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Can you bend your thumb backwards¹ until it touches your wrist? If you join your hands behind your back, can you lift them over your head without letting go?
你是否能将大拇指向后弯曲直至其触碰到手腕部位？你将双手在背后紧握，是否能够将其举过头顶而不松手？

If you can, you might be what some people call “double-jointed.” But, A Moment of Science wonders, how could anyone have double joints³?
如果你能做到，那么你可能拥有双关节。但是，《科学一刻》想知道，一个人怎么可能拥有双关节呢？

Range Of Motion
活动范围

Double-jointed people don’t really have two joints. Any joint², such as the thumb joint or the shoulder, can be called “double-jointed” if the tendons and ligaments surrounding the joint allow a wider range of motion than is usual.
所谓双关节的人并非真正拥有两个关节。任何关节，如肩关节或拇指关节，若围绕其四周的韧带和肌腱的活动范围超出平常，则可被称为“双关节”。

Joints are created whenever 2 adjacent bones meet. They have fibrous bands of connective tissue called ligaments that stabilize⁴ the joint. Connective tissues also form the tendons that join the bones to the muscles.
关节是两骨相接而形成的。他们含有被称为韧带的结缔组织纤维带，韧带能够稳定关节。结缔组织同时也组成了连接肌肉与骨头的肌腱。

Ligaments and tendons usually restrict the movement of the joint, but in some people, these normally restrictive tissues can be quite flexible and allow the extra movement we call “double-jointed.” Practice can increase a joint’s natural flexibility⁵ just the way stretching can increase your muscles’ flexibility.
韧带与肌腱通常会限制关节的活动，但是对于某些人来说，这些结缔组织非常灵活，能做出更大范围的运动，也就是我们所称的“双关节”。通过锻炼能增加关节的自然柔韧性，正如伸展运动可加强肌肉的柔韧性一样。

Flexibility
柔韧性

While flexible joints are very useful to contortionists, some joints can become so loose that they are unstable⁶. This allows the bones of the joint to slip out of their sockets⁷ and can be very damaging.
灵活的关节对柔术演员来说至关重要，但他们的一些关节可能会变得十分不稳固。这很容易造成关节脱臼，对身体造成很大的伤害。

Strengthening the muscles around the joint can stabilize it and still allow you to show off your trick joint at parties.
加强关节周围肌肉的锻炼能够使关节稳固，并且你依然可以在各式派对上炫耀你的小把戏。