Are Guardian Caps effective at preventing concussions? Here’s what research shows

[ASFN Admin]

Every time a new concussion tool in the NFL catches the attention of the national public, it also catches the eye of sports medicine professionals who study said tool's effects on health and safety.

It happened with impact sensors, like wearable mouthguards, or Q-collars, a different device designed to reduce head impacts, that's worn around the neck.

Now, sports medicine professionals are studying Guardian Caps, a brand of soft-shell helmet covers that slip on a helmet's hard plastic shell.

Some college programs, like South Carolina, have worn the caps since 2013. That drew the attention of the NFL — and now the nation, with some parents and coaches outfitting their players in caps available for retail sale.

At least five universities have published studies — both laboratory- and field-based — since 2023 that involved Guardian Caps. Overall, the lab studies found Guardian Caps reduce impact over many hits. Field studies, though, found little evidence Guardian Caps reduced energy transfer to the head.

A 2022 announcement by the NFL said its injury data showed concussions among players who wore Guardian Caps dropped more than 50% that summer, leading many football faithful to assume Guardian Caps reduced concussions.

But Erin Hanson, co-founder of Guardian Sports, the material science business that creates the caps, said the company never purported to prevent concussions.

"We call concussions the 'C' word in our office," she said. "When people do studies saying, 'Well, these don't (prevent concussions).' It's like, 'Yeah. We never said we did that.' That was the NFL's experience and that's amazing that at those speed the hits with their specific helmets being studied — that's what they saw.

"That's not what we set out to do, nor what we claim whatsoever. So it's really, really difficult to do these short-term studies on these very small sample sizes and get a picture of what you're doing for long term health."

More: Should your athlete be wearing a Guardian Cap? We asked experts about the debate

Here's what the five university studies found:

• Stanford University (published March 2023): The university did both a field and laboratory impact testing. The lab used a mechanical battering device on three helmet models of six impact locations and three impact velocities with and without the padded helmet shell cover. The study states it used "the Generation 2 Guardian Cap," or the XT model — the one used among high school and youth athletes.
  • The field testing found "no significant difference in any measure of head impact magnitude" between bare helmet impacts and those with the Guardian Cap padding.
  • The laboratory tests found Head Acceleration Response Metric, or HARM values, on helmets outfitted with Guardian Caps were reduced in laboratory impact tests by an average of 25% at 3.5 meters per second. However, as speed increased, the reduction of impact became less.
  • "We don't think it's the padding that's providing most of the protection; we think it's the sliding of the cap along the helmet's surface," said Nick Cecchi, the lead author of the study said in a Stanford Medicine report regarding laboratory impacts. "It redirects some of the impact energy when hit at the right angle."
• University of North Carolina (published October 2023): A field study had players wear helmets equipped with Head Impact Telemetry Systems — a system designed by helmet-maker Riddell to measure force — to compare head impact kinetics, or the transfer of energy resulting from impacts to a head, among 10 Division I football players. Five wore Guardian Caps and five didn't. Researchers collected data during 14 practices on 968 "video-confirmed" head impacts. The study used the Guardian Cap NXT, the model used in the NFL.
  • It found "protective soft-shell padding did not reduce head liner or rotational accelerations" among the players who wore the caps versus the ones who didn't.
• University of Nevada Reno (published June 2024): Researchers conducted a field study using mouthguards to test preseason workout on-field head kinematics, or the energy resulting from the movement of the head, with 25 NCAA Division I football players. The study did not distinguish which Guardian Cap model was used.
  • It also found no reduction in acceleration and total impacts between traditional helmets and ones with Guardian Caps. It did not track concussions.
• University of Wisconsin (published January 2025): The first field study to collect data of high school athletes to determine if Guardian Cap use during practice was associated with lower risk of sports-related concussions during practices and games. The study used the Guardian Cap XT model.
  • It found that using Guardian Caps during practice did not mean a decreased risk of sustaining sports-related concussions in practice or games.
• University of Georgia (published June 2025): The study included 54 participants from two college programs — 11 who wore a Guardian Cap and 43 who didn't — for one season. "Instrument mouthguards" recorded on-field head impact kinematics across 7,509 recorded impacts.
  • It found the Guardian Cap had "no on-field effect on head impact magnitude or frequency, but impact location patterns presented between wearers and nonwearers, suggesting that Guardian Cap usage could influence how players use their head during collisions. …"

A fifth study was published in July 2025, but it wasn't done by a university. It was conducted by Biotechnical Consulting & Research, also known as Biocore, based in Charlottesville, Va. It's the same lab that annually conducts the NFL's helmet laboratory testing.

The lab used the NFL's Injury Surveillance System Data and concussions rates in preseason practice during the 2018 to 2023 preseasons "compared before and after the NFL required players in certain positions ... to wear the Guardian Cap NXT during practices in 2022."

Dr. Allen Sills, the NFL's Chief Medical Officer who is an author of the paper, said the primary finding is "Concussion counts in those players that we required for the Cap decreased 53%," the same finding the NFL touted in 2022.

The paper's secondary finding: "For concussions involving the helmet shell (as opposed to the facemask), there was no significant association between Guardian Cap NXT use and the concussion rate, indicating that the reduction in concussions could not be attributed solely to the energy-absorbing effects of the Guardian Cap NXT."

Meaning it wasn't solely the Guardian Cap that reduced the apparent concussion rate.

"If you remember back in 2023, there was a lot of controversy around us introducing these caps," Sills said. "We weren't exactly getting thank you notes from people saying, 'Thank you for making me wear this.' And what we simply said was, 'Hey, look. We're not saying we've figured out all aspects of this problem, but the early results are encouraging. We think we should continue this.'"

So the NFL extended the mandate to more positions and then for another year. Wearing the Guardian Caps NXT model still remains a league mandate unless a player decides to wear the top-safety rated helmet.

"Basically, when we look at our data, our number one goal is to make injuries go down," Sills said. "It would certainly not make sense to us to change that policy when we've seen such favorable injury reduction."

Stephanie Kuzydym is an enterprise and investigative sports reporter. Reach her at [email protected] or on social at @stephkuzy.

This article originally appeared on Louisville Courier Journal: Are NFL Guardian Caps effective? What independent studies show

Continue reading...