Concussion in sports has gained increasing media and research focus in the past two decades. Concerns over the long-term effects of repeated head impacts and concussions fueled a drive for improvements in protective headgear and for strategies to reduce athlete head impact exposure.17,18,28 In 2019, the Annals of Biomedical Engineering (ABME) began publishing special issues on topics related to concussion in sports each fall. The 2019 issue featured research funded through a partnership between the National Collegiate Athletic Association (NCAA) and the U.S. Department of Defense (DoD).25 The partnership, known as the NCAA-DoD Grand Alliance and Concussion Assessment, Research, and Education (CARE) Consortium, was established in 2014 with the goal of informing policy decisions on concussion and its long-term consequences. In the fall of 2020, ABME published a special issue focused on concussion biomechanics in football.24 The papers in that issue covered the most recent advances in our understanding of concussion biomechanics through analysis of real-world impacts in football and new technologies to measure head impact kinematics. Applications of head impact biomechanics to concussion reduction strategies were also developed and reviewed. The current and third issue in this series expands on previous issues by including concussion research in all sports and recreational activities.

The NCAA-DoD issue included papers on concussion biomechanics and head impact exposure, as well as different aspects of concussion diagnosis and recovery. Several papers evaluated factors that may contribute to biomechanical concussion tolerance, since individual concussive impacts often seem unremarkable in a large dataset. Stemper et al. evaluated the effects of cumulative head impact exposure and risk-weighted exposure on concussion tolerance, and found that greater exposure may reduce individual concussion tolerance.30 Other factors that influence an individual athlete’s concussion tolerance were also explored by Campolettano et al. and Rowson et al.7,27 They confirmed that head impact exposure explains some of the variance in concussion tolerance, along with other physical characteristics like height, mass, age, race, and concussion history. These findings all support the need for a more individualized approach to evaluating concussion tolerance, either assessing risk on a population or individual basis. Although concussion tolerance varies by athlete, reducing head impact exposure decreases the odds of concussion for all athletes. However, rules and regulations to reduce head impact exposure should be data-driven to ensure effectiveness. An NCAA rule eliminated 2-a-day practices during the college football preseason to reduce head impact exposure, but the number of contact days increased, resulting in an overall increase in head impact exposure after the ruling.29 Other studies have quantified head impact exposure by practice drill type, which provides a basis for rulings that reduce avoidable head impacts.1,6 On-field head impact measurements have been important for reducing head injury risk to athletes by quantifying head impact exposure and concussion tolerance, but accurate measurements are difficult to obtain with current technology, especially in non-helmeted sports. Rich et al. developed and tested a custom instrumented mouthpiece to address the limitations of currently available sensors.23

In addition to concussion biomechanics, the NCAA-DoD partnership addressed concussion screening tests that are commonly used. Chrisman et al. showed how factors like learning disorders and primary home language can influence baseline results for one sideline concussion screening test, the King-Devick test.8 Houston et al. showed poor agreement between human-rated and automated instrument-rated Balance Error Scoring System (BESS) scores.11 Both studies suggest that caution should be used when comparing concussion screening test results between individuals and between different test administrators. Some screening tests such as BESS are also used to monitor athlete recovery following a concussion, but it is important to consider not only the patient’s physical recovery, but also their perception of mental and physical recovery.31

The 2020 special issue on concussion in football focused on characterizing the biomechanics of concussion through on-field data collection, and applying that data to helmet evaluation and concussion prevention strategies. Laboratory reconstructions of on-field impacts and head impact sensors used to instrument athletes are the most commonly used methods to characterize concussions in sports, but both methods have unique challenges. Head kinematics can be accurately measured with laboratory reconstructions, but determining impact conditions from video and using dummies to recreate impacts introduces error. Funk et al. showed that even using complex methods for video analysis to determine impact conditions, and closely matching conditions through video and iterative reconstruction testing, there were still substantial errors in helmet kinematics.9 These findings support the need for accurate wearable sensors to better understand concussion biomechanics. A variety of sensor types are available, but even if a sensor performs well in laboratory tests, it may not be accurate with on-field use.22 This is especially concerning since head impact sensor data is often taken at face value without using video to verify that all recorded impacts were true positives.20 To provide an objective evaluation of both laboratory and on-field performance of head impact sensors, Kieffer et al. developed a two-phase approach to sensor testing.13 The first phase used a pendulum impactor to test laboratory performance, followed by the second phase of on-field testing from video verification of impacts if the sensor performed adequately in the laboratory. A custom-fit mouthguard sensor had high accuracy in both laboratory and on-field testing. Other studies also confirmed the accuracy and usability of some available mouthguard sensors.5,10,15

Data collected from instrumented athletes have been applied to strategies to reduce concussion incidence. A review of regulations and interventions targeted at reducing concussion rates in football showed mixed results in terms of reductions in head impact exposure.21 These findings support the need for making data-driven decisions in concussion interventions. Maerlender et al. evaluated the safety and effectiveness of teaching athletes proper tackling technique through a rating system.16 The ratings showed that proper technique was both safe (less chance of head contact) and effective (fewer yards after contact), demonstrating that interventions to reduce concussions do not have to reduce player skill and performance. Rowson et al. developed a method for determining differences in individual concussion risk from real-world head impact data.26 Concussion risk varies with different physical characteristics, so a more individualized risk assessment could be used to inform new concussion reduction strategies. In the future, concussion biomechanics may also play a role in predicting clinical recovery, but the complex nature of concussion symptoms and variability between athletes make it challenging to determine any associations.19

On-field head impact data has also been used to develop helmet evaluation protocols to drive improvements in helmet design. In the Concussion Biomechanics in Football special issue, Bailey et al. published new evaluation methods for football helmets based on video analysis and laboratory reconstructions of National Football League (NFL) concussive impacts.3,4 When comparing laboratory performance with these evaluation methods to concussion rates in NFL players by helmet type, helmets that reduced impact severity in the lab also had lower concussion rates on the field.2 These findings suggest that helmets designed to perform better in the lab can also reduce real-world concussion rates. Further analysis of concussive NFL impacts showed differences in impact conditions by player position, and unique features of helmet-to-ground impacts that could be used for more specific helmet evaluation and design.12,14

This special issue on concussions in sports will build upon the previous issues by including studies on concussion biomechanics in a variety of sports and recreational activities. These studies are focused on real-world concussion biomechanics, helmet evaluation, laboratory reconstruction of impacts, head injury prediction metrics, and new head impact sensor technologies. By including all sports, the research presented here will have a broad impact on head injury mitigation strategies for different populations, and can provide a basis for future advancements in the field.