Quantifying Kinematic and Vibrational Responses in Hockey-Style Baseball Helmets

Introduction: : The most common cause of concussions in baseball is foul-tip balls that impact catchers’ masks. Consequently, MLB catchers experience a concussion rate of 3 per 100,000 athletic exposures (AE), with foul tips being the primary cause. A catcher’s mask is typically made of steel or aluminum wire. The wiring design differs between traditional masks, used since the 1800s, and the increasingly popular hockey-style helmets. Hockey-style helmets cover a greater area of the head than the traditional masks and manage impact energy with their shell and interior padding. These helmets are effective in reducing the accelerations catchers experience when hit by the ball. Because of this, the design of hockey-style helmets and masks has not changed much since their initial use in the sport. However, concussions are still experienced by catchers despite the helmets' ability to reduce accelerations to the head. Thus, the goal of this study was to quantify kinematic and vibrational responses to identify potential areas for improvement in hockey-style catchers’ helmets when experiencing direct impacts to their masks.

Materials and

Methods: : This study compared the kinematic and vibrational response of a bare head, a catcher's mask, and a modified mask to baseball impacts. Regulation high school baseballs were launched at 70 mph by a custom pitching machine into a medium NOCSAE headform mounted on a Hybrid III 50th percentile neck and a 16 kg sliding torso mass. The instrumented headform used three linear accelerometers and a triaxial angular rate sensor. Impacts targeted the center of the forehead just above the eyebrow line for all conditions. Three trials per condition, conducted with a ± 3 mph tolerance, were captured by a laser speed gate. Masks were modified with materials offering potential dampening qualities. As such, seven impact conditions were evaluated: one bare head impact as a control, one unmodified mask impact, and five modified mask impacts with different dampening materials (labeled A-E) for each condition. For the padded impacts, a floating mask proof-of-concept technique was used, and 1.5x1x1-inch blocks of material were attached between the floating mask and helmet. Three blocks of material were placed on the helmet’s forehead for the direct impact of the baseball/mask to the helmet, and one block was placed at the chin of the helmet. Only x-acceleration was analyzed as it captures most frontal impact signals. Acceleration measurements were compared in the time domain as peak accelerations, and in the frequency domain as percentage change in the area under the curve. The integral was evaluated between 0-250 Hz and 250-2000 Hz.

Results, Conclusions, and Discussions:: The bare head impacts showed high oscillatory responses in the time domain around 1000 Hz. With accelerations reaching greater than 350 g’s, only one test was conducted with the bare head impact at 70 mph. In this impact, a peak x acceleration of 528 g’s was measured (Figure 2), suggesting risk of a skull fracture. With the standard hockey-style helmet, the peak x-acceleration was reduced to 28.5 g (Figure 3); however, an oscillatory response was still present. As notable signal content was found in the entirety of the 0-2000 Hz range, distinctions were made above and below 250 Hz. Signal content above 250 Hz decreased 93.2-97.5% in the helmet configurations compared to the bare head. However, signal content below 250 Hz only had a decreased signal content by 36.6-55.5%. With the floating mask and dampening materials, the following observations were made compared to the unaltered helmet. Peak x-accelerations were increased by materials A, B, and C, and decreased by materials D and E. Though, all impacts did remain within a 7.4 g range. Also, all materials decreased signal content by 57.5-63.5% more than the unaltered helmet above 250 Hz. However, all materials increased signal content by 17.1-42.4% compared to the unaltered helmet below 250 Hz. Overall, there appears to be tradeoffs in changing the mask design. Though headform vibration can be mitigated in higher frequencies through altering the design, it is possible that the addition of certain dampening materials may increase peak accelerations as well as increase signal content in lower frequency ranges.

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