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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：

試想象您是這名選手，在空中翻跟斗。您的眼睛已不夠迅速，您只能靠感覺。

我是維爾斯博士。從側空翻到后空翻，平衡木上的體操選手看似會摔倒，但她們最終仍成功地站在僅十厘米寬的平衡木上。怎么可能這么準確呢？

她們依靠的是本體感覺。其中的原理是：肌肉和關節有一種叫本體感受器的神經細胞。即使我們自己看不見，大腦仍能接受到四肢位置的信息。它就像一部內置聲波定位儀。這和蝙蝠在黑暗中飛翔而不受碰撞是一樣的道理。

但她們也需要所謂過度伸展能力。我們的脊椎包含33塊骨頭保護著脊髓，椎骨之間有片狀組織，它可以吸收震蕩力。正常背脊可以過度伸展20至35度，但體操選手可以伸展到正常背脊的兩倍。

但過度伸展有其風險。過度伸展時椎骨會滑動移位，若壓迫到神經會引起劇痛，甚至可能需要手術治療。約6%的人會出現此癥。但體操選手的罹患率卻幾乎高達50%。

聲波定位儀，過度伸展。這樣高飛相當危險，但卻是奪金的唯一途徑。

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（Video provided by GE 編輯：Julie）