當頭向左偏時，頸部右邊肌肉是緊的；當頭向右偏時，頸部左邊肌肉是緊的；當低頭時，頸部後邊肌肉是緊的；當仰頭時，頸部前邊肌肉是緊的；若當頭正不偏不倚時，頸部四周肌肉是鬆的。此時整個重量若於頸椎上，頸椎骨頭支撐整個頭部，頭部重量向下，得到一個反作用力向上，如此頭部的感覺如同頂上有一線懸著，謂之頂頭懸。
(資料來源: mypaper.pchome.com.tw)
從物理角度來看，頭部重量向下，便會得到一個反作用力向上，稱為"作用力---反作用力對"。
In terms of Physics, the weight of the head presses down, there is an opposite reaction force acting from cervical vertebra to the head. It is called action-reaction pair.
In terms of Physics, the weight of the head presses down, there is an opposite reaction force acting from cervical vertebra to the head. It is called action-reaction pair.
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