Overview

The National Academies of Sciences, Engineering, and Medicine’s Forum on Traumatic Brain Injury hosted a virtual public workshop on April 15–16, 2025, to examine exposure to repeated head impacts (RHI) among youth under age 18, how RHI is defined and measured, its potential connections to short- and long-term health outcomes, and strategies to reduce exposure or mitigate risk, while identifying examples of major open questions and research needs (Rivara). Head impacts occur in both sport and nonsport contexts; the workshop focused particularly on repeated impacts not resulting in concussions, such as routine collisions in contact sports. No consensus definition of RHI and its effects exists, and measurement approaches vary across studies. Considering and clarifying what is currently known and unknown was a core aim of the meeting (Rivara, Yeates). This high-level overview provides a synthesis of major ideas shared during the workshop. Expanded summaries of workshop presentations and discussions, along with additional points highlighted by individual speakers, are found in subsequent chapters.¹

Reducing youth exposure to RHI is a broadly supported health goal that depends on evidence-based interventions, translation of research knowledge to policy and practice, and collaboration among families, educators,

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¹ This overview represents the rapporteurs’ summary of points made by individual speakers during the workshop convened in April 2025 and does not necessarily represent the views of all workshop participants, the planning committee, or the National Academies of Sciences, Engineering, and Medicine. For additional information, including the agenda and speaker biographies, visit https://www.nationalacademies.org/projects/HMD-HSP-24-11/event/44347.

coaches, health care providers, and researchers (Caccese, Yeates). Weighing RHI risks versus the benefits of an activity such as a sport requires individualized decisions aligning information and values amid uncertainty, honest reporting if symptoms develop, and youth engagement supported by coaches, parents, and clinicians (Culver, Kroshus-Havril, Master, Riggs, Warren). Families navigating questions about brain injury risk want clear, practical guidance to recognize concerning symptoms, make informed choices, and alter participation when warranted (Culver, Mays, Riggs). Several speakers identified elements that could form part of a pragmatic approach, including minimizing RHI exposure and reducing head-impact risk through improved sport technique, appropriate equipment, policy change, and adherence to safety protocols while preserving the physical, mental, and social benefits of sports (Caccese, Emery, Gioia, Master, McCrea, Meehan, Swartz).

Defining and measuring RHI remains a foundational challenge. To better tease out risks associated with repeated impacts that do not lead to the signs and symptoms of a concussion, a working description of RHI during the workshop was “repeated head acceleration events that occur in the absence of diagnosed injury” (Arbogast). RHI exposure is highly individualized, and proxy measures such as years played or age of first exposure may not capture true exposure (Arbogast, Chandran). Sensor-based tools show promise in detecting RHI yet vary in how they measure and process data, making it difficult to compare findings; video confirmation substantially reduces false events, further underscoring the need for harmonized acquisition and reporting standards (Arbogast, Yang). RHI exposure is also shaped by intersecting factors including sex, age, race, and broader social and structural factors such as access to protective equipment, training, and oversight (Chandran, Register-Mihalik). The evidence base overrepresents male athletes in helmeted sports; expanding research among female athletes, para-athletes, nonhelmeted sports, nonsport activities, and children under 11 is needed to inform more broadly inclusive prevention and mitigation strategies (Register-Mihalik, Yang).

Several speakers discussed the available evidence on relationships between exposure to RHI and health outcomes, noting that substantial gaps remain. Current evidence has not consistently shown short-term clinical symptoms following youth exposure to RHI (Rose), and long-term population risk has yet to be clearly determined or quantified (McCrea). Presenters shared data on short-term, measurable biological responses to RHI exposure, such as blood biomarkers, eye-tracking changes, and neuroimaging findings, yet most studies do not demonstrate consistent changes in clinical symptoms, cognition, balance, mood, or behavior; when differences appear following RHI exposure, they tend to be subtle and may be accounted for by factors such as ADHD, anxiety, or depression (Kawata,

Rose). Effects are often individualized: the head acceleration that results in concussion varies across individuals, and the timing and clustering of head impacts may be important contributors to injury risk.

Better characterizing cumulative exposure using quantitative metrics will be essential for understanding the effects of RHI (Gabler, Kawata). Neuropathological studies have found a dose–response relationship between cumulative head impacts and chronic traumatic encephalopathy (CTE) risk and severity (Mez); cohort studies of former contact sport athletes generally show long-term clinical outcomes comparable to the general population (McCrea). Where reported, elevated cognitive and mental health risks have been observed primarily among athletes with very high concussion or RHI exposure histories, but not universally (McCrea). These findings indicate the need for further research to determine risk predictors and outcomes. Current clinical tools, mostly developed for concussion, may be too coarse to detect subtler sequelae of RHI, reinforcing the need to develop appropriate outcome measures (Alosco, Rose).

A number of speakers highlighted that closing evidence gaps would take large, prospective, longitudinal studies that track diverse participants over time with validated exposure measures (harmonized sensors plus impact confirmation) and the use of multimodal outcomes drawing on biomarkers, imaging, physiological, scholastic, and functional domains and supported by advanced analytics attuned to brain development (Alosco, Bazarian, Brett, Gioia, Hunt, Mannix, McCrea, Mez, Rose). Research on how biological, lifestyle, and sociodemographic factors—such as physiological response, stress, access to care, and community support—amplify or mitigate RHI effects and shape long-range outcomes is also needed (Brett, Giza, Hunt, McCrea). Additional examples of suggested actions include determining which short-term, biological changes are clinically important, particularly when imaging or biomarker changes do not align with symptoms (Brett, Mannix); refining cumulative-exposure tracking and dose–response relationships (Bazarian); and strengthening cross-sector collaboration and investment (Gabler, Marinelli). In parallel, practical steps include school–health care partnerships (Keyes), targeting engineering and educational interventions to practice settings where most impacts occur and using coach-led, skill-based technique training (Swartz, Urban, Warren), pragmatic adjustments to equipment and game rules to lower impact forces (Chiampas), and community codesign of materials and interventions implemented with attention to organizational readiness and the realities of youth-sports governance (Chiampas, Emery, Gioia, Urban).

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