Author Mark Hyman of The New York Times recently published an article about two studies that have shown curveballs are no more stressful on the arm than fastballs. Hyman uses this information to openly question the wisdom that says curveballs are bad for young arms. [Click here to read Hyman's article in full.]
The chief problem with Hyman's article is that he seems to misinterpret the study's conclusion. The study found no link between curveballs and injuries, but Hyman appears to have interpretted this to mean that curveballs conclusively do not lead to injury. This is a logical fallacy.
It's unclear whether Hyman has an opinion of his own, but he did seek the opinions of Dr. Glenn Fleisig and Dr. James Andrews. He offers these opposing quotes from Dr. Fleisig and Dr. Andrews about the studies.
"I don’t think throwing curveballs at any age is the factor that is going to lead to an injury." - Fleisig
Dr. Fleisig's quotes in the article clearly indicate that he doesn't believe throwing a curveball is any worse than throwing fastballs or change-ups. They may be taken out of context, but Hyman sure makes it seem like Dr. Fleisig is very confident with this position.
"It may do more harm than good — quote me on that." - Andrews
Dr. Andrews, on the other hand, seems to have a deeper understanding of what the studies actually reveal. While the studies did not reveal an obvious link between curveballs and injuries, Dr. Andrews recognizes that a link may still exist outside the scope of these studies.
Obviously, a more stressful pitch is more risky than a less stressful pitch. That's just not all there is to it.
The two recent studies were inspired by a study published in 2006 by Dr. Fleisig, Dr. Andrews, et al. That study's clinical relevance was summed up in its abstract:
Because the resultant joint loads were similar between the fastball and curveball, this study did not indicate that either pitch was more stressful or potentially dangerous for a collegiate pitcher. The low kinetics in the change-up implies that it is the safest. - "Kinetic Comparison Among the Fastball, Curveball, Change-up, and Slider in Collegiate Baseball Pitchers." American Journal of Sports Medicine. 2006.
Essentially, this means that the slower your arm moves, the safer the pitch. This principle carried over into the follow-up studies on youth pitchers, and it's the main flaw with all three.
The studies measure raw joint torques but they don't account for basic mechanical or functional differences between the pitches which already vary from pitcher to pitcher anyway.
The key factor that is essentially unaccounted for in these studies is forearm action - pronation versus supination. A properly pronated pitch is not equivalent to a supinated pitch no matter how similar the kinetic measurements may be.
The muscles of the flexor-pronator mass can provide support against the valgus force that damages the ulnar collateral ligament (UCL). When a pitch is thrown with the forearm in a supinated position throughout the delivery - as most pitchers throw their curveballs - these muscles do not provide the same support for the UCL. This makes UCL tears more likely even if there is no difference in the measured stress levels between pitches.
Additionally, powerful pronation through release helps decelerate elbow extension and helps prevent the olecranon process from slamming into the olecranon fossa on the back of the elbow. When the elbow slams closed it can lead to inflammation of the hyaline cartilage and excessive bone growth including lengthening of the ulna, bone spurs, and bone chips.
When a supinated curveball is thrown, a pitcher risks injury in a number of ways. Without paying attention to what the pitcher is actually doing with his body, these studies simply do not reveal much. They certainly don't give carte blanche to start flipping curveballs like they're going out of style.