It is conventional wisdom in baseball to teach or believe that arm speed is what determines throwing velocity and there are many training methods that are built around the ability to develop arm speed or arm strength, but the evidence tells a different story. It is a very controversial statement that increasing arm speed may increase velocity but will eventually lead to a higher rate of injury. Most would suggest that this is fake news but it isn’t, it is a very logical statement based on the evidence that suggests increasing arm speed to throw harder will eventually destroy the arm.
In my research on this topic I have discovered that the internal rotation velocity of the shoulder at a high-velocity pitch is the fastest human movement ever recorded (Pappas, Zawacki, & Sullivan, 1985). It has been recorded up to 7,000 degrees per second which means the arm could spin around the shoulder 21 times in one second. If you asked the question to a room of baseball players at what level of the game would these arm speeds only exist, most would say the professional level and they would be wrong. Evidence suggests the college level of baseball is where the arm speeds are the highest (Fleisig, Barrentine, Zheng, Escamilla, & Andrews, 1999). The college-level is also the level of baseball where the internal rotation torques on the throwing arm are by far the highest (Aguinaldo, Buttermore, & Chambers, 2007). This is more than likely the result of high arm speeds. Evidence also suggests baseball players who throw with higher arm torques have a higher rate of arm injury (Anz et al., 2010).
"Arm Speed" Training Physics
To better understand why increasing arm speed eventually injuries the throwing arm you need to understand the physics involved. Evidence suggests that for the humorous to spin faster in internal rotation the external rotators must weaken (Wasserberger, Downs, Giordano, Brittain, & Oliver, 2019). In layman’s terms the humorous is like a pipe spinning in the shoulder socket and the internal rotators are the gas pedal and the external rotators are the brakes. If you want to spin the pipe forward like the wheels of a car in the shoulder socket, then you need to take your foot off the brakes and push down on the gas pedal. The problem is when it is time to stop the pipe from spinning itself out of the shoulder socket the brakes must come back on at full force in just enough time to stop 7,000 degrees per second in a split second. The continuous process of turning off the brakes and then slamming them back on again at full force will eventually destroy the braking system.
You can watch a video demonstrating the negative effects of this "Arm Speed" science:
Once the baseball player learns of this evidence exposing the misconception that elite pitchers have elite arm speeds the question usually becomes then how do professional pitchers throw harder than college pitchers while putting less stress on their arms? First of all they not only put less stress on their arms at higher throwing velocities but based on body weight they put fewer torques on their throwing arms than all levels of the game even youth pitchers. The reason they can do this is suggested in the research by Aguinaldo, Buttermore, and Chambers, 2007. The professional pitchers are delaying their trunks more than lower-level pitchers. This more technically means they are better sequencing the kinetic chain. Evidence also suggests those who poorly sequence the kinetic chain put more forces on their throwing arms (Sakiko et al., 2014). Kibler and Chandler, 1995 calculated that a 20% decrease in kinetic energy delivered from the hip and trunk to the arm requires a 34% increase in the rotational velocity of the shoulder to impart the same amount of force to the hand.
The moral of the story here is if a pitcher wants to make it to the professional level of the game of baseball he must learn to use as much kinetic energy as possible and avoid using as much arm speed as possible. This approach converts the arm into more of a regulator or funnel of energy than a generator of energy. This also means the arm will have a better chance of doing the other important jobs it has as a pitcher like griping the pitch for a good spin on the ball or applying pressure at a specific time in the forearm movements to add movement to the ball or supporting the location of the pitch. I only hope that one day this topic will no longer be a controversial topic but conventional wisdom.
References
Aguinaldo, A. L., Buttermore, J., & Chambers, H. (2007). Effects of Upper Trunk Rotation on Shoulder Joint Torque Among Baseball Pitchers of Various Levels. Journal of Applied Biomechanics, 23(1), 42.
Anz, A. W., Bushnell, B. D., Griffin, L. P., Noonan, T. J., Torry, M. R., & Hawkins, R. J. (2010). Correlation of Torque and Elbow Injury in Professional Baseball Pitchers. American Journal of Sports Medicine, 38(7), 1368–1374.
Fleisig, G. S., Barrentine, S. W., Zheng, N., Escamilla, R. F., & Andrews, J. R. (1999). Kinematic and kinetic comparison of baseball pitching among various levels of development. Journal of Biomechanics, 32(12), 1371–1375. https://doi.org/10.1016/S0021-9290(99)00127-X
Harding, J. L., Picha, K. J., & Bliven, K. C. H. (2018). Pitch Volume and Glenohumeral and Hip Motion and Strength in Youth Baseball Pitchers. Journal of Athletic Training, 53(1), 60–65. https://doi.org/10.4085/1062-6050-323-16
Kibler, W. B., & Chandler, J. (1995). Baseball and tennis. In: Griffin LY, ed. Rehabilitation of the Injured Knee, 219-226.
Pappas, A. M., Zawacki, R. M., & Sullivan, T. J. (1985). Biomechanics of baseball pitching: a preliminary report. / Biomecanique du lancer en baseball. Un rapport preliminaire. American Journal of Sports Medicine, 13(4), 216-222.
Sakiko, O., Bing, Y., Troy J., B., Darin A., P., Li, L., & Joseph B., M. (2014). Improper Trunk Rotation Sequence Is Associated With Increased Maximal Shoulder External Rotation Angle and Shoulder Joint Force in High School Baseball Pitchers. American Journal of Sports Medicine, 42(9), 2089–2094. https://doi.org/10.1177/0363546514536871 Wasserberger, K., Downs, J., Giordano, K., Brittain, A., & Oliver, G. (2019). Associations between glenohumeral rotation strength and select kinetic parameters during the baseball pitch in adolescent baseball pitchers. Sports Medicine and Movement Laboratory, 37(1) 41-44.