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Are Six-Man Rotations Ever Worth It?

Noah Syndergaard, the New York Mets’ phenomenal young starting pitcher, throws 99 mph fastballs and hits home runs to straight center field. Dillon Gee, the guy Syndergaard replaced in the rotation in May, does not do those things. After a trip to the disabled list, Gee is back, but the Mets couldn’t just send a rookie nicknamed Thor back down to the minors. They also couldn’t keep Gee, a perfectly solid back-of-the-rotation pitcher, there either.

So, flush with pitching talent, the Mets are trying something novel: They are deploying a six-man rotation. It might just be a brilliant way to safeguard the health of their pitchers.

Earlier this year, sabermetrician Russell Carleton wrote that six-man rotations offer few obvious benefits in terms of pitching performance: The extra day of rest doesn’t seem to increase pitcher strikeout rates or reduce walk rates. And, because the extra man entails splitting up the workload among a larger group of people, it tends to dilute the effect of truly great starting pitchers. Over a full season, a six-man rotation results in about 30-50 fewer innings per starter. For a top-heavy Mets rotation that can send Matt Harvey and Noah Syndergaard to the mound, reducing their workload appears costly and counterintuitive.1

If there is a potential benefit of a six-man rotation, then, it must be its health effects. And the Mets could use some preventive medicine. As others have noted, the number of pitchers with elbow injuries has spiked.2 The injury concern goes double for the Mets, whose rotation relies on a recently recovered Harvey, 42-year-old Bartolo Colón and fresh-off-the-DL Gee. (The Mets also saw another young pitcher, Zack Wheeler, undergo Tommy John surgery this year.) Since the greatest predictor of future pitcher injury is previous injury, the Mets are in a precarious position.

Previous attempts to figure out whether six-man rotations help pitchers’ health haven’t suggested much of a connection. When Carleton did it, he couldn’t find any benefit, but he looked over a long timeframe (going all the way to the 1950s). Because we have detailed injury data going back only about 10 years, Carleton had to use a model that incorporated both injuries and other factors that might remove a pitcher from the rotation (such as poor performance). Accordingly, Carleton found only a modest effect on injury probability for starters going on three days’ rest, and only in the past couple of decades.

On the other hand, Eno Sarris pointed out that six-man rotations are standard in Japan’s highest professional league, and the rate of Tommy John surgeries is much lower there. This lower rate exists despite a similarly abusive3 schedule for young pitchers. Furthering Sarris’s point, Yu Darvish, the Texas Rangers ace and recent victim of elbow surgery, argued that such a change might reduce wear and tear on the arm. Nevertheless, there are many distinctions between Japanese professional baseball and MLB, and it’s hard to confidently pin the responsibility for fewer injuries on the rotation strategy alone.

I took a more direct look, using injury information accrued by Baseball Prospectus (specifically, Corey Dawkins) from 2006 through 2014. Over this period, starting pitchers have been primarily used in a five-man rotation, usually getting 4.2 to 4.5 days of rest, on average, over the course of a year. As a result, we need to look at individual pitcher outings to see some evidence of an injury-prevention effect. By linking the injury data with the time between starts of every pitcher,4 we can get an idea about whether a six-man rotation would help reduce injury risk.

I found that there is a strong link between rest and injury rates. Looking at starts on three days of rest, 1.7 percent of pitchers suffered a reported injury within the next two weeks.5 At four days of rest, the typical amount in the modern age, that number drops precipitously to 1.0 percent. (Maybe that helps explain why the five-man rotation came to be.) Then the injury risk falls even further: at five days of rest — which would be standard for a six-man rotation — just 0.8 percent of pitchers are injured in the next 14 days, for a 20 percent decrease compared with four days of rest. That is a potentially meaningful drop in injury risk.6

Despite the drop in injury risk, when injuries were suffered, they were no more severe for pitchers operating on short rest. On either four or five days’ rest, pitchers lost a median of about 21 days of time.7 So more rest may prevent injuries, but injuries on shorter rest are no worse when they do happen.

There are still potential issues of correlation and causation. Managers might change usage patterns for pitchers depending on their injury risks. Alternatively, injury risk could be correlated with some other factor that dictates usage patterns. Furthermore, though the additional day of rest seems to reduce short-term injury risk, there’s no guarantee that it would work as well in the long term — perhaps more rest merely delays the inevitable.8

Even if you accept that longer rest periods lead to fewer injuries, it’s difficult to come to any hard and fast conclusions regarding the optimal strategy. Although it appears that starting with more rest is correlated with a lower injury probability, the benefit that might be gained will be different for every team and every rotation. Top-heavy rotations that deploy a Cy Young candidate will suffer from seeing their excellent pitcher throw fewer innings, but at the same time, they may be guarding against that pitcher’s suffering an untimely injury (at least in the short term).

This is the situation in which the Mets find themselves. With Harvey anchored as a dominant starter who’s also recovering from a dangerous injury, the reward (potentially reducing his risk of relapse) could outweigh the risk (losing some of his innings in the near term). And, as noted above, Harvey is not the only injury risk on the staff.

Regardless of whether the six-man rotation is a good idea for other teams, it seems to fit the Mets and their injury-prone rotation. The question now becomes whether their slick strategy will come to cost them a win or two, as Harvey or Syndergaard gives way to the less-talented Gee. Perched on the edge of playoff contention this year, but with a still-brighter future ahead, the Mets must carefully balance the reduced risk of injury with the possibility of a surprise October run.


  1. Carleton estimates a team’s cost of this reduced workload, for an average No. 1 starter, at about 1.6 wins (and Harvey might be better than an average No. 1 starter).

  2. Even so, the epidemic is not as bad as it seems.

  3. Or perhaps even worse than abusive

  4. Using data from Retrosheet

  5. These results hold for time windows going out to about 60 days, after which the correlation between rest and injury rates trails off.

  6. The difference in injury probability over two weeks hovers right at the edge of statistical significance (p=.06, using a two-tailed Fisher’s exact test), partially because the probability of an injury occurring over any two-week span is quite low. If you extend the test to consider longer time windows (such as 21 days), the p-value drops below .05.

  7. There were too few injuries after three days’ rest to make any comparison meaningful.

  8. You might expect this scenario if pitcher injury results primarily from the progressive buildup of damage in the ligaments of the arm. If that’s the case, you can delay the date of injury by reducing the frequency of starts and the workload, but you can’t really prevent it from happening.

Rob Arthur is FiveThirtyEight’s baseball columnist and also writes about crime.