The independent Atlantic League of Professional Baseball will experiment with two revolutionary rule changes in the 2021 season, limiting teams’ use of the designated-hitter position to when their starting pitcher is in the game and lengthening the distance between home plate and the pitcher’s rubber by one foot.

These changes have been implemented by Major League Baseball as part of an ongoing partnership between the two leagues that began in 2019 to examine the effects of various rule and equipment alterations. Coupled with the rule changes that will be tested at the affiliated Minor League levels in 2021, they will provide data and discussion for potential future use in MLB.

“The Atlantic League is an important step in the pipeline for potential rule changes at the Major League level,” said MLB executive vice president of baseball operations Morgan Sword, “and we look forward to seeing them brought to life in a competitive environment.”

The DH rule, to be known as the “Double-Hook” rule (because the DH is removed from the game at the same time as the starting pitcher), will be in effect the entirety of the 2021 season, while the change to the mound distance will be made in the second half.

The Atlantic League will also continue to use the automatic ball-strike system -- the so-called “robot ump” that assists the home-plate umpire in judging the strike zone. That system will also be in effect in the affiliated Low-A Southeast League this season.

“We are pleased to play a critical role in Major League Baseball’s tests and evaluation of experimental rules,” Atlantic League president Rick White said. “The ALPB is a forward-thinking league, and it is satisfying to our teams and players to be leaders determining the future of our sport.”

Previous Atlantic League experiments include the three-batter minimum (which has since become a Major League rule), restrictions on defensive positioning (which will be used in Double-A this year) and 18-inch square bases (which will be used in Triple-A this year).

With the next collective bargaining agreement talks looming, MLB felt it important to utilize 2021 for a variety of rule experiments. It is safe to assume, however, that neither of these Atlantic League rules would jump straight from indy ball to the big leagues. If they are deemed worthy of future use, they would be further tested in the Minors first.

“Fans, players and many others in the baseball community have expressed an interest in seeing more regular action on the field,” said MLB rules consultant Theo Epstein, former GM of the Cubs and Red Sox. “Therefore, it’s important that we use the 2021 season to explore various ways to create more frequent contact -- and the increased action and athleticism on display that will follow. We are grateful that the Atlantic League — which has been at the forefront of successful rule experiments in the past — has agreed to test a 12-inch increase in the distance between the pitching rubber and home plate during the second half of the season. We expect to learn a great deal about the impacts of such a change and whether an adjustment to this critical field dimension is worth potential future consideration at other levels of professional baseball.”

The Atlantic League features eight teams playing between 118 and 120 games apiece from May 27 through Oct. 10.

Here is a more in-depth look at the two Atlantic League changes for 2021 and what MLB hopes they can accomplish:

THE “DOUBLE-HOOK” DH RULE

The baseball world has typically been divided between DH devotees and those who prefer the old-school National League rules that -- aside from the temporary use of the universal DH as part of coronavirus-related health and safety protocols in 2020 -- require pitchers to hit.

This “Double-Hook” rule would be a compromise, of sorts. It utilizes the DH spot that has been in effect in the American League since 1973. But once a team’s starting pitcher is replaced, the pitching spot takes over the DH spot in the batting order. So from that point on, the team would be required to either use a pinch-hitter in that spot or let the relief pitcher bat.

The impetus behind this idea is to incentivize teams to push their starters deeper into games. Nearly 90% of pitching starts in the 2020 MLB season lasted less than seven innings. The increasingly early use of relievers -- including the use of the “opener” strategy -- diminishes the role of the starter. Yet there is widespread agreement that having identifiable starters -- workhorses around whom fans schedule their viewership or attendance -- is good for the sport.

In preventing starting pitchers from batting, unnecessary injuries, such as the forearm fracture D-backs ace Zac Gallen suffered while taking batting practice this spring, would be avoided. And the combination of expanding the DH to another league and compelling clubs to use their starters longer in games would conceivably improve offensive performance across the board.

Plus, lovers of the NL rules would still get the late-inning managerial strategy they desire. So the “Double-Hook” takes the best of both leagues and blends it all together.

MOVING THE PITCHING RUBBER (SECOND HALF ONLY)

Baseball’s dedicated distance from the rubber to the rear of home plate has been 60 feet, 6 inches since 1893. So the new Atlantic League distance of 61 feet, 6 inches will be considered by some to be an audacious expansion -- one small step for pitchers, one giant leap for baseball itself.

But several factors contribute to the idea of adjusting that seemingly sacred space between pitcher and batter.

First and foremost, Major League strikeout rates have risen every season since 2005 (the league-wide K rate was 16.4% in ’05 and was 24.9% through Monday’s play). Having roughly one-quarter of all plate appearances end in a strikeout is generally agreed to be detrimental to the game’s entertainment value.

The strikeouts increase is in part due to a dramatic increase in velocity (the average four-seam fastball speed thus far in 2021 is 93.6 mph, versus a 91.9 mph average in 2008). Additionally, research by The Ringer found that modern MLB pitchers, weighted by workload, are more than four inches taller, on average, than they were when the 60’6” distance was implemented in 1893.

All of which is to say that professional baseball may have arguably outgrown 60’6”.

Moving the pitching rubber back a foot will provide batters with more time to react to pitches, potentially improving contact rates and thereby injecting more action into the game. MLB found that the reaction time on a 93.3 mph fastball (the average four-seam velocity in 2020) thrown from 61’6” is equivalent to a 91.6 mph fastball (about the average fastball velo in 2011) thrown from 60’6”.

MLB also determined that a 12-inch increase would be the minimum interval needed to evaluate a change in mound distance. While a larger increase in the distance was considered, the goal is to make a change without disruption or added injury risk.

The American Sports Medicine Institute conducted a study in 2019 that found that collegiate players throwing from distances of 60’6”, 62’6” and 63’8” demonstrated no significant differences in key measures of rotational motion or acceleration. Velocity and strike percentage remained consistent, as well. As part of the mound change experiment, the TrackMan technologies installed in Atlantic League parks will be upgraded to project and measure pitches and evaluate outcomes.

A 12-inch increase in the distance might not be as drastic as it sounds. As part of its analysis, MLB found the standard deviation in how catchers set up behind the plate to be seven inches. Some catchers set up as much as three feet further back from the plate than their peers.

The 60’6” standard, therefore, is not as standard as we assume.

Why will the mound distance only be moved for the second half of the Atlantic League season? Because doing so will provide a control group. The first-half data will be compared with the second-half data to determine the true effects of the change.

It is worth noting that when 60’6” became the National League standard in 1893 -- a full five feet farther than the mound had sat the year prior -- the league-wide strikeout rate declined from 8.5% to 5.2% and batting average increased from .245 to .280. MLB also lowered the height of the mound from 15” to 10” in 1969, helping to lower the strikeout rate from 15.8% to 15.2% and raise batting average from .237 to .248.

Mound modifications aimed at improving offensive performance, therefore, are not without precedent.