British Journal of Sports Medicine
July 23, 2025
WHAT IS ALREADY KNOWN ON THIS TOPIC?
Helmet add-on devices like the Guardian Cap have been shown to reduce impact forces in laboratory settings.
WHAT ARE THE FINDINGS?
Guardian Cap use in high school American footballers during practice was not associated with a decreased risk of sustaining a sport-related concussion during practice or games.
HOW MIGHT IT IMPACT CLINICAL PRACTICE IN THE FUTURE?
The effectiveness of Guardian Caps and similar helmet covers should be established in real-world environments to more accurately inform use.
Background
American football (henceforth referred to as ‘football’) is one of the most popular high school sports in the USA with over 1 million participants.1 Unfortunately, football players experience a high rate of sport-related concussion (SRC) and head impacts compared with other high school athletes.2 3 SRCs are a major concern due to short-term and long-term symptoms, disablement and cost of the injury.4–8 Preventing SRC injuries remains a priority for athletes, parents, medical providers and sport governing organisations. Additionally, the Concussion in Sport Group shares this objective and has identified primary prevention as a focus area for SRC research.9 10
To that end, progress has been made in reducing the risk of SRCs in high school football. When policies aimed at limiting contact in football practices were implemented, practice-related SRC rates were reduced by 64%.10 Coach participation in a comprehensive football safety standards training programme also reduced the rate of SRCs in practices and games by 50%.11 Modifications of helmets to increase padding over the zygoma/mandible were associated with a 31% lower rate of SRC (figure 1).12 Interest in further improving the effectiveness of football helmets has continued.
Aftermarket add-on shell devices for football helmets, such as the Guardian Cap (GC), have been developed to reduce head impact forces (figure 2). The use of these devices was mandated during the preseason in certain position groups by the NFL in 2022 on the premise of reducing the cumulative impact forces to the head.13 Subsequently, the NFL attributed a significant reduction in concussion rates during the preseason to GCs.14 Laboratory studies investigating the efficacy of GCs to reduce impact forces have had mixed results, and no studies have investigated the real-world effectiveness of GCs to prevent SRC in high school football players.15–17 The objective of this study was to determine if GC use during practice was associated with a lower risk of SRC during practices or games among high school football players.
A convenience sample of 2610 football players from 41 high schools in Wisconsin participated in this prospective cohort study during the 2023 football season. One additional high school was recruited but ultimately excluded from the data analysis because team GC use changed midway through the season (figure 3). Study team members travelled to participating schools to present the study to the players and their parents at preseason team meetings. To be eligible to take part in the study, players had to be (1) a member of an interscholastic football team, (2) enrolled in grades 9 to 12 (ages 14 to 18 years) and (3) able to fully participate in preseason practice on the day of study recruitment. Prior to the start of the season, players reported their demographic data (sex, age, grade in school, height, weight), history of SRC, and previous football playing experience. Players also completed a head injury Symptom and Severity score from the Sports Concussion Assessment Tool V.5 (SCAT5).18
Data collection
Licensed athletic trainers (ATs) employed at each school reported the football helmet model, GC use, football exposures and injuries for all players from the first practice through to the last game. An exposure was defined as one player participating in one practice or game. Athletes were classified as ‘GC’ if they wore a helmet and GC during practice and ‘Non-GC’ if they only wore a helmet. GC versus Non-GC designation was captured per each exposure. Injury data were collected by the ATs and included injury characteristics (mechanism, football activity and playing surface), SCAT5 Symptom and Severity score, and confirmation of whether a GC was in use at the time of injury.
Sport-related concussion
A player was determined to have sustained an SRC using the definition provided by Amsterdam International Consensus Statement on Concussion in Sport.9 The school ATs administered the SCAT5 within 72 hours of the onset of the injury to any player suspected of having an SRC. This included the Symptom Evaluation which measures the severity of 22 SRC symptoms on a 6-point scale. When appropriate, injured players were referred to their primary care provider for further evaluation and treatment. Only the first SRC for each player was included in the analysis.
Players recovering from an SRC were permitted to gradually return to full unrestricted sport participation in accordance with consensus-based guidelines.9 ATs monitored all participants who sustained an SRC from the date of the onset of the injury, and recorded all missed practices and games and documented days from the time of injury to full recovery.
Statistical analysis
A power analysis was completed to determine the minimum number of players needed to test for 40% change in the proportion of athletes who sustained an SRC this season. This was based on previous work using similar methods in the same population where 5.8% of athletes sustained an SRC over the course of one season.6 It was determined that to achieve 80% power a minimum of 2630 players would be needed to find a small to medium effect size significant (Cohen’s d=0.31). Statistical analyses were conducted using SAS V.9.4. Data were summarised via mean (SD) or frequency (%) for the GC and Non-GC cohorts. Player and SRC exposure characteristics were compared between groups of interest (SRC vs Non-SRC and GC vs Non-GC) using χ2 tests of association and unpaired t-tests. Univariable and multivariable (controlling for previous SRC within the last year) generalised linear mixed effects models using a Poisson regression were built to model SRC rates, with school as a random effect to account for school-level traits. Player exposures were included as an offset to account for differences in number of exposures between athletes. Relative risks (RR) and 95% confidence intervals (CI) are reported.
Patient and public involvement statement
No patients were involved in the design or interpretation of this study. Letters of support were elicited from the Wisconsin Intercollegiate Athletic Association who expressed interest in the objectives of the study. All participation was voluntary. Players were not asked to assess the burden of their participation. Study results will be shared with stakeholders after peer review and publication.
Equity, diversity, and inclusion statement
We intentionally recruited athletes from urban, suburban and rural schools around the state of Wisconsin. The investigator team consists of three women and four men from different disciplines (medicine, biostatistics, athletic training, physical therapy and biomechanics). It included two junior researchers (ET and AC). There were very few female athletes included in the study, an issue we have expanded on in the discussion.
Results
There were 2610 high school football players (mean age 15.9, 99.1% male) included in the study, of which 1422 (54.5%) wore GCs (table 1). No differences in baseline player characteristics were identified (p values ≥0.06). Players participated in 131 048 practice exposures and 25 538 game exposures. 23 teams required all of their players (n=22) to wear GCs during practices. One school required 14 offensive or defensive lineman to wear GCs while the other 61 athletes did not. Another school required athletes whose helmets were compatible with GCs (n=72) to wear them while 13 wide receivers, punters and kickers did not because their helmets were not compatible. All athletes who wore GCs used the Football Guardian Caps XT model, which is available to high school athletes. Our study included 18 teams with players who did not wear GCs at any time.