Maintaining this vertical shin angle is vital in creating a pitching delivery that transfers energy sequentially and efficiently. Without the trail leg holding this vertical position through the knee, posture and strength would be lost through the hips as the system of energy would get too far in front of the lead leg. This idea begins by allowing the heel of the trail leg to stay in the ground as long as possible. Holding ground contact with the heel allows strength to stay in the back hip versus the opposite of the back heel lifting and pushing the body forwards.
Creating a position of strength as demonstrated by Scherzer above allows for direction of the hips and shoulders as well as counter rotation of the core. Once the athlete reaches a point in their delivery where the lead leg begins to land, the angle of the shin begins to get lateral as the hips start to rotate. Keeping a vertical shin angle early delays the early rotation of the hips which keeps the energy stored for more rotational power into the lead leg at the time of foot strike.
When evaluating and developing movement patterns of an athlete, understanding how different aspects of the delivery affect future aspects is critical. Oftentimes a good place to start is look how the athletes at the highest level move and compare that to others. If there is a common movement or position that they arrive at, it is most likely something that needs to be implemented into the delivery of everyone. The vertical shin angle of the trail leg is one of those movements that need to be implemented into the delivery of every pitcher. It is difficult to find a pitcher with a high output delivery who does not achieve this position through their delivery.