Throwing a baseball repeatedly as hard as you can they way major league pitchers do is a brutal and unnatural act. Basically, you are cocking the ligaments in your shoulder, elbow, and wrist as if you were stretching giant rubber bands, and then you are releasing that kinetic energy in rapid sequence: (shoulder) *snap* (elbow) *snap* (wrist) *snap* Whooooosh, the ball goes flying.
But the elbow bears the brunt of the force and does the lion's share of the work. The pitcher's arm, cocked completely so that his hand almost touches his scapula, snaps forward through the elbow until it is completely extended.
Not good for the elbow. Time was, pitchers would hurl until their elbow was all used up (maybe their late 20s or early 30s if they were lucky), and then they would retire and go home. Only freaks of nature could pitch into their 40s. But then there was "Tommy John" Surgery. Elbows could be "rebuilt." Over and over. Suffice to say, nowadays pitching into one's 40s has become routine.
Tommy John Surgery, officially known in medical jargon as ulnar collateral ligament reconstruction, is a surgical procedure to rebuild the ulnar collateral ligament in a person's elbow, usually by replacing it with a ligament taken from the person's wrist.
If ligaments in the wrist are not available (typically because they have been used in previous Tommy John procedures), they can also be taken from the knee, foot, or hamstring. The new ligament is literally stapled onto the bone and eventually attaches itself after a healing process.
"Tommy John Surgery" gets its name from famed Dodgers pitcher Tommy John, who was the first to have the procedure performed on him back in 1974, by legendary pioneering sports surgeon Frank Jobe. Prior to John's surgery, a torn ligament in a pitcher's elbow had meant the abrupt end of his career, but John told Jobe he was willing to try anything if it would mean he could pitch again, so Jobe thought about it and thought taking a ligament from somewhere else and stapling it to the bone just might work.
Jobe famously gave the procedure a 1 in 100 chance of working.
But Tommy John decided to try the surgery, was back on the field 18 months later, and pitched for 13 more years, actually winning more games after the surgery than before it.
Over the years the surgery has gotten better and better. John's surgery took a whole day to perform. Today the surgery is a routine outpatient procedure which takes less than one hour, and pitchers can be back on the field in a little under a year. Even twenty years ago, about one third of the pitchers who had the procedure were never really able to pitch again, and another one third or so were never as good after it as they were before. Today it is unheard of for a pitcher not to come back healthy and ready to pitch, and most amazing of all, a significant percentage of pitchers actually pitch better after the surgery, having added 3-4 miles per hour to their pitching speed!
The reasons for this increase are debated, but it is probably due to the fact that the replacement ligament is actually bigger and stronger than the original (so the pitcher is essentially snapping a bigger, stronger rubber band), and possibly also due to the rigorous arm strengthening procedures undergone as part of the rehabilitation process.
The noticeable improvement many pitchers display post-Tommy-John, combined with the low risk associated with the most up-to-date techniques, often leads other pitchers to joke that they hope they blow out their elbows so they can come back throwing harder too.