Remember that kid in grade school who could bend her thumb backwards until it touched her wrist? Or the one who could wrap his legs behind his head? Did they tell you they were double-jointed? Did you ever wonder what that means?
It might sound as though they have two joints in places others have only one, or that they have twice the normal amount of motion, but these are myths. As members of the same species, the vast majority of humans have the same number of bones and joints. While it's true that some people have more flexibility in their joints than others, the expression "double-jointed" implies something that isn’t true.
"Double-Jointed" Means Hypermobile
People with more than normal flexibility of their joints are considered to have hypermobility — which simply means their joints (and surrounding structures, including ligaments and tendons) are able to bend farther than "normal," or, more accurately, farther than average. In most cases, the reason for the extra mobility is not known. The most important questions are whether a person’s hypermobility is a health liability in some way, or whether it’s simply a curiosity. Although the latter is more common than the former, there are many examples of both.
Keep in mind that flexibility can change over time; in particular, it tends to decline with age. Through stretching and other exercises, one can maintain flexibility well into advanced age, but on average, people over age 75 will have less flexibility in their joints than people in their 20s. The specific causes are not clear, although tiny injuries in tendons and ligaments (with limited ability of these tissues to repair damage accumulated over time), diminished muscle mass and tone, and biochemical changes in aging connective tissues all may play a role. It is not clear, however, that loss of flexibility is inevitable in all people. In fact, there are notable examples of people of advanced age maintaining remarkable flexibility.
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Conditions Associated With Hypermobility
Most people with hypermobility are otherwise normal and healthy. It's also possible that they have an as-yet-undiscovered gene mutation, especially when others in the family have hypermobility as well. They may be able to bend forward and touch their palms on the floor, extend their elbows well past straight, or bend their fingers back toward the back of the hand farther than would be comfortable for others. Although club foot or dislocation of the shoulder, hip, or kneecap occasionally complicate hypermobility, most people who have these dislocations are not hypermobile. While hypermobility is usually harmless, there are notable exceptions, such as Marfan’s syndrome and Ehlers-Danlos syndrome, which are associated with hypermobility as well as other potentially serious problems.
Marfan’s syndrome is an inherited condition in which a gene mutation leads to abnormal connective tissue such as tendons, ligaments, blood vessels, bones and cartilage. Because structural support in these tissues is reduced, flexibility may be increased, but other problems, including reduced vision (because ligaments holding the lens and other elastic tissues around the eye are abnormal), chest-wall deformity, leaky heart valves and rupture of large blood vessels may develop. Affected people tend to be tall, with long, slender fingers because of abnormal bone development.
There are at least 10 types of Ehlers-Danlos syndrome, each with different manifestations, since different gene mutations affect different tissues. Most, but not all, cause hypermobile joints. Other problems include easy bruising, rupture of arteries, lax skin, scoliosis (curvature of the spine) and eye problems.
For people with one of the more serious inherited conditions, screening for complications and genetic testing are recommended. For some, doctors may recommend that affected people avoid strenuous exercise or contact sports, have regular eye examinations, and undergo tests to monitor the heart and large blood vessels. For the rest, however, there is usually no need for specific treatment or concern.
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Does Hypermobility Matter?
Hypermobility is common; in one study, 1% of athletes were hypermobile; another study found that 30% of healthy medical students were unusually flexible. When it is not associated with more serious, systemic (body-wide) abnormalities in connective tissue (such as Marfan’s syndrome), hypermobility may cause no symptoms at all or cause annoying (but not life-threatening) joint pain, muscle pain, fibromyalgia, osteoarthritis and back pain. These may be treated in a similar way to people who are not hypermobile, with exercises and/or braces, pain medicines, anti-inflammatory medications and muscle relaxants.
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The Bottom Line
If you have a family history of highly flexible joints or other features suggestive of an inherited connective-tissue disease, learn more about Marfan’s syndrome and Ehlers Danlos syndrome at the links below. There are important ways to prevent complications in these conditions. Recognize, though that most people with hypermobility have nothing so serious.
The notion of being “double-jointed” suggests more than the normal allotment of joints or that the joints have twice the normal motion, but neither is the case. It is true that different species have evolved to have different numbers of bones and joints with different capacities for movement, having adapted over centuries to thrive in their environments. But humans have a set number of bones and joints and only rarely is there significant variation.
For more information, visit the following sites:
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Robert H. Shmerling, M.D. is associate physician at Beth Israel Deaconess Medical Center and associate professor at Harvard Medical School. He has been a practicing rheumatologist for over 20 years at Beth Israel Deaconess Medical Center. He is an active teacher in the Internal Medicine Residency Program, serving as the Robinson Firm Chief. He is also a teacher in the Rheumatology Fellowship Program.