Striking Concerns: Reduced Protection in Older Kendo Helmets

Background: Protective headgear in many sports, such as cycling and skiing, is subject to clear recommendations regarding replacement intervals due to material degradation over time. In contrast, kendo helmets (men) lack evidence-based guidance on service life, despite repeated exposure to high-energy impacts during regular practice. In our prior study, we noted that new kendo helmets of different stitch patterns absorb kinetic energies differently. Objective: To evaluate the effectiveness of older kendo helmets compared to new helmets in mitigating impact forces that could lead to sports-related concussions in kendo. Methods: We collected data from seven kendo practitioners and collected over 1,900 strikes using a bamboo sword (shinai) on the same sensor-equipped mannequin from our prior study on five helmets over five years old with a 6-mm stitch pattern, tested under controlled conditions simulating realistic impacts encountered in kendo practice, using the same protocol as our prior study. Results: Older helmets consistently demonstrated lower energy absorption, exhibiting statistically significant higher mean g-forces, with a 95% confidence interval (CI), compared to new helmets with different stitch patterns (2 mm, 6 mm, and 9 mm) from our prior study (p < 0.001). Older helmets recorded an average of 17.9 g, compared with prior study values of 13.5 g for 9-mm helmets, 13.9 g for 6-mm helmets, and 14.7 g for 2-mm helmets (p < 0.001). This corresponds to respective decreases in energy absorption of 32.8%, 29.0%, and 22.0%. A comparison of the worst-performing old helmet (Old Helmet 3) with the best-performing new helmet (9-mm stitching) showed a reduction of 36.7% (18.5 g vs. 13.5 g). Conclusions: These findings indicate that older kendo helmets provide substantially reduced protection (22%-36.7%) compared to new helmets. Given the frequency of head impacts in kendo and the reduced energy absorption of older helmets, continued use of aged equipment may pose a safety risk. The results highlight the need for evidence-based recommendations regarding helmet replacement intervals in kendo to enhance practitioner safety.