In Gait, running and other weight bearing activities, understanding begins with the foot. In normal Gait when taking a step, the heel hits the ground and the calcaneus strikes on its lateral aspect. The calcaneus then everts (tips inward--frontal plane motion) secondary to gravity and ground reaction forces. Along with the eversion, the subtalar joint abducts (Transverse plane motion). Dorsiflexion then occurs by the lower leg moving across the planted foot (sagittal plane motion). These combined motions describe pronation movement in all three planes at any joint to load the muscles. Pronation is also occurring at the midtarsal joint. This is described as inversion, abduction and dorsiflexion of the forefoot in relation to the rearfoot. Pronation at the rearfoot loads or lengthens the posterior tibialis muscle and the calf muscles (including the Achilles tendon). Pronation at the forefoot lengthens the peroneus longus.
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Calcaneal eversion, and the resultant pronation at the foot, causes pronation to occur along the entire lower extremity kinetic chain. Higher up the chain, the knee flexes (sagittal plane), abducts (Frontal plane) and the internally rotates (transverse plane). As a result, the hip follows by flexing (sagittal plane), adducting (frontal plane) and internally rotating (transverse plane). This pronation at the hip lengthens or loads the hip extensors including the hamstrings, the abductors, the iliotibial band (IT band) and the external rotators.
This knowledge of biomechanics demonstrates that stretching/lengthening a muscle in a single plane does not achieve full lengthening of that muscle. Additionally, most athletic activity and activities of daily living occur with one or two feet on the ground. The importance of the interaction of the joints in all three planes, the resultant lengthening of the musculature, and greater range of motion of the joints which occurs in the weight bearing positions (and does not occur with the foot off the ground) enforces the need to perform stretching in both a weight bearing position and in all three planes of motion.