A Test Bed to Examine Helmet Fit and Retention and Biomechanical Measures of Head and Neck Injury in Simulated Impact

Summary

Using an anthropometric head and neck, optical fiber-based fit force transducers, an array of head acceleration and neck force/moment transducers, and a dual high speed camera system, we present a test bed to study helmet retention and effects on biomechanical measures of head and neck injury secondary to head impact.

Abstract

Conventional wisdom and the language in international helmet testing and certification standards suggest that appropriate helmet fit and retention during an impact are important factors in protecting the helmet wearer from impact-induced injury. This manuscript 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 fiber. To simulate impacts, an instrumented headform and flexible neck fall along a linear guide rail onto an anvil. The test bed allows simulation of head impact at speeds up to 8.3 m/s, onto impact surfaces that are both flat and angled. The headform 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. 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 the risk of severe life threatening and/or mild diffuse brain injury and osteoligamentous neck injury. To our knowledge, the presented test-bed is the first developed specifically to assess biomechanical effects on head and neck injury relative to helmet fit and retention.

Introduction

Most epidemiological evidence suggests bicycle helmets provide protection against head injuries for cyclists of all ages¹. The biomechanical literature presents the consistent theme that the helmeted head sustains relatively less severe head/brain injuries secondary to impact, relative to the unprotected (un-helmeted) head². Some research suggests that poor helmet fit is associated with an increased risk of head injury³, implying that helmets are most effective when fit properly. Depending on the criteria used to define good helmet fit, incorrect helmet use was found to be as high as 64% among hel....

Protocol

1. Helmet Fit Scenarios Arrangement

1. Define fit scenarios to be studied on an anthropometric test device head and neck (Hybrid III 50th percentile male) with a head circumference of 575 mm.
   NOTE: An example of four fit scenarios is shown in Table 1 with helmet positions corresponding to Figure 1. The forward and backward fit scenarios were based on definitions of correct helmet use from previous epidemiological studies, which specified proper helmet position as not covering the eyebrows or exposing the forehead³.
2. For each scenario, mark each helmet position on the headfo....

Results

Fit Force Measurement
For each fit scenario, fit force measurement was performed at each sensor location (Figure 12) and a t-test, assuming unequal variances, was performed to determine significance (p < 0.05). The average standard deviation across all measurements was ± 0.14 N. Higher fit forces indicate a tighter fit.

Head Kinematic and Neck Kinetic Data

Discussion

Here, methods for investigating helmet fit in simulated helmeted head impacts are presented. Helmet fit was quantified with fit force sensors, impacts were simulated with an ATD headform and neck on a guided drop tower, and helmet movement was tracked with high speed video. Different impact scenarios were simulated under different fit scenarios to investigate the effects on biomechanical measures of helmet fit.

The helmet fit sensors are capable of distinguishing differences in fit forces betw.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Hybrid III Headform	Humanetics or Jasti-Utama	N/A	50th Percentile ATD, for impact simulation
Hybrid III Neck	Humanetics or Jasti-Utama	N/A	50th Percentile ATD, for impact simulation
Linear Accelerometers	Measurement Specialties	64C-2000-360	for head acceleration measurement
Upper Neck Load Cell	mg Sensor	N6ALB11A	for neck load measurement
High Speed Camera	Vision Research	v611	for motion capture
Camera Lens	Carl Zeiss	N/A	50 mm f1/.4, for motion capture
Camera Lens	Carl Zeiss	N/A	100 mm f/2.0, for motion capture
Bicycle Helmet	Bell	N/A	Traverse
Data Acquisition System	National Instruments	PXI 6251	for Hybrid III signal acquisition
Head Impact Drop Tower	University of Alberta	N/A	Custom-designed, for impact simulation
Optical Interrogator	Smart Fibres Ltd.	N/A	SmartScan, for optical sensor force measurement
Fit Force Sensor	University of Alberta	N/A	Custom-designed, for measuring helmet fit forces