New Mound Distance, Modified DH Among Atlantic League Rule Changes For 2021

The Atlantic League is moving back the mound at the request of Major League Baseball, and this time it really means it.

Moving back the pitching rubber is one of three rules the Atlantic League is set to roll out in 2021 as part of its partnership with MLB. The Atlantic League season is set to begin on April 27.

In addition to moving the mound back by one foot during the second half of the season, the Atlantic League will use a modified designated hitter rule in 2021. The automated strike zone the league uses, known as ABS for Automated Ball-Strike technology, will be adjusted as well.

In the Atlantic League this year, the DH will be in effect until the starting pitcher has been removed from the game. At that point, relief pitchers will have to hit for themselves or teams will have to pinch hit for them. 

The rule is expected to encourage teams to stick longer with their starting pitcher, especially if they are slated to bat in the next half inning. It also may lead teams to alter their roster composition, since they will need enough hitters and defensive replacements for pinch hitting and double switching in late innings.

The strike zone will once again be called off of Trackman readings, with the ball-strike call being fed into an earpiece worn by the home plate umpire. The strike zone will be a two-dimensional box at the front of the plate, rather than the three-dimensional strike zone used in 2019. The automated strike zone will also be used in the affiliated Low-A Southeast League this year. Like the Low-A Southeast League, the strike zone will be set as a percentage of a player’s height.


That formula for the strike zone will be different from the Low-A Southeast formula. The Atlantic League strike zone will be wider, but not as tall. The top of the strike zone will be set a little lower than it was in 2019. At the same time, the strike zone will grow a little larger on the sides, adding an inch to the sides of the rectangular strike zone. The intent of this change is to encourage more contact since pitches at the very top of the strike zone were found to lead to less contact than pitches thrown on the inner and outer edge of the plate.

Neither of those rules will likely engender nearly as much controversy as the mound move. This is MLB and the Atlantic League’s second time trying to move the mound back.

In 2019, MLB and the Atlantic League announced a series of experimental rules changes, the most significant of which was the decision to move the pitching rubber to 62 feet, 6 inches from home plate halfway through the season.

But faced with plenty concerns from Atlantic League pitchers about injuries, the mound move ended up being postponed. The coronavirus pandemic meant there was no Atlantic League season in 2020.

This time the mound is moving back to 61 feet, 6 inches at the halfway point of the season. By moving the mound midway through the season, the Atlantic League and MLB will be able to use the first half of the season as a control group to see how much the move alters play.

MLB’s hope with moving the mound back is that it will give hitters more time to react to the ever-increasing velocity in the game. A foot of increased distance is expected to effectively serve as the equivalent of reducing a pitch’s velocity by 1.5 mph. 

It is not known yet if that will actually lead to more contact, as the increased distance also leads to more pitch movement.

Getting acceptance from Atlantic League pitchers may be difficult. Atlantic League pitchers Baseball America talked to in 2019 said that if the mound move had been made then they would have considered leaving the league and going elsewhere to play. Some cited the risk of potential injury.

It will also create some issues for stadiums. Beginning on Aug. 3, facilities will need to have a mound at a non-standard distance while other teams using the same stadium will need a mound with the pitching rubber 60 feet, 6 inches feet from home plate. Some Atlantic League clubs have a moveable mound that can be set at either distance, but not all do.

Since that 2019 proposal, there has been research that may allay some of the injury concerns. Dr. Glenn Fleisig and the American Sports Medicine Institute conducted a randomized study that found that pitchers’ biomechanics did not change when they threw from the traditional 60.5 feet, 62.5 feet or 63.7 feet.

In that study, 26 college pitchers were asked to throw five fastballs each from the three different distances in a randomized order. The study found their mechanics were not altered by the differences in distance.

The summary of that paper, published in ScienceDirect, stated that “No significant differences in pitching kinetics and kinematics were observed among the varying pitching distances. Ball velocity and strike percentage were also not significantly different among the pitching distances, however, the duration of ball flight and horizontal and vertical break significantly increased with pitching distance … In conclusion, it is unlikely that moving the mound backwards would significantly affect pitching biomechanics and injury risk; however, the effects on pitching and hitting performance are unknown.”

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