Pitching has been the story of the Twins' season thus far, and the bullpen has been as a big a part as any. The Twins have had a strong bullpen according to just above every metric under the sun. The 2.49 ERA they carried into Monday's game was fourth in baseball. In terms of WAR, Fangraphs pegged the Twins' bullpen third at 1.3. They're fourth in FIP as well (2.98) and 11th in K/BB ratio (2.78).
With a strong bullpen performance, it's not surprising to see them with a good showing in one-run games (5-3). They're not quite to the level of the 2012 Orioles (29-9 in one-run games), but they've been instrumental in a respectable start.
Unlike the rotation, however, the Twins bullpen isn't generating much in the way of grounders. Their 36.1 percent ground-ball rate is the lowest among all Major League bullpens. Not surprisingly, their fly-ball percentage is near the top at 44.4 percent -- good for fourth-highest in baseball. To their credit, nearly 18 percent of those airborne outs have been infield flies, but that doesn't change the fact that the Twins relief corps is putting the ball in the air an awful lot.
Despite their sky-high fly-ball rate (awful pun intended), Twins relievers have allowed just four homers on the year. Just 4.4 percent of fly-balls hit against the Twins' pen have left the yard, whereas the average Major League pitcher has given up a homer on 11.3 percent of his fly-balls this season (or 9.8 percent, if we want to stick solely to relief pitchers).
A large home park and frigid weather in cities like Minneapolis and Chicago thus far have probably helped the relievers keep the ball in the yard, but as the weather warms up the trend could become more problematic. Casey Fien has been the chief offender, with just four of the 23 balls against him being of the ground-ball variety. Ryan Pressly could improve his 35 percent mark, but his ground-ball rate steadily declined as he continued to climb up the Minor League ladder.
Right now, only Anthony Swarzak and Jared Burton have above-average ground-ball rates. Brian Duensing, Josh Roenicke and Glen Perkins all have career rates around the league average, so there's hope for the group as a whole to turn things around. If they're going to continue their success in close games though, it's going to have to happen with a return to those career rates. The bullpen has excelled at keeping the ball in the yard thus far, but allowing fewer home runs on more fly balls than just about any team in baseball isn't a trend they should bank on continuing.
If they are able to get back to their career norms, then there's no reason that the core of this group -- Perkins, Burton, Duensing, Fien and Swarzak -- can't function as an effective unit for the duration of the season.