L2 - Influence of helmet surface protrusions on linear and rotational head kinematics in youth bicycle helmets

Introduction: : According to the US Bicycling Participation Benchmarking Study, 53% of Americans aged 3 to 9 rode a bicycle at least once in 2022 [1]. Concussions are due to impact-induced linear and rotational head accelerations. Helmets designed to minimize these accelerations can reduce concussion risk. Bicycle helmets are comprised of many design elements; however, protrusions are unique features of youth helmets. Protrusions are constructed of thermoplastic rubber (TPR), a deformable material, affixed to the helmet’s exterior. Typically, these protrusions are designed to mimic animal faces or mohawks. A study by Jung et al. noted that the presence of protrusions at the impact location improved the helmet’s ability to reduce peak linear accelerations (PLA) and peak rotational accelerations (PRA) [2]. However, the study had a low sample size of helmets with protrusions and did not directly compare impact locations with and without protrusions. The objective of this study was to compare PLA and PRA at location-matched impacts between helmets with and without protrusions.

Materials and

Methods: : Oblique impact testing was completed using guided drop tests of a helmeted small NOCSAE headform onto a 25-degree anvil with 80-grit sandpaper. This setup simulates falling while traveling forward and contacting pavement. Six youth bicycle helmet models with protrusions were selected. Four samples per helmet model were used; two had protrusions intact and two had the protrusions removed (Figure 1). Each helmet sample was tested at 4.2 m/s in three locations, for a total of twelve tests per model, totaling 72 tests. For each helmet model, impact locations were selected so the protrusion would be the first part of the helmet to contact the anvil. The headform was instrumented with a six-degree-of-freedom sensor package mounted at the headform center of gravity that consisted of three accelerometers and a triaxial angular rate sensor. The impact duration and resultant PLA and PRA were calculated for each test. Linear mixed-effects regression models (LMER) were conducted separately for PLA, PRA, and impact duration using a significance level of α < 0.05. Protrusion presence was the fixed effect, with helmet model and location (nested within helmet model) treated as random effects. Post hoc comparisons were completed using least squares means to compare pairwise differences for protrusions in the LMER models. Means, standard deviations, and 95% confidence intervals (CI) were computed by helmet model and protrusion presence. A previously published youth-specific risk function was used to estimate concussion risk based on PLA and PRA. Cumulative risk for each helmet was computed by summing risk values across impact conditions.

Results, Conclusions, and Discussions:: Across the six youth bicycle helmet models tested, PLA values ranged from 66.8 to 147.4 g with protrusions and from 115.2 to 194.1 g without protrusions, while PRA values ranged from 1991 to 7045 rad/s2 with protrusions and 1146 to 7917 rad/s2 without protrusions (Figure 2). When looking at each model, adding protrusions to the helmet made it perform equal to or better than it did without protrusions. Presence of protrusions (p < 0.001) had strong effects on PLA, PRA, and impact duration. Protrusions decreased PLA by 45.6 g (CI 39.5 – 51.6 g) and PRA by 1079 rad/s2 (CI 663 – 1495 rad/s2) and increased impact duration by 5.0 ms (CI 4.5 – 5.6 ms). Reductions in PLA and PRA from helmet samples without and with protrusions varied between helmet models (Figure 2). The Raskullz Stego reduced PLA the most (-53.4 g) while the Raskullz Unicorn reduced PLA the least (-26.3 g). The Raskullz Bolt reduced PRA the most (-1385 rad/s2) while the Raskullz Unicorn reduced PRA the least (-458 rad/s2). Percent change of concussion risk from helmet samples without and with protrusions also varied between helmet models (Figure 3). The Raskullz Llama had the least percent change in concussion risk (-29.8%) while the Raskullz Bolt had the greatest percent change in concussion risk (-61.9%). One limitation of this study is that it did not quantify different design characteristics of the protrusions, such as depth off helmet shell, surface area, and amount of protrusion deformation during impacts, and relate them back to PLA and PRA performance. However, this study indicates that protrusions play a beneficial role in reducing peak head kinematics experienced by youth bicyclists. Additionally, there is no indication that helmets designed with protrusions similar to those tested in this study will increase concussion risk among youth bicyclists.

Acknowledgements and/or References (Optional):: Sources
[1] “2022 U.S. Bicycling Participation Study: A Survey of U.S. Residents.” Accessed: Mar. 31, 2025. [Online]. Available: https://resources.peopleforbikes.org/product/biennial-pfb-bicycling-participation-report
[2] C. Jung, N. E.-P. Stark, S. M. Gagliardi, M. T. Begonia, and S. Rowson, “Quantifying Effects of Design Features on Youth Bicycle Helmet Performance During Oblique Impacts,” Ann. Biomed. Eng., Apr. 2025, doi: 10.1007/s10439-025-03730-1.