Will conditioning programs or strength training improve pitching velocity in baseball pitchers?
Many coaches and players alike think that strength training or power exercises will automatically improve pitching velocity. The idea that most coaches continue to advise is that for pitchers to improve velocity, they must get bigger and stronger.

Stronger or Bigger Legs Do Not Help Improve Pitching Velocity
Thus, they advice them to go into the weight room to build more strength, especially leg strength, by exercises, such as heavy squats, leg press, leg curls, or leg extensions. There is no evidence that stronger legs or a bigger lower body will improve either pitching velocity or overall better performance.

Also, some programs would like pitchers to believe that by doing explosive medicine ball exercises or explosive jumping that velocity will also improve dramatically.
However, in both cases, whether heavy weight training or explosive full body exercises, the research does not support automatic improvements in pitching velocity.

Specifically conditioning the baseball pitcher by doing explosive exercises is designed to help improve his ability to do more work, while reducing the risk of injury. However, velocity improvements must come from improved pitching mechanics.

Strength training has not proven to provide benefits for improving pitching velocity or for improving any baseball player’s performance.

Pitchers Should Do Explosive Fully Body Exercises, Not Heavy Weight Training
If pitchers are going to condition, then they should specifically do full-body explosive exercises that help the body get into more specific condition for pitching. These full-body explosive exercises along with both sprints and aerobic training, will help the pitcher do more work while reducing the risk of injury.

Below is a study that proves this. It is also important to understand that in order to improve pitching velocity, pitchers must fully understand pitching mechanics, and how the body actually produces velocity.

It has been proven that pitching velocity is the result of the momentum of body into a long stride that produces more elastic energy so that, upon landing, the trunk is positioned properly, and the front leg and hip provide bracing action to help speed up the trunk which whips the arm through at high speed. This must be achieved from improved mechanics.
Pitching velocity improvements clearly must be achieved by improving mechanics and the ability of the pitcher to continue to improve his speed of movement as well as the proper sequencing on his body.

Kluckhuhn, K. L., Signorile, J. F., Miller, P. C., Webber, B. C., & Garcia, M. (1997). An analysis of high-speed isokinetics and pitching. Medicine and Science in Sports and Exercise, 29(5), Supplement abstract 1273.

The importance of strength, power, and acceleration were investigated in baseball fastball pitching. The only variables that related to pitching were those associated with speed. Acceleration and power at the fastest movement speeds accounted for almost all fastball pitching variance (99 percent). Strength was not related to speed of movement and, therefore, should not be a variable considered when training for speed.

Implication. The use of strength training, which does not allow maximum speed, will not enhance speed of movement. To improve pitching speed it is necessary to train for speed first. This is best done by consistently trying to move and exceed previous maximum velocity. Any training that does not allow maximum speed in a trial will be useless and could be detrimental.