I am a little league coach and father of our team's best pitcher. The major cause of this sports injury is excessive throwing combined with the repetitive twisting and whipping of the elbow.
Typical symptoms are stiffness and soreness around the elbow and pain when throwing.
That is, the insignificant breakdown of tissue that normally occurs during physical activity is so unremitting that the damage exceeds the body's ability to make repairs. The break-in period for pitchers should include a carefully planned progression that increases the number, distance, and intensity of throws. Of course, pitchers must always warm up carefully during each practice, before a game, and prior to taking the mound. Limit the number of pitches thrown. Reports in The Physician and Sportsmedicine recommend no more than about 100 pitches per game or in practice.