In order for a pitcher to produce significant force through the trunk and into the arm, he needs to be able to load a stable yet mobile hip, knee, ankle and heel. As the stride leg rises up & then down towards the plate the pitcher has the opportunity to load his joints, muscles and tissues into an optimal position for force production. At the hip there should be internal rotation & adduction, the knee should flex and the ankle dorsiflex along with heel eversion. This will allow for the pitcher to lean his trunk towards centerfield and keep his front shoulder closed as he strides towards the plate. The mobility at the foot is the key because the combination of dorsiflexion and heel eversion will unlock the knee and allow for more muscles to be loaded so that they can exert force into the rubber. The idea of loading the stance leg is not so that the pitcher can jump towards the plate but so that he can exert force into the rubber and have that force return up through his body to be transmitted into the rest of the pitching sequence.
Clinical Application: This is a pitcher that came in with some elbow issues. When I analyzed his mechanics it was obvious that he 
wasn’t loading his stance leg. His lack of ankle mobility kept his knee extended and his hip went into external rotation. This meant he just fell towards the plate without any stance leg contribution. The video below contains some task specific hip mobility/stability drills and a simple step down variation I had him work on in addition to manual mobility treatments. As you can see in the bottom photo he is now able to load the back leg and drive towards home plate.
Give these drills a try if you see similar mobility limitations in your pitchers and post to comments.
Chris Butler Sports PT 