The evolution of the bullpen and 2017 as the golden age of relief pitching


BULLPENFor the more novice readers of FunGraphs, it’s important to go over some definitions before delving into the meat of the article. A baseball game spans nine innings. “Starters” are the pitchers who start the game, and the object for them is to pitch as much of those nine innings as effectively as possible. “Relievers” are the pitchers who come in to replace the starters as they become ineffective and/or fatigued. Relievers don’t actually sit in the dugout with the rest of the team, but rather in the “bullpen”, an area often cordoned off behind the outfield walls or situated in foul territory. While the etymology of the term bullpen offers no true consensus, the most popular theory is that, back in the old days, fans could buy discounted tickets that gave them the right to stand in a roped off area in foul territory. Apparently some fans felt like they were being herded like cattle in a pen, so this area became known as the bullpen. Eventually teams realized it’s probably a bad idea to put fans on the field of play, so they scuttled the discounted ticket idea in favor of plopping the team’s relief pitchers in the same spot. But the term bullpen stuck.

Baseball’s original rules (by original, I mean at a time when Rutherford B. Hayes was president) did not allow for player substitutions, so the concept of a relief pitcher was a non-starter (no pun intended…seriously). After this rule was amended in 1889 relief appearances became more common, but these typically consisted of a starting pitcher getting a bit of work in between starts. The first pitcher to truly earn the designation as a relief specialist was Firpo Mayberry, owner of a great old baseball name. Mayberry pitched from 1924 to 1936 and accrued a majority of his 2,067 career innings in relief.

Despite Mayberry’s trailblazing, the notion a pitcher devoted solely to pitching in one or two inning relief increments was slow to gain acceptance. Most relief pitchers were simply guys who didn’t cut it as starters (although, as we’ll see, this trend persists today). However, after the MLB lowered the height of the pitching mound and shrunk the strike zone in the late 1960s, teams began to focus more on developing relief specialists in an effort to neutralize the offense-favored rule changes. Some guys, known as ‘LOOGYS’, were left-handed pitchers brought in with the sole purpose of pitching to one or two left-handed batters on the opposing teams. ‘Closers’ were often regarded as the best pitchers in a bullpen and would pitch in the last inning of a game to close it out.

SP InningsThe advent of specialized relief pitching allowed starters to favor maximizing their efforts over five or six innings rather than conserving their energy to pitch into the seventh and eighth innings. Sure enough, starters have gone from averaging 6 2/3 innings per start in 1968 to 5 2/3 thus far in 2017 (Source: Baseball Reference).


RP Stasts

Relief pitchers have picked up that slack, and they are doing so with increasing effectiveness. In 1968 relief pitchers were worse at preventing runs than starters according to ERA Index, which is simply relief pitcher ERA divided by starter pitcher ERA for a given season. This provides additional evidence that teams didn’t take relief pitching seriously at the time, because there is no way that starters should be better than relievers at preventing runs on a per inning basis. Why? Relief pitchers generally hurl at higher velocities since they only need to throw 1/5 the amount of pitches in an appearance, and they also benefit from a ‘novelty factor’ since opposing hitters don’t have six innings to get used to the relief pitcher’s delivery and repertoire.

However, that story changed by the 1970s, as specialized relief pitchers began to outperform starters on a run prevention basis. Since 1980, the ERA index has hovered in the 89-92 range, which means that relief pitchers have allowed about 8-11% fewer runs than starters on a per inning basis. At first I was a bit surprised to see that relief pitchers haven’t continued to improve in ERA Index since freaks of nature like Aroldis Chapman weren’t around 30 years ago.

Yet, when considering the IP share, which is simply the percentage of innings pitched by relief pitchers, the calculus makes more sense. Relievers pitched 26% of all innings in 1968, which has steadily climbed to 37% today. As relief pitchers continue to shoulder more of the load, starters will accrue more velocity benefits, as they can pitch more to max effort with a shorter work day, as well as the novelty factor benefits, since opposing hitters don’t get a third and fourth chance to hit against them in the same game. So the fact that relief pitchers have kept their ERA index steady against starters indicates that they have kept pace with the improvement they have afforded starters.

BEST RP BULLDigging a bit further into the numbers, it looks like 2017 is ushering in a golden age of relief pitching. While 2017’s ERA Index for relief pitchers is only at 91.9, up from 87.5 in 2012, there are signs that certain teams have begun to master the art of crafting a bullpen. For example, the 2017 Cleveland Indians’ bullpen currently has the best ERA- (not the same as ERA Index), which adjusts ERA for league run scoring environment as well as park factors, of all-time at 56 (meaning the Indians bullpen has allowed 44% fewer runs than league average pitching in 2017, across both starters and relievers). The 2017 Los Angeles Dodgers are currently eighth all-time with an ERA- of 65.

But we’re still a bit early in the 2017 season. Luck factors are certainly at play, and Indians and Dodgers could be getting a bit lucky in regards to batted ball luck while other bullpens are getting unlucky. FIP, fielder independent ERA, is an earned run measure that calculates an expected ERA based solely on factors the pitcher can control – strikeouts, walks and home runs. And by FIP-, which adjusts FIP to league run scoring and ballpark in a similar way that ERA- does for ERA, 2017 is an absolutely banner year for relievers. Six bullpens from 2017 – the Indians, Dodgers, Yankees, Astros, Red Sox and Rockies – populate the top 10 all-time list. While the Indians and Dodgers have already translated this success to ERA, don’t be surprised if Yankees, Astros and Red Sox end up on the top 10 ERA- list in short order.

A subsequent article will explore the reasons underlying 2017’s relief pitching renaissance. One of the main factors is that teams have become more adept at converting failed or mediocre starters to relievers, with top-end bullpen arms Aroldis Chapman, Dellin Betances and Andrew Miller each following that path. While teams in the 1960s simply tossed a failed starter into the bullpen with little thought, organizations are now working with these players to maximize their strengths via mechanical adjustments and changing pitch repertoire. Stay tuned for further analysis.



One thought on “The evolution of the bullpen and 2017 as the golden age of relief pitching

  1. Relief pitching has become increasingly sophisticated & important t. Pitchers used to go nine innings all the time 30 years ago. A really fun time for the Yankees. 10 runs a game.


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