The arm drag (elbow drop below the plane of the shoulder) during acceleration: is it a function of arm mechanics or hip-core-trunk mechanics?
I would argue that the chain begins with the foot-leg to create force for the core and shoulder to generate speed. Therefore, the shoulder is a conduit for speed not force generation. Moreover, the shoulder reacts to hip-pelvis rotation that results in a significant drag effect before mid acceleration of the shoulder. If the hips are known to create compressive loading (normal) on the shoulder during trunk rotation then excessive compression will result in the elbow dropping because of excessive tension in the shoulder. Resulting in reduced ball velocity, inaccuracy, and strain to the elbow and rotator cuff.
Rule out the hip to core function before looking at the shoulder-elbow as the primary culprits for the dysfunctional pitcher. Especially novice, fatigued, and injury pitchers.
thoughts from others?