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January 6, 2014

Baseball Therapy

The Five-Man Rotation: The Appendix of Baseball

by Russell A. Carleton


Over the last few weeks, I’ve been trying to answer a question. Fifty years ago, it was routine for teams to carry only four starters, and for those four starters to complete a good chunk of their games. Pitching on three days’ rest was common, and pitchers regularly posted pitch counts that would get a manager fired today if he let it happen once. What happened?

Well, we know that the dip in pitch counts has been slow and gradual but fairly consistent over time, and that managers have developed a quicker hook because a fresh reliever is better than a tiring starter. The four-man rotation began to disappear in the mid-1970s, and by the 1990s, pitching on three days had mostly been eradicated. (The evidence suggests that it was not actually necessary to do this, but that now that pitchers have been trained in the “every four days” ecosystem, when they try to go on three days’ rest, it’s a risk factor for injury.) At the end of the last article, I wondered whether there was another factor at work. Perhaps the reason that pitchers have been switched to four days of rest is that teams saw that pitchers who had particular skills needed more rest, but that these skills were valuable enough (and abundant enough) to warrant the tradeoff.

There have been two major trends in pitching over the past few decades that are worth noting. One is that strikeout rates have increased sharply in the past 20 years. In 1993, pitchers struck out 5.8 batters per nine innings. In 2013, that number was up to 7.6 per nine innings. The other trend in pitching, also peaking around the same time, is that pitchers are getting bigger. Here’s a graph of the body-mass index of starters (more than half of their appearances as starts) of all pitchers in MLB for each year. Body-mass index (BMI), for the uninitiated, is a ratio of weight to height—actually, height squared—that is a standard measure of whether someone carrying too much weight, medically. BMI is widely used in medical and public health research. Scores under 19 and over 25 (sometimes 26) are associated with an increased risk for health problems.

The average BMI for a starter (relievers had the same basic graph) went from the mid-24s from 1950 until the mid-1990s, and then rose sharply. This is a pattern that’s also been observed among position players. For what it’s worth, I’m using the players’ listed (which, um, could be a lie) height and weight in the Lahman database, meaning that the player maintains that listed height and weight for the entirety of his career. Having personally been both 24 and 34—let’s just say that’s not always the case. But I think the point comes across.

Oddly enough, the trends toward bigger pitchers and higher strikeout rates do not seem to be one and the same. We don’t have velocity data back into the 1950s, but bigger pitchers generally throw harder. However, BMI and strikeout rate for each pitcher-year from 2000 to 2012 were completely uncorrelated (r = .006). There were also small correlations between BMI and walk rate (r = .127) and HR rate (r = -.097). Bigger pitchers aren’t necessarily better pitchers. But perhaps bigger pitchers are slower to recover from pitching and need an extra day of rest?

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Premium Article Minor League Update: I... (01/06)
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Premium Article Baseball Therapy: Rest... (12/26)
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2014 Draft Prep and Yo... (01/06)

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