In baseball, a middle reliever, or middle relief pitcher, is a relief pitcher who typically pitches during the fifth, sixth, and seventh innings of a standard baseball game.
Middle-relief pitchers are usually tasked to pitch one, two, or three innings.
Several factors determine this, such as who’s winning, the score, eligible bullpen pitchers remaining, the importance of the current game, etc.
When they’re replaced, it is partly due to deprivation of stamina and/or effectiveness but also characteristics, such as pitch arsenal, speed of pitches, which arm they throw with, and who’s up to bat next for the opposing team.
Middle relievers may pitch in these later innings, especially during games tied or close in score.