The first thing that struck me when I saw footage of Yu Darvish pitching last year was that he uses a lot of arm to throw that hard and that his mechanics are similar to those of Kerry Wood, Mark Prior, and Adam Wainwright, all of whose faulty mechanics resulted in elbow injuries that required Tommy John surgery.
The excellent website of Chris O’Leary, a St. Louis–based pitching and hitting instructor, has dozens of helpful and informative animated GIFs, including of the three pitchers mentioned above. On his page about Adam Wainwright and the “inverted W”, O’Leary pinpoints what he thinks Wainwright’s mechanical error was, or at least the source of his problems: he keeps moving his elbow upward for far too long, resulting in his elbow being higher than the ball for too long and even significantly higher than his shoulder for a while. This requires his elbow to twist too much too fast and his throwing hand to whip around and forward too quickly, which puts way too much strain on the elbow over time.
Mark Prior’s mechanics were even worse and make me cringe just looking at them. O’Leary rightly calls Prior’s mechanics a train wreck. O’Leary even created another page comparing Mark Prior’s mechanics to Greg Maddux’s and Nolan Ryan’s, two pitchers who never had serious arm problems (though Nolan Ryan is kind of a freak of nature, so I’m not sure any comparison to him would be fair. I’d love a Roger Clemens comparison because I always considered him to have nearly perfect mechanics.) The story is the same for Mark Prior: elbow too high too late, ball dragging behind it, whipping around too fast too late.
Kerry Wood is among the most famous injured pitchers of the last couple decades, and O’Leary’s Kerry Wood page details basically the same problems that hurt his elbow as hurt the other two. Added to that is Kerry Wood’s somewhat across-body throwing style that presumably helped him snap off his curve ball so effectively. Maybe Kerry Wood’s curve ball contributed more to his elbow problems than his “inverted L” arm motion did, but I think I generally agree with O’Leary’s analysis.
I have little doubt Yu Darvish and Craig Kimbrel will suffer similar fates because I see the same faults in their mechanics: their elbows are ahead of their throwing hands, their elbows come up and back too far, and they have to whip the ball around too fast at the last minute, which puts excessive torque strain on the elbow. Their mechanics seem particularly worrisome because you don’t need GIFs or slow-motion video to detect their faults; they are obvious in real time on TV. Here are a couple photographs of each pitching:
Kimbrel is far worse than Darvish, and I don’t think the fewer innings he will throw as a reliever will save him. It isn’t as evident in those photographs of Darvish as it is in video, but he seems to whip or twist his arm around much more than most pitchers, possibly similar to Kerry Wood in the 1990’s (though I haven’t seen video of pre-injury Kerry Wood in a long time, certainly not enough of it to study it).