It's the same reason that a radiator has long coils, to increase surface area compared to volume to let heat out, and because the leg is like a pendulum, the longer and thinner it is at the extremity, the more energy-efficient it is to swing.
这与散热器有很长的线圈 去增加相对于体积的表面积 而使热量散出是同样的原因， 因为腿就像一个钟摆， 更长和更细， 摇摆就有更多的能量效率。