Recently, trauma suffered by NFL stars has raised fresh concerns about the league’s ongoing issues with head injuries — and raised questions about whether there is more the NFL could do to protect its players.

In early September, Tua Tagovailoa, a quarterback for the Miami Dolphins, suffered a frightening third (diagnosed) concussion, prompting some fans and former players to urge his retirement. In the wake of that injury, former Green Bay Packers star quarterback Brett Favre disclosed a diagnosis of Parkinson’s disease during congressional testimony on Tuesday, noting that repeated head trauma was likely a major factor. And on Thursday, Malik Nabers, a rookie wide receiver for the New York Giants, set a receiving record before leaving the game with a concussion.

It isn’t news that professional football can be dangerous: The NFL first admitted the connection between football and CTE — chronic traumatic encephalopathy, a degenerative brain condition — in 2016. CTE is most often found in athletes who suffer repeated hits to the head and members of the military who are exposed to explosive blasts, and has been tied to the deaths of famous football players, including Andre Waters and Mike Webster. More than 300 former football players have been diagnosed with CTE following their deaths. (The condition requires a brain autopsy to accurately diagnose.)

Due to concerns about CTE and other conditions related to brain trauma, the league has invested in improving the equipment that players wear, changed rules for practice and game day, and promoted techniques intended to reduce head contact. This season, those changes include allowing players to wear new headgear meant to better protect players’ brains. This week, however, was a reminder that major problems remain for America’s most popular sport.

In recent years, the NFL has made major changes to helmets and protective gear, as well as shifts to in-game rules aimed at reducing collisions, NFL Chief Medical Officer Allen Sills told Vox.

Perhaps one of the most noticeable this season is the introduction of a new type of headgear called “Guardian Caps” — layers of foam padding worn over a helmet — during games. The caps, which most players have been required to wear when practicing since 2022, are intended to reduce the impact that players experience if they get hit in the head, potentially by about 10 percent, according to the NFL.

There are a few issues with the caps, however. One, players aren’t required to wear them during games, and relatively few players have chosen to do so thus far, limiting their impact.

And independent studies also haven’t confirmed whether they’re effective. Despite the NFL’s findings, separate teams of researchers from the University of North Carolina and the University of Nevada-Reno both found limited reductions in force for players wearing the caps, while a third study, from Stanford, saw force reductions in the lab but not when athletes wore them on the field. Sills argues this variability comes from differences in the researchers’ methodology and the NFL’s, and notes that the NFL intends to publish its research within the next few months.

The NFL also claims its researchers found that Guardian Caps reduced concussions by roughly 50 percent when worn in practice, a finding that some physicians have been skeptical of.

One main issue, doctors told the New York Times, is that better helmets and Guardian Caps shield the head, but they don’t shield the neck — which can be critical for preventing concussions. Hits and twisting of the neck play a major role in causing concussions, they note.

As Jamshid Ghajar, a neurosurgeon, told the Times, race car drivers’ helmets, like those used in Formula 1 racing, are more effective at preventing concussions because they stabilize the neck even in cases of a crash. The NFL has disputed the Times’s characterization of concussions, and Sills argued that concussions aren’t predominantly caused by what he characterized as “neck forces.”

The NFL has also touted its use of 12 new helmet models this season, which are supposed to protect players from position-specific blows, and a new rule meant to make kickoffs — a part of the game in which players run toward one another, often at high speeds — safer. The effectiveness of these changes is still unclear, and it’s important to note that even with these changes, the fundamental issue of football involving bodily collisions and head contact remains.

As Julie Stamm, a neuroscientist at the University of Wisconsin Madison, notes, “the best way to prevent [head trauma] is to not have impacts.”