CHARLOTTE, N.C. --
CHARLOTTE, N.C. --Carolina Panthers center Matt Paradis was among about 100 NFL players who arrived for the first padded practice of training camp wearing a large, soft-shelled cover over his helmet that gave him the appearance of a "Star Wars" stormtrooper.
"Squishy helmet," Paradis said with a laugh.
The Guardian Cap, officially.
Paradis, 31, was offered the opportunity last season to wear the shell, which is designed to reduce the energy of impact and limit head injuries. He didn't, but as he thought about it during the offseason, he came to this simple conclusion: "The science says it helps, so it's not really a debate at that point.''
But what have scientific studies actually revealed about the effectiveness of Guardian Caps in reducing the energy of impacts to the heads of NFL players?
ESPN.com obtained a copy of a memo the NFL sent to team physicians, head athletic trainers and equipment managers in August 2020 when it decided to move forward with on-the-field experimentation of the cap.
The memo explained that two add-on caps -- the Guardian Cap and Defend Your Head ProTech Helmet Cap -- were chosen in a collaborative effort between the NFL and NFLPA to be tested by Biomechanics Consulting and Research under Dr. Ann Bailey Good, a senior mechanical engineer for Biocore.
The study tested helmets with and without the cap under conditions that simulated impacts NFL players sustained during games from 2015 through 2019. The tests were conducted using the same type of linear-impactor machine used for annual NFL-NFLPA performance evaluations of bare helmets.
In an attempt to duplicate the average impact speed and concussive impacts that offensive and defensive linemen experience in games, the tests used two impact velocities and three impact locations: front, side and upper.
Statistical analysis of the weighted results showed the helmet with the Guardian Cap on average showed a 9% reduction in force impact compared to a bare helmet. The average reduction was 5% for the ProTech. Additional testing performed this year revealed an average reduction of 10% for the Guardian Cap.
For high school players who aren't as big or fast, Erin and Lee Hanson, who invented the Guardian Cap a decade ago, claim the reduction has been tested up to 33%.
After the Jacksonville Jaguars experimented with them in 2020, the NFL approved use of the cap in practice for offensive and defensive linemen this year. It was determined those groups take more helmet collisions during a week of practice than they do in a game. So, though it is not marketed as a concussion-preventing device, the cap could theoretically help reduce the rate of concussions.
The NFL didn't move forward with the caps without approval from the NFL Players Association. The NFLPA, a spokesperson told ESPN.com, allowed players to use the cap for practice on a voluntary basis in 2021. Twenty-three teams began camp with the caps on an experimental basis.
"Ten percent is a substantial amount of force to be reduced when somebody has a helmet contact,'' said Jeff Miller, the NFL's executive vice president of health and safety initiatives. "We'll have a better appreciation for it after this year, when we have a better sense of how many players use it, how frequently they've used it, and how our concussion numbers at those player positions look.''
Early stages of Guardian Cap development
Erin and Lee Hanson began a material science company in the mid-1990s to give them freedom to create products and solve problems for other companies. They weren't looking to get into the football world until they were approached in 2010 about developing a flexible shell for helmets.
This was a year before a Pittsburgh injury law firm became the first to sue the NFL over concussions, filing for 120 former players, and five years before the movie "Concussion,'' in which forensic pathologist Dr. Bennet Omalu, played by Will Smith, battled the NFL about suppressing his research on chronic traumatic encephalopathy (CTE), a debilitating brain disease that can cause a range of symptoms including memory loss.
"It was way ahead of its time,'' Erin Hansen said of the cap.
The initial cap was connected with a fixed strap, but there was so much motion that the straps broke. Next came elastic straps that allowed movement, but the facemask still popped off the snaps.
In 2017, a rubber flap was added over the straps along with Velcro for double reinforcement. That year, the cap won the inaugural NFL HeadHealthTECH Challenge, a competition funded by the league to promote the development of equipment.
Since then, the helmet has undergone extensive testing by the league. The current cap, called Guardian NXT, was made specifically for the NFL. It has additional padding and slides over the helmet so it is basically "floating'' on top of the helmet.
That floating helps the cap move with the helmet to reduce the risk of neck injury.
Will add-on helmet caps eventually be used in NFL games?
It could be several years before the NFL has enough data to determine the full value of the caps and whether they might be effective enough to wear in games. One drawback has been the caps don't deflect off each other as easily as the hard helmets that create a glancing blow. That slightly increases the chances of a neck injury.
"There's things we still don't understand about how that device might react in an uncontrolled environment,'' Good said. "It'll be important to collect that feedback before making a decision on whether it should be used in games.''
"Understanding the chaotic environment is a little less predictable.''
Dr. Katherine Breedlove, whose work as a research associate at Brigham and Women's Hospital in Boston focuses on sports-related concussions, determined during her 2017 independent study that the risk of neck injury was minimal.
She said the other concern is that the foam used in the cap doesn't have a quick rebound "so that each time it's hit it was in its best shape.
"The data from my paper pretty solidly says, 'Yes, maybe [it helps limit concussions],' and so we need to find out more,'' she said. "It's not a conclusive, no. But it's not a conclusive yes. Maybe this will be helpful for the NFL to do this targeted sample.''
Popularity of add-on shells increasing at all levels of football
Youth leagues and high schools began using the shells in the early stages of development. Colleges soon followed, with South Carolina leading the way in 2012, when Steve Spurrier was the coach and Clint Haggard the athletic trainer.
Now, according to Guardian, more than 200 colleges use them.
But it wasn't until the NFL sent a memo announcing the league tested the cap for linemen and teams were permitted to use them that the caps got the national exposure they have now.
Several former NFL stars, such as Atlanta-based Jerome Bettis, Tim Lester and Fred McCrary, have come to the Hansons to talk about getting the caps for their children and youth teams because they saw what the real world looked like when they were players.
The Hansons spent time recently with former NFL cornerback Deion Sanders, now the head coach at Jackson State, demonstrating the cap.
The Los Angeles Rams weren't using the Guardian Cap when training camp began. That changed after quarterback Matthew Stafford hit his surgically repaired thumb on top of a defensive lineman's hard helmet.
So the soft cap serves a purpose there as well.
Coach Sean McVay called it a "heightened sense of urgency.''
"It was a freak thing, but it is something that consistently occurs,'' McVay said. "It's one of those things where I'm saying to myself, 'Man, I feel stupid that I didn't implement some of the things to prevent that.' ''
The sounds in practice alone tell you there's a difference. Instead of the usual sharp crack when two helmets collide, there is a dull thump.
"Yeah, it sounds different when players aren't wearing them,'' Paradis said.
That helped convince the center to continue wearing the cap, though several teammates stopped wearing them because they are hotter and heavier than just the helmet. One who stopped was 2020 first-round pick Derrick Brown, who called it a personal decision.
"I mean, we're talking about the brain,'' Paradis said. "There's so much that goes into that and so much we don't know. It's kind of like, at this point, it's available, it helps statistically, it helps reduce that force, so I might as well give it a shot.''
Does reducing head impacts also reduce concussions?
Dr. Kristy Arbogas is a concussion research specialist who has spent the past six years as an NFLPA consultant on injury prevention. She also is the director of the injury and prevention research center at the Children's Hospital of Philadelphia with a focus on sports concussions for children.
She said the Guardian Cap is one of many layers -- from reduced padded practices to improved helmet design to rule changes such as penalizing helmet-to-helmet contact -- that have been introduced to help reduce concussions.
"We really do think it's a meaningful part,'' Arbogast said of the cap. "We wouldn't have recommended it to head coaches if we didn't. Can I give you a number that it's going to reduce five concussions? No.''
But Arbogast is optimistic the cap can help.
"It's been clear for several decades that acceleration, either linear or rotational acceleration, is what leads to brain injury,'' she said. "What we've shown with these helmet covers is that it can reduce the amount of acceleration. We believe that will be better. It surely is not going to make it worse.
"Our approach is similar to all efforts we've made. That is if there's anything we can do to reduce high-acceleration impacts players take, it's a good thing. That's what we know this does.''
But overall there is no simple answer for how a 10% reduction in force might help in terms of concussions because there are so many varying factors around brain injury.
"It's easy to think that the harder the hit, the more likely and worse the concussion -- but studies show that many factors contribute to a concussion, including force, place of impact, brain development, and previous history of head injuries,'' said Dr. Samuel Quaynor, who completed a fellowship in sports neurology, headache and concussion at LifeBridge Health Sports Medicine Institute, in Baltimore.
"... A 10% reduction in force to the football helmet and head could be slightly beneficial for players in general but may not reduce concussion for every player because studies show there is not a direct relationship between force and concussion severity.''
Continued testing will determine future use of Guardian Caps
The data Carolina coach Matt Rhule heard over the summer from Dr. Allen Sills, the NFL's chief medical officer, was enough to convince him to offer the cap to his players.
"It just lowers the head force,'' Rhule said. "Some of the leadership guys on the offensive and defensive line said, 'Let's try it for our sake.' ''
More sophisticated testing, which better replicates the chaotic environment of NFL action, helped sell Rhule and others, as well.
"Way back in 2010, the standard for football helmets was called a drop test,'' Erin Hanson said, when asked why it took the NFL so long to come on board. "That's dropping a helmet right down onto a plate and measuring the force.
"But not a single football player out there drops out of the sky onto his head. So as testing has evolved, now they're doing rotational forces. The NFL has invested a lot of money in replicating hits.''
That change began around 2017, when researchers began using a pneumatic ram and adjustable table to determine the effectiveness of the cap -- and helmet -- on impacts from all different angles.
NFL technology has advanced to the point that some players wear sensors in their mouth guard to provide data on impact. That's information that can't be replicated in a laboratory.
"We've taken a look at more than a thousand concussions on-field and tried to repeat [the hits] in the laboratory,'' Miller said. "We added the Guardian Cap to the helmet impacts in the lab and measured the forces that those Guardian Caps mitigated from reaching the helmet in the head.''
Good and Breedlove agreed that the mouth-guard sensors provide more accurate data because they are positioned centrally closer to the brain than sensors that were on the surface of the helmet.
Miller said getting feedback and data from actual use will help even more as the NFL determines whether it will offer the caps to other positions next year.
Rams Dr. Casey Batten said players have given him no reason to suggest a downside to the caps.
"Other than the first few days when players complained of it slipping or how it looked funny in the shadows, we haven't heard any complaints,'' he said. "We haven't had any issues with it.''
Whether the caps will help prevent concussions or head trauma, Batten said the "frank answer is we don't know.''
He reminded the cap still is in the experimental stages and the results of this season will help determine whether the league expands use to other positions in the future.
"You'd have to look at several seasons here on out to really make heads or tails of whether the Guardian Cap really is reducing concussions or not,'' he said. "For us, we're just trying to see how this works in the real world.''
ESPN Los Angeles Rams reporter Lindsey Thiry contributed to this story.