The "average" human being exerts 7 to 10 times their normal body weight on each foot with each step. In the case of brake application, closed kinetic chain and vector forces become applicable in the recumbent position. Basically the gastrocs counter balance the quadraceps in extension of the leg and plantarflexion of the foot. Since the gastrocs and the soleus combine to form the triceps surae which forms the Achilles tendon and whose major function in the recumbent position is to plantarflex the foot as with brake application, the force is a factor of patient's weight, leverage from seat position and muscle girth/strength plus endorphin stimulation. If the brake is applied as an emergency response verses a calculated movement, it will be applied with more force, quicker and disregards most "natural" formulas. Example: we've all heard stories of mothers with super human strength lifting cars that have fallen off jacks and trapped family members. Normally these same indiviudals might be capable of nominal physical feats. Most of the strength is derived from the legs in this maneuver.
Disclaimer: I'm NOT a professional mechanic and don't pretend to be one in normal daily conversations but I play a bona fide doctor during the daytime, who knows the muscular functions of legs and feet!
