如果你把两瓣屁股用力往中间挤，试图使它变成一块浑然天成的坐垫，你会失望地发现，完全做不到。此时此刻弱小无助的你不得不独自面对一件令人细思极恐的事实：
必须是左右两瓣吗？想象一下替代方案：
如果屁股是上下两瓣，那么你走路的时候屁股就会噼啪作响，随着步伐的节奏而律动，非常尴尬。如果屁股是一整个圆乎乎的，你将看到一个个左右摇晃的不倒翁艰难地蠕动。
奇形怪状的屁股，原来屁股可以有这么多形态 / 图源：小红书
从你还是一个胚胎的时候，身体就已经被设定成了左右镜像模式。当妈妈怀孕第 4.5-5 周时，小婴儿的身体开始分节，像一节节高铁车厢，左右对称排列——这叫 " 体节 "。
体节在生长因子等信号波的精密调控下，严格遵循双侧对称原则发育 [ 1 ] 。发育的结果就是身体长成了看起来像复制粘贴的左右两半，左眼配右眼，左腿配右腿，屁股自然也得按这个模板长。
两半的身体不是人类所独有的，从鱼到鸟，外部双侧对称是脊椎动物的普遍特征 [ 3 ] [ 4 ] 。而这种对称性在脊椎动物出现之前，即数亿年前的埃迪卡拉纪就已经演化形成。双侧对称性让生物能分清前后左右，进行定向运动，也让感觉器官可以左右配合、立体感知 [ 5 ] [ 6 ] 。
动物如猴子和猩猩的屁股也是左右两瓣的，但是它们的屁股看起来更扁平 / 图虫创意
既然双侧对称性是生物界的普遍规律，动物和人类同样都有两瓣屁股，为什么偏偏人类的两瓣屁股更容易让人在意呢？
人的骨盆两侧，那两块像翅膀一样张开的骨头，叫髂骨。这是整个屁股结构的 " 地基 " [ 7 ] ，也是臀肌的起始点 [ 2 ] 。哈佛大学的一项研究表明，大约 500 万到 800 万年前，当人类的祖先从非洲猿类中分化出来时，控制骨盆发育的基因发生了关键突变，重新编程了骨骼生长方式 [ 8 ] 。
人类的骨盆与附着在上面的肌肉 / 图虫创意
黑猩猩的髂骨是高而窄的刀片状，适合攀爬时锚定肌肉；而人类的髂骨则横向张开，形成了独特的 " 碗状 " 骨盆 [ 8 ] 。也就是说，
前锋的臀部很发达，能在爆发性运动中维持身体平衡 / 图虫创意
髋关节的稳定性和肌肉发力，依赖于肌肉附着点在骨骼上的精准位置 [ 16 ] 。
罪魁祸首是椅子。现代久坐的生活方式使臀大肌与臀中肌处于持续拉伸状态，而前侧的髋屈肌因为一直弯着而缩短紧张 [ 18 ] 。一边被拉长放松，一边持续紧张，屁股变得又扁又塌，天生的 " 蜜桃臀 " 慢慢变成了 " 方形臀 "。长此以往，大脑适应了这种状态，神经系统会减少发送信号，逐渐忘记如何激活臀肌。
当屁股 " 失忆 " 后，腰部肌肉、大腿后侧的腘绳肌、膝盖周围的肌肉都得顶上来分担原本属于臀肌的工作，不得不疯狂加班 [ 19 ] [ 21 ] 。结果就是，腰痛、膝痛、骨盆前倾全家桶统统找上门来 [ 19 ] [ 21 ] 。
但是健身大佬们并不服输。为了拯救失忆的屁股，侧卧抬腿、单腿臀桥、蚌式开合、四足位后抬腿、单腿罗马尼亚硬拉，让臀肌回忆起工作的状态。好消息是，屁股确实可以在可以练习中恢复 " 饱满 " [ 18 ] 。
健身大佬拼命挽救中 ...... / 图虫创意
所以说老板应该把办公椅改成床，躺着上班，为了尊重屁股的劳动成果，也是为了让打工人拥有更曼妙的屁股。
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