Every athlete realizes the importance of their feet. The bio-mechanics between the foot and the body have been documented scientifically for years. Chronic medical conditions related to the function of the foot , knee and spine during weight-bearing activity will ultimately affect performance.
Even though the importance of this functional relationship associated with placement of the foot during athletic activities is clearly identified by science, medicine, and virtually every athlete, only two or three bio-mechanical researchers currently understand the mechanical forces involved.
The problem involves the anatomical relationship of the lower extremity and the mechanical forces that result during closed kinetic chain function.
Intrauterine development of the human fetus results in a frontal plane angular deformity of the tibia at birth. Interestingly, this varus (inward) curvature of the leg is present in almost everyone, and is what I believe to be the cause of excessive pronation of the foot and the internal rotation of the leg and knee that are observed during ambulation. This angular deviation of the lower leg requires pronation of the foot to place the foot on the supportive surface. Twenty degrees of curvature in the leg, identified in almost all human beings, demands 20 degrees of compensatory pronation in the foot.
Basically, the varus wedge, present in all functional orthotics, brings the ground up to the foot and compensates for the angular deformity in the leg. When this wedge is used during weight-bearing activity, the foot no longer needs to pronate, and internal rotation of the leg and knee does not take place. Orthotics supinate the foot and externally rotate the leg and knee as we walk, stand, and run.
Function of this varus wedge is determined by the change in angle of the wedge.
For every degree of varus wedge, applied during foot function, more pronation of the foot and internal rotation of the leg is stopped. When the full 20 degrees of angular deformity present in the tibia is compensated for by the wedge, optimal alignment of the knee, with optimal bio-mechanical function occurs.
During Swing Phase of Gait, the foot is off the ground and orthotics provide no change in foot function.
Stance Phase of Gait is divided into three parts. Complete understanding of Foot function and control requires examination of the forces during each phase of gait.
During Contact Phase, foot function cannot be controlled. The shape of the heel bottom is round and the corresponding part of the orthotic is cupped. Wedges placed here, only rotate the “cup” of the orthotic and provide no effect on foot function.
During Mid-stance a wedge with highest point centered near the T-N joint (apex of the medial longitudinal arch) will compensate for the angular deviation of the lower leg from just after contact to just before you propel off the ball of your foot, where the wedge terminates, in most prescription and OTC orthotics.
During Propulsive Phase of gait, only wedges placed directly under the metatarsal heads will change the plane of support and bio-mechanical function. No matter how good you think your orthotics are, when you run, bike without a propulsive phase wedge, you are just carrying dead weight.
Brent A. Jarrett D.P.M.