Temperature-Dependent Impact Dynamics of Padding Materials in American Football Helmets

Introduction: : Football helmets are critical in reducing the risk of head injuries during high-impact collisions in contact sports. Effectiveness of these helmets largely depends on the mechanical properties of the padding, which is engineered to absorb impact energy. However, the performance of padding materials may be affected by environmental factors, particularly temperature. An understanding of the temperature-dependent behavior of helmet padding is critical for optimizing design and maintaining protection across a range of environmental condition. Athletes compete in various weather conditions, from freezing winters games to hot summer practice days [1], yet limited research has evaluated the effect of temperature on the protective performance of helmet padding. Different helmet manufacturers utilize diverse rate-dependent materials, including specialized energy absorber foams, 3D-printed elastomeric polyurethane (EPU) or thermoplastic polyurethane (TPU). Each material may exhibit temperature-dependent mechanical behavior, potentially altering impact response and safety. Given that nearly 15% of NFL games are played below 4.4°C and 4% above 27°C [1], understanding how temperature influences impact attenuation is crucial. Previous epidemiological studies have reported an increased incidence of concussions during colder weather [1-3], suggesting that low temperatures may compromise the protective function of helmet padding. Therefore, this study investigates the dynamic response of six modern football helmet padding materials under controlled temperature conditions using a drop tower setup. We assess impact-related metrics such as peak acceleration, transmitted force, deformation, and impact duration at five distinct cold and warm temperature conditions. This study aims to supports helmet design optimization and enhance player safety across varying environmental conditions.

Materials and

Methods: : Six padding samples were extracted from top-rated football helmets listed in the NFL’s 2024 published helmet rankings [4]. The tested padding materials included, from left to right in Figure 1: Vicis Zero 2 (two front cubical shell units), Riddell Diamond (rear 3D-printed lattice), Light Gladiator (side shock absorber), Riddell Axiom (crown padding), Xenith Shadow XR (Rheon-based unit), and Riddell SpeedFlex (side padding unit).
Impact testing was conducted using a twin-wire drop tower (Cadex, Inc.; St. Jean-sur-Richelieu, Canada), where a 4.9 kg mass was dropped from a height of 30 cm, producing an impact velocity of 2.4 m/s. The drop mass was instrumented with a tri-axis accelerometer (PCB Piezotronics Model 356B21). A 10 kN custom load cell (PCB Piezotronics Model 201B05) consisting of three force rings was placed beneath the padding to record transmitted force. A laser displacement sensor (Micro-Epsilon ILD1900-500) measured sample deformation.
Samples were conditioned at five temperatures: −10°C, 5°C, 17°C (room temperature), 28°C, and 38°C. Conditioning was achieved using a refrigerator and an oven (Quincy Lab, Inc.), by maintaining padding samples for 24 hours before testing at target temperature. Each sample underwent three successive impacts at each temperature.
Acceleration and force signals were filtered using a Butterworth fourth-order (1650 Hz) and second-order (300 Hz) filter, respectively. Key parameters—peak acceleration, peak transmitted force, maximum strain, and impact duration—were averaged across tests. Impact duration was defined as the interval between local minima before and after the acceleration peak, each falling below 30% of the peak value [5].

Results, Conclusions, and Discussions:: Temperature was found to have a substantial effect on the dynamic response of modern football helmet padding materials. Figure 2a presents representative acceleration traces for the Axiom padding sample across five temperature conditions. At lower temperatures (−10°C and 5°C), acceleration profiles displayed sharper peaks and shorter durations, indicating stiffer material behavior, while as temperature increased (28°C and 38°C), the traces showed lower peak values and longer durations, consistent with softer, more compliant responses. Impactor force-displacement curves (Figure 2b) further supported this pattern: cold conditions produced higher peak forces and lower deformation, while warm conditions showed reduced forces and increased displacement.
Figure 3 summarizes the temperature-dependent changes across all six tested paddings. Compared to room temperature (17°C), peak acceleration increased by 29–252% at −10°C and 6–115% at 5°C, but decreased by up to 30% at 28°C and 52% at 38°C. A similar trend was observed in peak transmitted force, which rose by 22–239% at −10°C and 5–111% at 5°C, but declined up to 33% and 58% at the higher temperatures. Peak strain decreased by 14–37% and 1–25% in cold conditions but increased up to 32% and 80% at 28°C and 38°C, respectively. Impact duration followed the same trend, becoming shorter in the cold, up to 52% reduction, and longer in the heat, up to 84% increase.
These results suggest that colder temperatures increase material stiffness, leading to greater transmitted forces and peak acceleration, while reduced strain and impact duration, factors known to elevate concussion risk, aligning with findings showing increased concussion rates in cold environments [1-3]. Conversely, while warmer temperatures may improve energy absorption by increasing material compliance, they raise the risk of bottoming out during high-energy impacts, potentially compromising protection. Approximately 15% of NFL games are played below 4.4°C, while 42% occur above 16°C [1], emphasizing the broad range of thermal conditions under which helmets must perform. These findings highlight the need for helmet manufacturers and regulatory bodies to incorporate temperature effects into design, certification, and testing protocols. Materials engineered for thermal stability could improve performance consistency and player safety across varied playing conditions.

Acknowledgements and/or References (Optional):: Acknowledgement: Funded by UTSA-SwRI CONNECT grant program.

References:
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