Conventional wisdom and the language in international helmet testing and certifi- cation standards suggest that appropriate helmet fit and retention during an impact are important factors in protecting the helmet wearer from impact-induced injury. This thesis aims to investigate impact-induced injury mechanisms in different helmet fit scenarios through analysis of simulated helmeted impacts with an anthropometric test device (ATD), an array of headform acceleration transducers and neck force/moment transducers, a dual high speed camera system, and helmet-fit force sensors developed in our research group based on Bragg gratings in optical fibre. To quantify fit and track dynamic helmet movement, novel methods were developed using fit force sensors and high speed cameras respectively. The development of these methods are described in this thesis. The application of these tools and existing practices are implemented in simulated helmet impacts.

To simulate impacts, an instrumented headform and flexible neck fall along a linear guide rail onto an anvil. An instrumented Hybrid III headform and neck is fit with a crash helmet and several fit scenarios can be simulated by making context specific adjustments to the helmet position index and/or helmet size. Specifically, 4 fit scenarios were studied:​ a normal, oversized, forward, and backward fit. Impact condition simulate a variety of scenarios, including a low (4 m/s) and high (6 m/s) impact velocity, a flat and angled anvil, as well as head and torso-first impacts. To quantify helmet retention, the movement of the helmet on the head is quantified using post-hoc image analysis. To quantify head and neck injury potential, biomechanical measures based on headform acceleration and neck force/moment are measured. These biomechanical measures, through comparison with established human tolerance curves, can estimate risk of severe life threatening and/or mild diffuse brain injury and osteoligamentous neck injury. Poor helmet fit did not significantly increase risk of skull fracture based on measured linear head acceleration. A backward fit was shown to increase the likelihood of brain injuries in certain torso-first impacts. Neck injury was found to be consistent between fit conditions in all tested impact scenarios. Helmet movement was found to be greatest in the backward fit scenario, with the greatest helmet displacements observed in torso first impacts indicating that in torso impacts more of the head could be exposed for subsequent impacts following a first impact.

In summary, helmets remained effective in mitigating risk of head and neck injury indicating that as long as the helmet is retained on the head during the first impact, it is an effective protection device. Poor fit did affect helmet retention, suggesting that poor fit in some cases could lead to head exposure and increased likelihood of injury in a second subsequent impact. The results in this thesis document trends in biomechanical measures from a laboratory study with several limitations. These results should not be construed to indicate deficiency in the design of the helmets used.