The reason you have been told not to biceps as a pitcher comes down to the anatomy of the bicep. The biceps tendon insertion point is at the superior aspect of the Labrum. Now why does this mean you shouldn’t get your biceps jacked….

Ever stop to ask why? If you look at the mechanics of each you will notice one requires that the athlete learns how to stop momentum, and brace through rotation. The other you must create forward momentum, accelerate momentum forward, rotate, post up on one leg and decelerate. They may seem similar but they are very different frontal plan movements.