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Fractures of the os calcis are due to two factors: direct violence by a fall on the sole of the foot, and a strong contraction of the gastrocnemius, making a sudden upward pull, thereby putting tension on the Achilles tendon. We have to deal here with a short fragment attached to one of the strongest muscles of the body, tending to displace it gradually even further than it was carried by force of the direct violence, and in the same direction (from C to D; Fig. 1). This short posterior fragment forms the posterior third or quarter of the longitudinal arch of the foot (E to H), and, when displaced upward, loweres the arch of the foot (changing CG to DG), at the same time stretching or tearing the bowstring which holds this arch normally (GH) and is attached to the anterior surface of the posterior fragment of the os
MAGNUSON PB. MECHANICS OF FRACTURES OF THE OS CALCIS. JAMA. 1917;LXVIII(7):530-532. doi:10.1001/jama.1917.04270020194009