The decision to not allow unregistered participants, like military ruck marchers, to participate in the Boston Marathon course was made because of the increased field size and increase in the number of those planning to march, according to the Massachusetts Emergency Management Agency (MEMA).

“We began to develop these rules just within the last six weeks and by the time we started pulling together these rules we were aware of well more than 1,000 ruck marchers who had expressed a clear intention of marching along the course,” said MEMA director and undersecretary of homeland security for the state of Massachusetts, Kurt Schwartz. “When we brought it to the B.A.A. ‘Can we have 36,000 runners and more than 1,000 marchers on the course at the same time?’ the answer was clearly no. That’s going to really jeopardize the experience of the runners.”

Due to increased interest in this year’s Boston Marathon following the bombings that occurred at the finish line last year, the B.A.A. increased the field size by 9,000 runners.

“We want to embrace the military but for this year in particular we just can’t accommodate more,” Schwartz said.

Last week, the B.A.A. announced new stricter security guidelines for this year’s race including a rule that, “units or groups such as military ruck-marchers and cyclists, who have sometimes joined on the course, will not be allowed to participate.”

Schwartz said there has been more flexibility in past years but the number of ruck marchers in prior years has been much smaller.

“It wasn’t about them posing a threat,” Schwartz said. “Nobody is going to be on the course with backpacks. Even if they didn’t have backpacks there wasn’t enough room.”

The B.A.A. has formed a partnership with the National Guard of Massachusetts and has provided 130 official bibs for soldiers.

“The Guard will register those people and they will all be required to follow all of the same rules as runners,” Schwartz said.

The National Guard members will begin their march around 6 a.m. on Marathon Monday and will have a staggered start, marching along the course in single-file groups of 10.

“We’re very comfortable that the number is small enough,” Schwartz said.

Schwartz said it is unfortunate the B.A.A. and MEMA did not make the rules earlier, or else they would have been able to communicate with groups planning to march.

“It’s unfortunate that by the time we developed the rules some of these groups were way down the path of signing up marchers and putting together trips and plans,” he said. “We were not aware of the numbers.”