I scoured the internet for the complete list of players who have undergone the procedure and came across a massive list of 737 confirmed players (major and minor leagues) and crossed out everyone that was not a position player. I was left with a meager list of 29 names from the major leagues (minor league players were excluded because of the distinct differences from each minor league level). After removing even more names of players who may have appeared briefly in the major leagues or had the surgery and never returned to playing, I was left with just 15 confirmed names. Stars of the times like Paul Molitor, one of the very first recipients of the surgery, and lesser known players like Kyle Blanks both stood out on the list.
The next step in the process of unraveling the mystery behind the surgery was to figure out how the surgery affects the batters. In other words, I wanted to test if different tools were affected and in what ways. Did batters hit for the same amount of power as they did before? To begin, I collected data to test for three different measures of arm strength.
Batting Average on Balls in Play (BABIP) determines the rate at which balls put into play are turned into hits. While this is not entirely based on arm strength, arm strength is a large factor in placement of the ball coming off the bat. A more powerful swing will lead to more balls in play being turned into hits. More on that here.
Slugging percentage (SLG) was the next piece of data I tested for. If a batter could hit the ball further, then they could have more extra-base hits.
Similarly, I tested for Home Run to Fly Ball percentage (HR/FB). This measures the rate at which fly balls go over the outfield walls and become home runs. Another barrier to success, as can be seen in the image below, was that there was no recorded advanced fielding data prior to 2002. So it is possible that the HR/FB data is less diluted by sample size than the other measures.
Slugging percentage was the most impactful finding though because, of the 15 batters, 11 experienced a decrease in slugging percentage. A reminder that each surgery occurred at different points in the batters' careers, meaning that natural weakening with age should be filtered out. Overall, the data combined to form a 0.419 drop in slugging percentage or an average 0.028 decrease post-Tommy John surgery. 2.8% less hits were extra-base hits for the remainder of these batters' careers. A significant amount when considering that some of these batters had careers lasting fifteen years or more.
Home Run to Fly Ball rate had to be adjusted to take into account the emergence of fly ball data in 2002 (I removed the home runs hit before 2002 before calculating). Of the 9 batters tested, now 7 of them experienced a decrease in their HR/FB rates. This all comes out to be a 0.018 decrease, meaning 1.8% less fly balls zoomed out of the park and into the stands. The major league average usually stands at 10% but these batters saw their power drop from 10.1% to 8.3% after the surgery.
The only thing left to say is that analysts and fans alike need to recognize the fact that Tommy John surgery does have a negative effect on a batters' power. Mostly though, I'm disappointed Miguel Sano's power will never be what it could have been.
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