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I have moved this from the Midfoot Stiffness thread to start a new discussion:

You must remember, that during the late midstance and propulsion phases of walking gait, that if the center of pressure (CoP) is lateral to the subtalar joint (STJ) axis, this does not necessarily mean that there is a net STJ pronation moment occurring. STJ moments do not just occur from ground reaction force and the positioning of CoP relative to the STJ axis. STJ moments also arise from muscular forces that cross the STJ. The gastrocnemius-soleus-achilles tendon (GSAT) complex, being medial to the STJ axis in nearly all feet, exerts a strong STJ supination moment in late midstance and propulsion. This is the reason why the CoP will nearly always be lateral to the STJ axis during relaxed bipedal standing if the individual is standing with their center of mass (CoM) balanced anterior to the ankle joint axis. It is at this position of balance that the GSAT complex must be exerting an ankle joint plantarflexion moment to keep the CoM balanced anterior to the ankle joint axis and also will be creating a STJ supination moment that keeps the CoP lateral to the STJ axis.