Pitchers who know how difficult it is to do the same thing over and over again, while going through the start and finish of their delivery, pay particular attention to the development and sustain the health of their abdominal muscle group.
This muscle group sustains and supports every muscle group from sternum on up. The sternum is that part of the body, upper center of the chest where the breastplate forms by the joining of both sides of the rib cage.
So important are the abdominal muscle group that if a lack of development will lead to serious injuries to the shoulder and pitching arm, stiffness in back of the neck, strains with the trapezoid muscle group, lumbar, and groin muscle group.
This muscle group (abdominal) actually allows a pitching coach to fine tune a pitcher's progression, note flaws in his/her pelvic muscle group's contribution to the pitching motion(s), and above all - NOTES SPRAINS AND INJURIES TO OTHER PART OF THE BODY.
Think of this for a moment - if you were watching a eight (8) year old pitch, compared to a twenty two (22) year old seasoned, matured, professionally coached pitcher, you'd be able to spot the difference without any hesitation. I know this is an extreme example, but it should drive home the emphasis of just how important this muscle group (abdominal) is. The mature pitcher will use his/her abdominal muscles to support his setup, his/her drive down the mound, his/her rotating the shoulders, and finally balance off his/her release and recovery.
A professional scout/professional pitching coach can spot immediately an athlete with properly developed and sustained abdominal muscles and the associated support muscles.
Exercises and workouts to strengthen this muscle group are specific for each individual, age bracket, physical demeanor, health history, and other particulars. Chose your workout plan carefully and with purpose of use and sustained tolerance - not for that 6 pack for the beach to impress the ladies.