which parts of your body must be flexible to be effective whne pitching?
Everything must be flexible the main parts are the hip flexor, arm and lat's. Try to go head to toe with your stretches.
everything. stretch out your legs, your back, your arms, everything. only as strong as the weakest link if you think about it.
And also make sure that those parts are in good physical condition. Often a pitcher's side will become tired even after a good stretch before the game. To add more stamina to those vital areas, make sure that they're well stretched and well conditioned.