This is a study I found:
Toyoshima, Hoshikawa, Miyashita, and Oguri (1974), who had seven adult males throw hard rubber balls weighing 100, 200, 300, 400, and 500 g. found a not unexpected negative correlation between ball velocity and weight. The contribution of body segments, however, appeared to be independent of the weight of the ball. By comparison to the velocity of a normal throw (100%), the velocity without a step was 84%, with the lower body, upper body , and upper arm immobilized, ball velocities were 64%, 53% and 43% of the criterion, respectively.
Similar restraint conditions were imposed by Hoshikawa and Toyoshima (1976) upon four adult males who threw a 250 g. hard rubber ball. The ball velocities at release without the initial step, without hip and trunk rotation and using the forearm alone were 82%, 50%, and 37% respectively, for the preferred and 88%, 60% and 45% for the non-preferred hand. Following 15 weeks of training, the release velocity for the non-perferred hand increased but the relative contributions of the segments remained essentially the same.
Peterson (1973), investigating throwing in 20 highly skilled baseball players, reported the following contributions to baseball velocity calculated over a 9.15m distance immediately following release: legs and hips 37%, trunk and shoulder rotation 15%, arm 24% and hand 24%.
Thus, the consensus of research employing the restraint protocol in overarm throwing is that approximately half the release velocity of the ball is a result of the initial step and rotation of the trunk.
In fast bowling in cricket, an action in which there is a complete rotation of the humerus in the sagittal plane and in which flexion of the elbow is illegal, Davis and Blanksby (1976) indicated that for 19 skilled bowlers the run-up contributed 19% to the total ball velocity at release, leg action and hip rotation 23%, trunk flexion and rotation of the shoulder girdle 11%, arm action 41%, and the hand 5%. Again the importance of the run-up and trunk rotation in determining the speed of the ball leaving the performer’s hand was evident.
Which says that at least 50% of a baseball’s velocity is due to the shoulder to finger links if the kinetic chain.
In other words, arm strength contributes at least 50% of a pitchers throwing velocity.
And in most cases it is greater than 50% because in the kinetic chain composed of body and arm, the smallest (weakest) muscles are in the shoulder-arm-wrist links. Therefore if you want to be able to use the power developed by the larger muscles of the legs, hips and torso, the shoulder, arm, forearm, wrist and fingers must be STRONGER relative to the body. In other works , arm strength is MORE IMPORTANT than core strength because the arm has less muscle mass than the body.