Hard Evidence on Soft Shells
Helmet shells promise to reduce high rates of head injuries in football, but are they effective?
Injuries are a constant in the modern era of American football. Whether they are hamstring strains, twisted ankles, or broken bones, the violent nature of football renders these kinds of injuries nearly inevitable. Head-related injuries have become a particular concern, especially the high rate of concussions. This has led to recent efforts to build and deploy protective devices to prevent or reduce the likelihood of concussions.
One conspicuous device is the “Guardian Cap,” which is a foam device that can be slipped over a normal football helmet. While it certainly won’t score points for style, the intention of the Guardian Cap is to decrease the force of a hit upon a player’s head.
But how does the design of the Guardian Cap seek to accomplish this goal? The physics behind impacts can help explain the thinking behind the design of Guardian Caps.
When two objects collide, the amount of force experienced is proportional to the amount of time the impact takes to occur. This is modeled by the equation FΔt = mΔv, where F is force, t is time, m is mass, and v is velocity. In the case of head collisions in football, the mass and velocity of a given player’s head during a specific play are constant no matter which type of helmet is being used. So, the only variable that can be changed is the duration of the impact. That is, if the time for collision is longer, then the amount of peak force decreases.
The Guardian Cap is supposed to increase this time due to the foam padding on the helmet shell, which compresses as a collision takes place. This ultimately means that the collision is extended over more time (i.e., the impact is slowed), leading to less peak force being experienced by a player.
Guardian Caps have been used by many players at multiple levels for over a decade. The key question, then, is – are they effective? Unfortunately, the research evidence is not very promising.
For example, a 2024 study published in The American Journal of Sports Medicine found that Guardian Caps may not be very effective at reducing concussions (Funk et al., 2024). The researchers analyzed concussion rates in the NFL preseason before and after a Guardian Cap requirement was instated, which spanned over 213,500 preseason practices. They found that, while the amount of concussions per year decreased for positions that required Guardian Caps, the amount of concussions per year increased for positions that did not require Guardian Caps. This may initially seem to be an indication that Guardian Caps are effective at reducing concussions. However, the authors pointed out two implications of their findings that raise concerns about the effectiveness of the protective device.
First, the use of Guardian Caps should decrease concussions even for players that did not use them. In other words, at minimum two players are involved in head-to-head collisions and a Guardian Cap should increase the collision time (and thus decrease peak force) for both players – albeit this time increase would be smaller for the player without the device. Therefore, rates of concussions should have dropped for all positions in the study if Guardian Caps were successful in decreasing concussions.
Second, the study noted that when only ‘shell’ impacts were considered – collisions where the Guardian Cap is directly involved, as opposed to facemask collisions – there was no significant relationship between Guardian Cap usage and a lower concussion rate.
Another 2025 study published in the British Journal of Sport Medicine focused on high school football players (Hammer et al, 2025). This study spanned over 2600 players from 41 high schools in Wisconsin. Players self-selected into two groups: those wearing Guardian Caps during practice (23 teams, 1422 players) and those who did not (18 teams, 1188 players). Similar to the study on NFL players during preseason games, this study also failed to find that Guardian Cap use was associated with a decreased risk of sustaining a sport-related concussion.
The promise of Guardian Caps in reducing concussions remains unfulfilled. While ‘the physics’ of reducing force does make sense and laboratory results for Guardian Caps were supportive, real world conditions are complex and have failed to demonstrate Guardian Cap effectiveness. Until other technology can be brought to the field, the most viable ways to reduce head injuries in footballs seem to be rule changes and on-field testing protocols.
