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英文字幕：

Imagine you're this athlete, flipping through space. You can't trust your eyes. All you can do is feel.

Hi, I'm Dr Greg Wells. From no-hand cartwheels to no-look backflips, gymnasts on the balance beam seem like they're spinning off into the void. But then, somehow, they manage to land on a narrow beam just 10 centimetres wide. So how do they possibly pull it off?

They rely on what's called proprioception. Here's how it works: Muscles and joints have nerve cells called proprioceptors. They shoot messages to our brain about where our limbs are in space -even if our eyes can't see them! It's an amazing internal sonar -the same way bats fly in the dark without crashing.

But they also need what's called hyperextension. Spines have 33 bones, or vertebrae protecting the spinal cord. They're separated by discs that act like shock absorbers. Normal backs have 20-35 degrees of hyper-extension, but gymnasts have at least twice that range!

There's a downside to hyper-extension. It happens when a vertebrae slips out of place in the spine. If it hits a nerve, it can be horribly painful, and might even need surgery to repair. It only happens to about 6% of us, but for gymnasts, the number climbs to almost 50%.

Internal sonar...Hyper-flexibility...It's risky flying this high, but that's the only way to gold!

中文字幕：

试想象您是这名选手，在空中翻跟斗。您的眼睛已不够迅速，您只能靠感觉。

我是维尔斯博士。从侧空翻到后空翻，平衡木上的体操选手看似会摔倒，但她们最终仍成功地站在仅十厘米宽的平衡木上。怎么可能这么准确呢？

她们依靠的是本体感觉。其中的原理是：肌肉和关节有一种叫本体感受器的神经细胞。即使我们自己看不见，大脑仍能接受到四肢位置的信息。它就像一部内置声波定位仪。这和蝙蝠在黑暗中飞翔而不受碰撞是一样的道理。

但她们也需要所谓过度伸展能力。我们的脊椎包含33块骨头保护着脊髓，椎骨之间有片状组织，它可以吸收震荡力。正常背脊可以过度伸展20至35度，但体操选手可以伸展到正常背脊的两倍。

但过度伸展有其风险。过度伸展时椎骨会滑动移位，若压迫到神经会引起剧痛，甚至可能需要手术治疗。约6%的人会出现此症。但体操选手的罹患率却几乎高达50%。

声波定位仪，过度伸展。这样高飞相当危险，但却是夺金的唯一途径。

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（Video provided by GE 编辑：Julie）