“There are many reasons a pitcher’s (baseball player’s) arm breaks down, why a ligament in the elbow detaches, all or part of the rotator cuff tears, or the shoulder needs to be reconstructed.
These could be the result of poor mechanics, a lack of monitoring [a relief pitcher at a Division 1 program told me that he threw in 14 consecutive games. Later that year he had Tommy John surgery], throwing with pain on a vulnerable arm, throwing too many pitches in one particular outing, throwing too many breaking balls, not enough recovery period time between games, or throwing curves in Little League.
The good news is that most of these problems are obvious and can be corrected.”
That excerpt is from Alan Jaeger in his article “Rediscovering The Lost Art of Long Toss” that was originally published in Collegiate Baseball Magazine, May, 1999. The entire article may be read online at http://baseball-articles.com/longtoss.html.
Alan Jaeger is one of the biggest authorities on pitching mechanics.