I got (back) into the world of baseball when my older son started pitching for his two baseball teams. I became his pitching coach and, by default, the pitching coach for his teammates.
In the first game of the season, I brought in a relief pitcher who threw two nice, hit-less innings. He didn't throw very hard, but he seemed to be able to drop the ball on top of the plate and cause hitters to swing over the top of the ball. As a result, before the next game I asked him if he wanted to be the starting pitcher. He told me that he wasn't sure if he should, because the inside of his elbow was killing him.
I didn't know much about pitching injuries at the time. As a result, I told his dad that I was going to shut him down while I tried to figure out what was going on with his elbow. After admittedly going a little crazy, reading literally every journal article that I could find on pitching elbow injuries, and studying the work of people like Dr. Mike Marshall, I came to understand that the root cause of his problem was likely that he was inadvertently supinating his forearm through the release point and throwing a slider as a result. That created the sharp vertical break on his pitches, but also put a tremendous load on the inside of his elbow. As a result, I taught him how to pronate all of his pitches and his elbow problems disappeared as fast as they appeared.
That fixed my pitcher's problem, but it also got me interested in the question of pitching injuries.
I stopped being able to play baseball due to problems that started to crop up when I was was in 7th and 8th grade. Realizing that I was much closer to the edge of the cliff than I thought I was, and not wanting what happened to me to happen to my son and his friends, I continued to dig into the world of pitching mechanics and injuries and tried to understand what distinguished healthy pitchers from injury-prone pitchers.
I knew from my experience troubleshooting large computer systems that the easiest way to see the difference between two systems is to compare things at the extremes, so I decided to compare two groups of pitchers. The first group was pitchers like Greg Maddux, Nolan Ryan, and Roger Clemens, who have had long, relatively injury-free careers. The second group was pitchers like Mark Prior, Chris Carpenter, and B.J. Ryan, who have had, short, injury-plagued careers.
As soon as I did this, I began to notice differences in the arm actions and timing of these two groups of pitchers.
It turns out that the pitching mechanics of long-lived pitchers like Greg Maddux, Nolan Ryan, and Roger Clemens are distinctly different from the pitching mechanics of injury-plagued pitchers like Mark Prior, Chris Carpenter, and B.J. Ryan. I think those patterns of difference in their pitching mechanics, which I describe using terms like the Inverted W, the Inverted L, and the Inverted V, go a long way toward explaining their different fates.