Limits on defensive shifts and pickoff attempts, bigger bases and “robot umps” are some of the changes coming to certain Minor League Baseball levels in 2021.

When the season begins this week, the Minors will be a testing ground for a variety of experimental playing rules aimed at creating more balls in play and action on the basepaths, improving the pace and length of games and reducing player injuries, Major League Baseball announced in March.

While none of the rule changes are guaranteed to eventually be implemented at the big league level, each was deemed worthy of more serious scrutiny by MLB’s competition and playing rules committees, who will report their findings to the Major League clubs after the conclusion of the season.

“The game on the field is constantly evolving,” said MLB consultant and former Cubs and Red Sox executive Theo Epstein, “and MLB must be thoughtful and intentional about progressing toward the very best version of baseball -- a version that is true to its essence and has enough consistent action and athleticism on display to entertain fans of all ages.”

The changes -- and the leagues in which they will apply -- are as follows (more detail on each can be found below):

• Slightly larger bases with a less-slippery surface (all Triple-A leagues)

• A requirement that all four infielders have their cleats within the outer boundary of the infield dirt when the pitch is delivered (all Double-A)

• A requirement that pitchers must step off the rubber to attempt a pickoff (all High-A)

• A limit of two pickoff attempts per plate appearance (all Low-A)

• A 15-second pitch clock (Low-A West only)

• An automatic ball-strike system (Low-A Southeast only)

The rules are scattered through the various levels and leagues in order to help clarify the effects of each one. MLB has previously partnered with the independent Atlantic League on some experimental rules, but the sport’s new Minor League structure allows for analysis within games that more closely align with the Major League product.

These changes were chosen after extensive fan research and opinion gathering from on-field and executive personnel.

“We are listening to our fans,” said Michael Hill, MLB’s president of on-field operations and former Marlins general manager. “This effort is an important step towards bringing to life rules changes aimed at creating more action and improving the pace of play.”

Here is a closer look at each rule and what it aims to accomplish.

LARGER BASES (ALL TRIPLE-A LEAGUES)

Just as 90 feet from base to base is the big league standard, so is the size of the base itself. Each side is 15 inches long.

But the Triple-A bases this year will be 18 inches square and will be composed of a material that is expected to perform better in wet conditions. (This applies to first, second and third base. Home plate remains the same size as ever.)

The subtle increase in size provides more room for players to operate around the bases, reducing the odds of the kinds of collisions that have caused foot and ankle injuries in the past. And the more grippy surface could potentially prevent injuries such as the knee issue Bryce Harper suffered in 2017, when he slipped on a wet first-base bag in rainy conditions.

This change also slightly decreases the distance between bases. While the impact is likely modest, the three inches would theoretically lead to more ground balls and bunts getting beat out at first base and, perhaps, more successful stolen-base attempts, thereby increasing both base traffic and action.

DEFENSIVE POSITIONING RESTRICTIONS (ALL DOUBLE-A)

“Infielder” will never be a misnomer in Double-A this year. The first baseman, second baseman, shortstop and third baseman will be required to have both feet completely in front of the outer infield dirt boundary when the pitch is delivered.

Note that this is different than the oft-discussed idea of forcing teams to have two infielders on each side of the second-base bag -- a rule that was part of past Atlantic League experimentation. So infielders can still shift to the other side of the diamond, but they must position themselves within the infield.

This is more easily enforced by umpires than a restriction on lateral movement in which players could conceivably rush from one side of the second-base bag to the other as the pitch crosses the plate. It is possible, though, that MLB may require two infielders on each side of second base in the second half of the Double-A season, depending on how the initial experiment goes in the first half.

The goal here is, simply, a higher batting average on balls in play. Under this new arrangement, for example, the scorching ground ball we so often see gobbled up by a second baseman positioned in shallow right is more likely to get through.

THE “STEP OFF” RULE (ALL HIGH-A)

This change is aimed at increasing the number of stolen-base attempts and, perhaps, the stolen-base success rate. By forcing pitchers to fully step off the rubber before attempting a pickoff, the move by left-handed pitchers -- think Andy Pettitte -- to raise the right knee up in the delivery and then throw to first is eliminated. Snap throws followed by the step off are prohibited.

When this rule was tested in the Atlantic League in 2019, runners were more ambitious with their leads and more successful with their stolen-base attempts.

Conceivably, this change could have an impact on the pitcher-hitter dynamic, as well. If the pitcher is more mindful of the running game, he may throw more fastballs in the zone, leading to more offensive action.

PICKOFF ATTEMPT LIMITS (ALL LOW-A)

The Low-A leagues will each see a combination of rule changes, with both leagues having a limit on pickoff attempts combined with one other rule change: either pitch clocks or robo umps.

This limit on pickoff attempts has the same intent as the step off rule in the High-A leagues but with a different method. It can become monotonous when a pitcher makes throw after throw after throw to first base in a vain attempt to pick off a pesky baserunner.

This rule will limit pitchers to just two “step offs” or pickoff attempts per plate appearance. On the third attempt, if the runner is not thrown out, the move is ruled a balk and any runners are automatically awarded the next base.

Depending on the preliminary results of this change, MLB will consider further reducing the limitation to a single “step off” or pickoff per plate appearance.

THE PITCH CLOCK (LOW-A WEST)

A 20-second pitch clock has been in effect in the Double-A and Triple-A levels since 2015. But while the clock did, in its first year, have a tangible impact immediately (reducing game times by 12 minutes from the previous year), game times have risen in years since as players found workarounds -- most notably by restarting the clock by stepping off the rubber.

This change will be a more aggressive, 15-second pitch clock. One timer will be located in the outfield and two behind home plate, between the dugouts. Inning breaks and pitching changes will also be timed.

The hope is that, when paired with the limit on pickoff attempts that will be in effect at the Low-A level, the pace of play will be quickened.

“ROBOT UMPS” (LOW-A SOUTHEAST)

Long proposed, the automated balls-and-strikes system will finally become a reality in affiliated ball with this experiment, though a human umpire will still be positioned behind home plate. The Hawk-Eye tracking system will be used to deliver an audio signal to the home-plate ump, who will then relay the ball or strike call.

The goal here, obviously, is improved accuracy and reduced controversy. The automated calls have been tested previously in the Atlantic League and Arizona Fall League, and the technology has progressed to the point where MLB wants to see it in action in a full-season league in order to properly assess its impact.

MLB is continuing its three-year partnership with the Atlantic League to test and adjust experimental rules and equipment changes. Any new experimental rules for the 2021 Atlantic League season will be announced in the coming weeks.