Helmet accommodation analysis using 3D laser scanning

A parametric head geometry model accounting for variation among adolescent and young adult populations

BACKGROUND AND OBJECTIVE

Modeling the size and shape of human skull and scalp is essential for head injury assessment, design of helmets and head-borne equipment, and many other safety applications. Finite element (FE) head models are important tools to assess injury risks and design personal protective equipment. However, current FE head models are mainly developed based on the midsize male, failing to account for the significant morphological variation that exists in the skull and brain. The objective of this study was to develop a statistical head geometry model that accounts for size and shape variations among the adolescent and young adult population.

METHODS

To represent subject-specific geometry using a homologous mesh, threshold-based segmentation of head CT scans of 101 subjects between 14 and 25 years of age was performed, followed by landmarking, mesh morphing, and projection. Skull and scalp statistical geometry models were then developed as functions of age, sex, stature, BMI, head length, head breadth, and tragion-to-top of head using generalized Procrustes analysis (GPA), principal component analysis (PCA) and multivariate regression analysis.

RESULTS

The statistical geometry models account for a high percentage of morphological variations in scalp geometry (R²=0.63), outer skull geometry (R²=0.66), inner skull geometry (R²=0.55), and skull thickness (error < 1 mm) CONCLUSIONS: Skull and scalp statistical geometry models accounts for size and shape variations among the adolescent and young adult population were developed as functions of subject covariates. These models may serve as the geometric basis to develop individualized head FE models for injury assessment and design of head-borne equipment.

A review of the methodology and applications of anthropometry in ergonomics and product design

Anthropometry is a key element of ergonomic studies for addressing the problem of fitting the tasks/products to user characteristics, but there is a gap between anthropometric data and their application for designing ergonomic products and environments. This research was conducted to review the literature on the methodology and applications of anthropometry for the ergonomic design of products and environments, and to identify where further research is needed to improve its application and evaluation protocols. One hundred and sixteen papers meeting the inclusion criteria were reviewed. Although a number of anthropometric investigations have been conducted to improve the design of products/environments for different users, further research seems to be necessary, particularly for special groups, such as children, the elderly and people with disabilities. Different anthropometric measurement methods/techniques and fitting criteria are discussed regarding their applicability for various design applications. This review also highlights methodological issues (sampling considerations and prototype evaluation and testing) that should be considered in future research to ensure a user-centred approach of the design process. Practitioner Summary: A literature review was conducted on the methodology and applications of anthropometry for the ergonomic design of products/environments. This review emphasises the need for anthropometric research to design for special groups, such as children, the elderly and people with disabilities, and methodological issues that should be considered in future research. Abbreviations: 1D: one-dimensional; 2D: two-dimensional; 3D: three-dimensional; HF/E: Human Factors/Ergonomics; PCA: Principal Components Analysis; CA: Cluster Analysis; DHM: Digital Human Modelling.

Fit, stability and comfort assessment of custom-fitted bicycle helmet inner liner designs, based on 3D anthropometric data

Research has demonstrated that a better-fitted bicycle helmet offers improved protection to the rider during an impact. Nowadays, bicycle helmets in the market that range in size from small/medium to medium/large might not fit the diverse range of human head shapes and dimensions. 3D scanning was used to create 3D head shape databases of 20 participants who volunteered for the study. We developed new custom-fitted helmet inner liners, based on the 3D head shape of two sub-groups of participants, to map their head sizes and contours closely to the conventional Medium (M) and Large (L) sizes as described in from AS/NZS 2512.1: 2009. The new custom-fitted helmet was compared with the helmet available in the market place in a dynamics stability test and from participants' subjective feedback. A significant reduction in the angle of helmet rotation on the headform in the lateral direction was recorded for the custom-fitted helmet. A Wilcoxon signed-rank test was conducted to evaluate participants' feedback on the helmets according to different area definitions. The overall fit and comfort and the top region of the new helmet were significantly improved. However, no difference was found at the significant level of 0.05 for the front and rear region of the new helmet.

Using subject-specific three-dimensional (3D) anthropometry data in digital human modelling: case study in hand motion simulation

Digital human modelling enables ergonomists and designers to consider ergonomic concerns and design alternatives in a timely and cost-efficient manner in the early stages of design. However, the reliability of the simulation could be limited due to the percentile-based approach used in constructing the digital human model. To enhance the accuracy of the size and shape of the models, we proposed a framework to generate digital human models using three-dimensional (3D) anthropometric data. The 3D scan data from specific subjects' hands were segmented based on the estimated centres of rotation. The segments were then driven in forward kinematics to perform several functional postures. The constructed hand models were then verified, thereby validating the feasibility of the framework. The proposed framework helps generate accurate subject-specific digital human models, which can be utilised to guide product design and workspace arrangement. Practitioner Summary: Subject-specific digital human models can be constructed under the proposed framework based on three-dimensional (3D) anthropometry. This approach enables more reliable digital human simulation to guide product design and workspace arrangement.

The Helmet Fit Index--An intelligent tool for fit assessment and design customisation

Helmet safety benefits are reduced if the headgear is poorly fitted on the wearer's head. At present, there are no industry standards available to assess objectively how a specific protective helmet fits a particular person. A proper fit is typically defined as a small and uniform distance between the helmet liner and the wearer's head shape, with a broad coverage of the head area. This paper presents a novel method to investigate and compare fitting accuracy of helmets based on 3D anthropometry, reverse engineering techniques and computational analysis. The Helmet Fit Index (HFI) that provides a fit score on a scale from 0 (excessively poor fit) to 100 (perfect fit) was compared with subjective fit assessments of surveyed cyclists. Results in this study showed that quantitative (HFI) and qualitative (participants' feelings) data were related when comparing three commercially available bicycle helmets. Findings also demonstrated that females and Asian people have lower fit scores than males and Caucasians, respectively. The HFI could provide detailed understanding of helmet efficiency regarding fit and could be used during helmet design and development phases.

Evaluation of an anthropometric shape model of the human scalp

This paper presents the evaluation a 3D shape model of the human head. A statistical shape model of the head is created from a set of 100 MRI scans. The ability of the shape model to predict new head shapes is evaluated by considering the prediction error distributions. The effect of using intuitive anthropometric measurements as parameters is examined and the sensitivity to measurement errors is determined. Using all anthropometric measurements, the average prediction error is 1.60 ± 0.36 mm, which shows the feasibility of the new parameters. The most sensitive measurement is the ear height, the least sensitive is the arc length. Finally, two applications of the anthropometric shape model are considered: the study of the male and female population and the design of a brain-computer interface headset. The results show that an anthropometric shape model can be a valuable tool for both research and design.

Factors affecting motorcycle helmet use: size selection, stability, and position

OBJECTIVES

One of the main requirements of a protective helmet is to provide and maintain appropriate and adequate coverage to the head. A helmet that is poorly fitted or fastened may become displaced during normal use or even ejected during a crash.

METHODS

Observations and measurements of head dimensions, helmet position, adjustment, and stability were made on 216 motorcyclists. Helmet details were recorded. Participants completed a questionnaire on helmet usability and their riding history. Helmet stability was assessed quasistatically.

RESULTS

Differences between the dimensions of ISO headforms and equivalent sized motorcyclists' heads were observed, especially head width. Almost all (94%) of the helmets were labeled to be compliant with AS/NZS 1698 (2006). The majority of riders were satisfied with the comfort, fit, and usability aspects of their helmets. The majority of helmets were deemed to have been worn correctly. Using quasistatic pull tests, it was found that helmet type (open-face or full-face) and the wearing correctness were among factors that affected the loads at which helmets became displaced. The forces required to displace the helmet were low, around 25 N.

CONCLUSIONS

The size of the in-use motorcycle helmets did not correspond well to the predicted size based on head dimensions, although motorcyclists were generally satisfied with comfort and fit. The in vivo stability tests appear to overpredict that helmets will come off in a crash, based on the measured forces, tangential forces measured in the oblique impact tests, and the actual rate of helmet ejection.

Bicycle helmet size, adjustment, and stability

OBJECTIVES

One of the main requirements of a protective bicycle helmet is to provide and maintain adequate coverage to the head. A poorly fitting or fastened helmet may be displaced during normal use or even ejected during a crash. The aims of the current study were to identify factors that influence the size of helmet worn, identify factors that influence helmet position and adjustment, and examine the effects of helmet size worn and adjustment on helmet stability.

METHODS

Recreational and commuter cyclists in Sydney were surveyed to determine how helmet size and/or adjustment affected helmet stability in the real world. Anthropometric characteristics of the head were measured and, to assess helmet stability, a test analogous to the requirements of the Australian bicycle helmet standard was undertaken.

RESULTS

Two hundred sixty-seven cyclists were recruited across all age groups and 91% wore an AS/NZS 2063-compliant helmet. The main ethnic group was Europeans (71%) followed by Asians (18%). The circumferences of the cyclists' heads matched well the circumference of the relevant ISO headform for the chosen helmet size, but the head shapes differed with respect to ISO headforms. Age and gender were associated with wearing an incorrectly sized helmet and helmet adjustment. Older males (>55 years) were most likely to wear an incorrectly sized helmet. Adult males in the 35-54 year age group were most likely to wear a correctly adjusted helmet. Using quasistatic helmet stability tests, it was found that the correctness of adjustment, rather than size, head dimensions, or shape, significantly affected helmet stability in all test directions.

CONCLUSIONS

Bicycle helmets worn by recreational and commuter cyclists are often the wrong size and are often worn and adjusted incorrectly, especially in children and young people. Cyclists need to be encouraged to adjust their helmets correctly. Current headforms used in standards testing may not be representative of cyclists' head shapes. This may create challenges to helmet suppliers if on one hand they optimize the helmet to meet tests on ISO-related headforms while on the other seeking to offer greater range of sizes.

A comparison between Chinese and Caucasian head shapes

Univariate anthropometric data have long documented a difference in head shape proportion between Chinese and Caucasian populations. This difference has made it impossible to create eyewear, helmets and facemasks that fit both groups well. However, it has been unknown to what extend and precisely how the two populations differ from each other in form. In this study, we applied geometric morphometrics to dense surface data to quantify and characterize the shape differences using a large data set from two recent 3D anthropometric surveys, one in North America and Europe, and one in China. The comparison showed the significant variations between head shapes of the two groups and results demonstrated that Chinese heads were rounder than Caucasian counterparts, with a flatter back and forehead. The quantitative measurements and analyses of these shape differences may be applied in many fields, including anthropometrics, product design, cranial surgery and cranial therapy.

Resolution Influence on 3D Anthropometric Data Clustering for Fitting Design

Sizing based on 3D anthropometric data may lead to significant improvement in fitting comfort of wearing products. However, the required computational load is a common problem in 3D data processing. In a previous study, wavelet analysis was adopted to establish a multi-resolution description of 3D anthropometric data to reduce computational load and modeling complexity. K-means clustering was subsequently performed on the decomposed 3D samples. This study further examines the influence of decomposition level on clustering results. As a case study, 378 face samples, 447 head samples and 432 upper head samples were analyzed. Cluster membership variation on five different resolution levels was examined by using Cluster Membership Accuracy Rate (CMAR), which denotes the clustering consistency on the decomposed levels compared with the clustering results on the original data sets. For the face data sets, the CMAR values on the five decomposition levels are 100, 99.21, 97.88, 93.92 and 93.39%, respectively; for upper heads, the CMAR values are 99.3, 99.1, 98.4, 92.1 and 84.3%, respectively; while for whole heads, the CMAR values are 99.3, 98.2, 95.1, 85.5 and 77.9%, respectively. These results indicate that clustering on the third decomposition level is proper for face and head scans in reducing computational load while maintaining at least 95% clustering accuracy.

Multi-resolution description of three-dimensional anthropometric data for design simplification

Three-dimensional (3D) anthropometry can provide rich information for ergonomic product design with better safety and health considerations. To reduce computational load and model complexity in product design when using 3D anthropometric data, wavelet analysis is adopted in this paper to establish multi-resolution mathematical description of 3D anthropometric data. A proper resolution can be selected for design reference according to the application purpose. To examine the approximation errors under difference resolutions, 510 upper head, whole head, and face samples of Chinese young men have been analyzed. Descriptives of approximation errors under different resolutions are presented. These data can be used as resolution selection guide. The application of the multi-resolution method in product design is illustrated by two examples. RELEVANCE TO INDUSTRY: Multi-resolution description of 3D anthropometric data would facilitate the analysis of and design with 3D anthropometric data to improve fitting comfort. The error data under different resolutions provide important reference for resolution selection.

Multi-resolution shape description and clustering of three-dimensional head data

The purpose of this study was to develop a 3-D anthropometric sizing method based on a clustering algorithm combined with a multi-resolution description and demonstrate the method with 3-D head data. Wavelet decomposition was adopted to provide flexible descriptions of 3-D shapes on different resolution levels. A block-division technique was then proposed to divide each decomposed 3-D surface into a predefined number of blocks. Afterwards, by using the block-distance metric, each decomposed surface was converted into a block-distance vector. Not only the size information but also the geometric information of the 3-D surfaces are contained in the vector. Finally, k-means clustering was performed on the vectors to segment the sample population into several groups. A total of 378 3-D upper head and face samples were analysed to illustrate the applicability of the method. Clustering was validated by using two measures, size-weighted variances and Clustering Validity Index. K-means clusterings of different variables were compared, including head length and head breadth, the top five principal components from principal component analysis (PCA) on the proposed block distance-based vectors and the block distance-based vectors directly. No obvious difference was found between clustering on the vectors with and without PCA. Lower values of the two measures when clustering on the block distance-based vectors indicated that the proposed block distance-based descriptor is superior to the traditional sizing dimensions of head length and head breadth. Unlike the traditional sizing methods based on key dimensions or derived variables, the method proposed in this study is based on the 3-D shape of the body surface. The proposed block-distance vector reflects not only the overall size but also the local spatial geometric features of a 3-D surface. The new method can be expected to improve the ergonomic design of those products requiring close fitting, such as face shields, goggles and helmets.