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Notice in Figure 2, that when the hip extensors are maximally stretched, a dynamic pull travels down the lateral leg and tugs on the tibialis anterior/peroneus longus stirrup.
Heel strike during running activates this antigravity stirrup system and delivers energy back up from through the biceps femoris and sacrotuberous ligament where it combines forces with other muscles to rotate the sacrum and pelvis and counter rotate the lumbar spine (figure 3). When working in perfect harmony with the other three spring systems (described in the Don’t Get Married article on my site), it winds-up the intervertebral joints and deep ligamentous structures creating the smooth cross-patterned gait we’re used to seeing in highly functioning individuals.
However, if the soles of the shoe are too cushioned or the running surface is too soft, i.e., sand, the tib anterior and peroneus longus stirrup can’t quite raise the arch and, therefore, is unable to properly distribute ground forces up through the biceps femoris and sacrotuberous ligaments to help secure the sacrum on the ipsilateral side. In an effort to get this stirrup spring system going, the biceps femoris must exert greater force to elevate the arch and this is where the problems begin. |