A discussion of running-related injuries necessitates a brief summary of the biomechanics of running. During running, each foot strikes the ground 50 to 70 times per minute for each foot. The force produced is two to four times the runner's body weight. This force is distributed through the runner's footwear, and transmitted upwards through the lower extremities and into the pelvis, sacrum, and spine, exposing these structures to increased axial stresses. There are two main phases of running: the support and airborne phases. The support phase consists of the heel strike, midstance, and toe-off. The airborne phase consists of the follow-through, forward swing, and descent. There are complex motions of the subtalar joint and other joints of the lower extremity during these phases. At heel strike, there is dorsiflexion and supination of the foot, and slight external rotation of the tibia. Following heel strike, the foot pronates during approximately 60% of the midstance phase, and there is internal rotation of the tibia on the talus . The excessive stresses of each of these complex but normal biomechanical phases of the weight-bearing phase of running may be magnified by altered biomechanics such as excessive pronation or supination of the foot, pes planus and pes cavus deformities, genu alignment deformities, leg-length discrepancies, and scoliosis .
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