31 July 2013

Is It Possible for a Bullpen to Be Too Rested?

Scrolling through the Orioles newsfeed the other day, an article written by Derek Wetmore on the team’s website caught my eye.  The article discussed the recent trend of Baltimore starters pitching deeper into games, which gives the relievers in the bullpen more time to rest.  It’s not breaking news to tell you that the overuse of a bullpen is a bad thing, but I did find it curious that the team would complain (complain is probably the wrong word to use here) about its bullpen receiving too much time off.    

Conventional wisdom states that overuse of a team’s bullpen can lead to relievers becoming ineffective, particularly during the home stretch of the long season.  The most recent example is the 2011 Atlanta Braves, who leaned on Craig Kimbrel, Jonny Venters, and Eric O’Flaherty heavily throughout the year (each appeared in at least 78 games).  This strategy worked well for Atlanta until September, when the team finished with a 9-18 record, and blew an 8.5 game lead in the wild card to the St. Louis Cardinals on quite possibly the greatest single day of baseball in history.  Kimbrel and Venters were a key part of that collapse, as Venters compiled a 5.11 ERA in the month, while Kimbrel had a 4.76 ERA along with 3 blown saves. Additionally, earlier this year, both O’Flaherty and Venters underwent Tommy John surgery.  While their heavy usage likely played a part in their September 2011 struggles, it’s unclear how much it contributed to Venters and O’Flaherty’s injuries (if at all), but it is worth noting.  

Back to the Orioles.  As manager Buck Showalter mentions in the article linked above, the line between too much rest and overuse is a delicate balance.  You don’t want to wear your guys down, but at the same time, baseball is like everything else in life, you have to practice to be good at it, and nothing can replace the experience of pitching in a live game.  At this point in the season, the starting rotation ranks 18th in innings pitched, while the relievers rank 12th.  There’s room for improvement, but they essentially seem to be in the middle of the pack.  The addition of Scott Feldman to the rotation in early July has certainly helped, as the rotation’s average innings per start in July is the highest it’s been all year.

They’ve been even better since the All-Star break, averaging 6.43 IP per start.  Combine the increased innings of the starting pitchers with the extra days off at the All-Star break, and you should have a very well rested bullpen.  But is that necessarily a good thing?

Let’s take a look at a graph that shows Baltimore’s bullpen ERA and FIP associated with the days of rest the relievers have received.  The number in parentheses included on the x-axis represent the innings pitched in those situations.  

In 2013 as a unit, the Orioles bullpen has performed best after 5 days of rest, while they have performed worst after 3 days of rest.  However, the 9.1 innings pitched with 5 days of rest is too small of a sample to confidently say that is the preferred amount of rest.  Additionally, the entire analysis over 4 months of the 2013 season may be too small of a sample.  With that in mind, let’s see how the Baltimore bullpen performance changed relative to days of rest in 2012 and 2011, and see if we can find any sort of pattern.

It’s interesting that both ERA and FIP spike upwards at 3 days of rest on all 3 graphs.  This would suggest that 0-2 days of rest would be optimal for bullpen performance. The varied results following 4+ days of rest could be a result of the small sample of innings pitched (innings pitched for 6+ days of rest are biased high, as explained below).

Admittedly, this analysis is very basic, and it’s important to remember that while these graphs tell us a lot, they don’t tell us everything.  First is that they only tell us how often a reliever throws in back to back games (zero days rest), but they don’t tell us how pitching with zero days rest multiple days in a row affects the results.  Additionally, they don’t tell us who is pitching these innings.  This is important because your best relievers are more likely to be pitching more often with less rest (and yielding better results), while the “mop up” guys will be pitching “mop up” innings, meaning your worst pitchers should normally be pitching with more rest.  Finally, since the number rest days only looks at major league days of rest, it includes innings from recent minor league call-ups who have probably pitched more recently than 6 days ago, therefore artificially inflating the innings pitched in that category.

As basic as this study was, it could easily be expanded to give more useful information.  It could include how pitching with zero days rest multiple days in a row affects results.  It could be expanded to include all teams over a longer period of time to give more confidence in the findings.  Individual pitchers could be looked at to see how each pitcher performs when given certain rest days.  Or you could even get into the details and see how rest days affect the velocity and movement of individual pitches, as Josh Kalk attempted to do for the Hardball Times.  It’s amazing how one simple question could lead to so many different studies. 

As for this study, it just reinforces the thought that Buck Showalter knows what he’s talking about.  But you probably knew that already.

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