As all engineers are required to know principles of calculus, so chiropractic biomechanical specialists need to know kinematics.
Contrarily, in the weight bearing position, this F-A joint becomes part of a closed kinematic system when the joints of the foot pronate, causing mandatory internal rotation of the knee and the hip. Another example of a closed kinematic system is the vertebral column, either weight bearing or non-weight bearing. Torsion on the "Y" axis of the head will cause rotation from the atlas (1st cervical) to the fifth lumbar. This is a mandatory response, although not always normal.
Two closed systems may interact not withstanding that biomechanical faults may exist in one or both systems. An example is a mandatory response of the lower extremity initiated by weight bearing pronation which will cause the femoral head to internally rotate. This will interact, although not mandatorily, with the pelvolumbar system creating anterior pelvic rotation and an increase in Ferguson's lumbosacral angle, a condition with which we are very familiar.
More than 4,000 references in chiropractic research archives collection (CRAC) have a bearing on chiropractic thought, yet only two articles deal with kinematics of the foot. One paper was written on rigid foot orthotics and the other by Dr. John Triano on heel lifts.
In the course of an average day each foot has a collision with the ground approximately 10,000 times. Every time heel strike occurs while walking or running, the body must absorb between one and ten times its weight. This calculates to be about five million pounds of shock which must be efficiently absorbed by the foot and leg each day. The body's major shock absorbing system is the pronation of the subtalar joint (STJ) causing the talus to adduct and plantar flex. This causes the ankle mortise to be lowered to the ground during heel strike. The drop in ankle height directly affects shock absorption by acting as a cushion the moment heel strike occurs. In order for the knee to flex, the tibia must rotate faster and farther then the femur. At heel strike, the calcaneus is rapidly causing plantar flexion and adduction of the talus. The motion unlocks the midtarsal joints (calcaneal-cuboid) on the lateral aspect and the talonavicular on the medial arch. Immediately following this first phase of gait, the talus begins to change direction like a car shifting gears, and it moves from adduction toward abduction and supination. At this point the mid-tarsal arch, which was unlocked and mobile, becomes locked, rigid and stable. This is the second phase ending at mid-stance and full weight bearing. The third phase through the fourth, which is propulsion and push off, the subtalar joint is supinated and the mid-tarsal joints locked and stable. Throughout propulsion, the body weight is transferred from the lateral forefoot to the medial. Weight is completely relieved from the fifth metatarsal head and transferred to the first through the third metatarsal head. The above scenario is one example of a closed kinematic system.
The goal of foothotic therapy is to minimize the destructive forces associated with abnormal foot function. This is accomplished by fabricating a device which can precisely control and accommodate the angular relationships of the bones and joints of the foot and ankle. Just as a concave lens is used to control and accommodate light rays in a myopic eye (eye-thotic), so is a precision-fitted foothotic used to control proper foot mechanics. We should remember the foot is three dimensional and store-bought shoes are two dimensional. "All" shoes would better serve the human foot if a precision adapter were inserted. Now we have created a three dimensional fit for a three dimensional foot. This really does make good sense when you think about it. By what authority is it decreed that store-bought shoes are properly adapted to each human foot? Research has strongly supported the premise that shoes are a manufacturer's best guess, and often are a maladaptive device. Until modified by a precision-fitted adapter, it cannot be assumed that any shoe correctly fits the human foot.