"I expect the real reason baseball will return to the four-man rotation will be the simplest of all: It helps win games. The five-man rotation is not on that evolutionary path; it's a digression, a dead-end alley. Just as baseball once believed that walking a lot of batters was better than throwing a home-run pitch, we are now chasing an illusion that our pitchers work better on four days' rest and that the five-man rotation significantly improves their future."
Craig Wright, The Diamond Appraised, 1989
First of all I have to say this post is based on a chapter out of the book Baseball Between The Numbers, written by the Baseball Prospectus team of experts. If you call yourself a baseball enthusiast, it's a must read.
Anyways, the chapter in the book argues that a team that implements a four-man rotation heightens their chances to win in the near and distant future. Here's why:
First of all it must be stated that the starting pitchers do not pitch nearly as far into games as they used to. The complete game percentage for major league pitchers has been below 5% for a decade now. The probable reason for that is that the hitters have gotten better, and pitchers have to put more into every pitch and every inning than in the past, which leads to quicker fatigue.
Another huge key to this idea is the fact that throwing is not dangerous to a pitcher's arm. Throwing while tired is dangerous to a pitcher's arm. That statement is vital to this whole concept.
Without getting into some of the complex math equations and whatnot, I'll give the basic structure of this type of thinking.
With a four-man rotation, you're best pitchers will make more starts, obviously. You wouldn't be running out a fifth starter like Kevin Correia who is going to get shelled all the time. Now you might have previously thought that having a pitcher make close to 40 starts a year would be torturous on their arm and lead to shortened careers, which certainly wouldn't help any team in the long run. However, the fact above throws that out of the window. Pitchers can handle more starts, as long as they don't throw many innings while their arms are tired.
This would, of course, lead to starting pitcher's being pulled earlier in games. A pitcher who throws every four days probably isn't going to have the arm to throw seven, eight, or nine innings without getting tired. The manager would be forced to pull the pitcher at the first sign of arm fatigue, which would often be in the fifth or sixth inning. That would require the team trying this theory to have a strong bullpen. That wouldn't be too hard to accomplish however, since they'd have an extra spot for a bullpen arm since they are only using four starters.
This next statistic is accurate as of 2005:
Since 1972, 175 pitchers have tossed 408 seasons with at least 8 starts on both three and four days rest. Weighting each pitcher's season (so that pitchers who routinely threw on three days rest--usually the best pitchers--are not overrepresented) shows that this group actually did better in most important categories on three days of rest than on four days of rest:
- Winning percentage was 0.014 higher (.539 vs. .525, a 2.60 percent improvement)
- ERA was 2.4 percent lower, 3.54 versus 3.62
- Strikeout rate (-0.44 percent) and walk rate (+0.7 percent) were each slightly worse
- Home runs per inning was 4.1 percent lower
- Opposing batters hit .254/.306/.269 against these pitchers on three days rest; they hit .254/.305/.274 on four days rest - virtually identical
- These pitchers threw just as deep into the game (6.90 innings on three days versus 6.86 innings on four days - a tiny difference)
From this study they looked at how these pitchers performed over the course of the season, to see if they got worse in the later stages of the season. They found that they did not show any signs of getting worse in the later months, and therefore didn't have worn-out arms despite starting more games than what is normal.
So now that you know that a four-man rotation is not more dangerous to a pitcher's arm (if implemented correctly), let's look at what comes out of making the switch:
- Thirteen starts currently being given to fifth starters or worse are instead made by the top three starters
- About 10 runs are saved by a combination of the better RA (run average, which is ERA that doesn't account for errors) of the top three starters and by the extra innings they pitch, taking innings away from the shallow end of the bullpen
- Bullpen pitchers have to throw fewer innings (helped by the extra roster spot), reducing the wear and tear on those pitchers who don't have the luxury of a predictable schedule
All of this amounts to a team that correctly implements the four-man rotation getting a 3-4 win boost because of it, which could be huge in a close playoff race.
That is all assuming that the mangers monitor pitcher fatigue very closely and follow a strict, healthy schedule for their pitchers between starts. If you let pitchers throw too much while they're tired, they have a greater chance of getting hurt and you're going to be in big trouble.
Every team in the big leagues has a five-man rotation these days. That is because of the misconception that the more innings a pitcher throws, the greater his chance of getting injured is. However, if teams listened to science and doctors, they would realize that a pitcher can throw basically however many innings he wants in a year, and won't have a heightened chance of injury as long as he doesn't throw too much while he's tired.
The four-man rotation seems like a great idea, however, we probably won't see it return to the game anytime soon. That's because people with jobs are afraid to lose those jobs. If a manager decides to go with the four-man rotation and it blows up in his face, he's almost certainly going to lose his job. If the manager uses a five-man rotation and it blows up in his face, you can't put the blame on him because he's just doing what everyone else is doing. The more unique you are, the more criticism you take if it doesn't work. I don't see anybody being gutsy enough to go out on a limb and try it any time soon.
Baseball has been evolving quickly, but I'm not sure this is ever going to catch on, even if it is logically and mathematically the better option.