The NFL has warned teams that 11-on-11 training camp drills were largely responsible for last year's spike in preseason concussions, an important data point as coaches finalize practice schedules for this summer's practices.

The league has stopped short of banning full-squad drills, which coaches consider essential to preparing for the contact that occurs during regular-season games. But NFL data showed that preseason concussions in 2017 increased by 73 percent, from 26 in 2016 to 45. Most of them occurred during full-team drills during the opening weeks of camp, before preseason games began.

Each team has received a customized data report describing how and when preseason concussions occurred in their camps last season, compared to the rest of the league. Jeff Miller, the NFL's executive vice president of health and safety initiatives, said Friday that he hopes the data "will inform what clubs do" when camps open. Miller said the league hopes to replicate its success in addressing a similar issue after the 2014 season. In 2015, camp concussions dropped by 33 percent.

"We didn't have a level and specificity of data then," Miller said, "but we went through a process led by our football operations to inform teams and talk to them and say, 'Hey, we noticed this data.' We brought it to their attention, and there was an effort by many of them to address the question. And we saw a drop the following year.

"Raising the consciousness level of that issue was definitely worthwhile. I don't want to take full credit for that decrease, but I think it informed what clubs did."

Dr. Allen Sills, hired last year as the NFL's chief medical officer, already has spoken with each team's medical staff. Sills and Miller will visit a select number of training camps as well but said they are not targeting or pressuring specific teams to change their practice schedules.

"It's not a matter of having five or six recidivistic clubs that we have to discipline into line," Sills said. "This is a league-wide issue where everyone has to understand it's on all of us to work on. It might sound trite to say, but any concussion we save is important to us. We want to put the awareness out there ... and make sure we're making it as safe as we can."

Although the camp concussions largely came in the opening weeks, Sills said there is no medical reason to think that players need more time to get acclimated to contact before advancing to 11-on-11 drills.

"We don't think about the brain as a hamstring, where you have to get it loose and flexible," he said. "I don't think we feel that. We think it's exposure: What behavior are you doing and how much are you doing of it?"

The focus on early-camp concussions is part of a larger effort this offseason to reduce concussion numbers across the league. Sills declared a "call to action" in February and followed up by working with the competition committee on two major rule changes.

First, the league made it a 15-yard penalty for lowering the head to initiate contact on any part of an opponent's body. Second, the league mandated a redesign of the kickoff, which has been as much as five times as likely to cause a concussion as the average play.

In addition, the NFL joined with the NFL Players Association to ban certain underperforming helmets over the next two seasons.