The Royals brought the emergence of the bullpen to prominence during their road to the 2015 World Series championship, underscored by a bullpen that allowed only one unearned run in the seventh inning or later during the five-game World Series success against the Mets.

The role of the reliever, however, has undergone a major transformation during the expansion era, particularly during the last two decades.

Bullpens are pitching more innings than ever, even though individual relievers aren't.

Now the Dodgers' Mike Marshall did set the record for innings pitched by a reliever with 208 1/3 in 1974, breaking the record he set the previous season with Montreal by 29 1/3 innings.

That, however, is a very big exception.

Only five other times has a reliever worked enough innings to even qualify for an ERA title -- 162 innings since the schedule expanded in 1961 and 154 innings prior to that. Bob Stanley worked 168 1/3 innings with the Red Sox in 1982, Bill Campbell 167 2/3 with the Twins in '76, Eddie Fisher 165 1/3 with the White Sox in '65, Andy Karl 164 2/3 innings with the Phillies in '45 and Hoyt Wilhelm 159 1/3 with the Giants in '52.

There has not, however, been a reliever who has even worked 100 innings in a season since Scott Proctor handled a 102 1/3-inning workload with the Yankees in 2006.

Bullpens, however, are working more often and more innings than ever, an outgrowth of increasing focus on pitch loads leading to starting pitchers no longer working deep into games on a regular basis, as well as expansion, which has added 14 teams since the 1961 season, prompting the expansion of the regular-season schedule from 154 to 162 games.

There have been 314 bullpens working at least 500 innings in a big league season, and 312 of them have come during the expansion era, beginning with the Los Angeles Angels in 1961, according to Stats. The two pre-expansion bullpens to handle 500 or more innings were the 1956 Washington Senators and the 1957 New York Giants.

And the trend has grown in recent years.

Prior to 1996, the most teams in a season with a bullpen working at least 500 innings was eight teams in '87.

Since 1996, there have been 238 bullpens work at least 500 innings, including a record-setting 657 innings by Rockies relievers in 2012. A record 19 teams had relievers work at least 500 innings in 2015.

Expansion has resulted in what once was a 16-team alignment becoming 30 teams, 15 in both the American League and the National League. With teams having traditionally gone with 10 pitchers, sometimes even as few as eight, and now regularly having a 12-man pitching staff, that has expanded the number of active pitchers in the big leagues at one time from roughly 160 to 360, more than double the number of big league pitchers prior to 1961.

Combine that with health issues that teams address in a number of ways, including limiting workloads.

In addition to the AL adding the DH in 1973, there were areas of change in the game that preceded the rise in the workload for bullpens -- height of the mound and pitch selection.

In 1968, pitchers dominated and baseball looked for a way to beef up the offense.

Carl Yastrzemski led the AL with a .301 batting average. Luis Tiant led the AL with a 1.60 ERA and Bob Gibson led all of Major League Baseball with a 1.12 ERA. The AL ERA of 2.98 and the NL ERA of 2.99 are the two lowest league ERAs since 1920.

The solution was to lower the mound from 15 inches to 10 inches with the idea it would eliminate an advantage of the pitcher. What it also did, however, was affect the pitcher's stride to a point that it increased the stress on the shoulder and elbow.

Throw in the rise of the usage of the slider and split-fingered pitch, which create a torque on the elbow, and the number of arm injuries to pitchers increased. The result in an attempt to limit stress on a pitcher's arm was lowered pitch counts, which at times would exceed 200 pitches previously. That meant starting pitchers would not go as deep into games.

And as teams adjusted to those elements, the use of the bullpen showed a steady increase.

After only two bullpens worked as many as 500 innings prior to 1961, 14 bullpens worked 500 or more innings in the 1960s; 11 in the '70s; 32 in the '80s despite a strike-shortened 1981 season that wiped out roughly 54 games for each team; 55 in the '90s when both the '94 and '95 seasons were shortened by labor stoppages; 128 from 2000-09; and 72 in the past six seasons.

What the Royals did was adjust to the demands of the time.

They put an emphasis on creating a bullpen that had the ability to overpower hitters for the last three innings of the game, adding two power arms to go with the closer, who during the evolution of the bullpen had become a one-inning specialist.

With a World Series championship to show for their effort, the Royals are becoming a model for other teams.

Tracy Ringolsby is a columnist for MLB.com.