Angular acceleration of the head has long been recognized as a biomechanical mechanism for traumatic brain injury. In sporting events as well as during bicycle and motorcycle accidents, this angular acceleration may occur with or without direct contact against a rigid surface. The objective of this research was to investigate the effect of combined indirect and direct loading on headform kinematic response and resulting brain injury predictors produced by finite element modeling. This was accomplished by conducting a series of guided free-fall tests that were designed to subject the headform to specific loading conditions—indirect loading (IL), direct loading (DL), and combined indirect/direct loading (IDL). Three different helmet types were selected for testing (bicycle, hockey and football). For the helmet types tested, the IDL condition resulted in significantly higher values of angular acceleration (6600-11,100 rad/s2), maximum principal strain (17.8-31.2), Von Mises stress (9.5-9.8 kPa), and cumulative strain damage measure at 15 % strain (2.7-7.7 %) than IL or DL alone. This trend was consistent across all helmet types and demonstrates the deleterious effects of combined loading on the brain tissue. These results also provide some insight into how seemingly similar impacts can produce substantial differences in brain injury outcome.
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