MATERIALS AND METHODS

The 3D Taiwanese bodybank is used for the development of 3D digital models. The bodybank were stratified into 15 strata. For each stratum, a representative subject was selected as the based model for 3D human digital model. In all, a set of 15 digital models were developed for each gender.

The digital model was constructed first by matching a stick-skeleton into the base model. The stick-skeleton was a simplified multi-linkage structure resembling our skeletal system. Each link is a representation of a segment and is defined by its proximal and distal nodes.

Secondly, the surface point coordinates of each segment was established as a function of its two nodes and two nearby nodes, so that if the stick-skeleton was moved, the stick-skeleton would brought about the movement of all surface points of that segment.

Finally, the digital models were manipulated to match real scanned data to evaluate the goodness of fit.

The base model: The 3D bodybank, consists of 270 subjects, 135 males and 135 females which were drawn from Taiwanese worker’s population, aged between 18 and 64. The distribution was stratified on 5 stature heights and 3 body weights (specified in BMI; body mass index) as listed in Table 1 (Department of Health, 2002). The male has an average height of 167.1±6.26 cm and BMI of 25.9±4.75 and the female has an average height 155.4±5.22 cm and BMI of 25.3±4.62. BMI was used instead of weight because it could be more representative than weight (Bahram and Shafizadeh, 2006; Gholamreza and Mohsen, 2007; Veghari and Golalipour, 2007; Hassan et al., 2008; Abu-Samak et al., 2008; Nemati et al., 2008; Kanaani et al., 2010; Hassanzadeh et al., 2011). The analyses of variances indicated that the Taiwanese 3D bodybank was not significantly differed from the previous survey (Yu et al., 2008).

The 3D bodybank was measured by ITRI Gemini 3D body scanner and a high resolution hand-foot scanner as illustrated in Fig. 1 (Chang et al., 2005). The precision and accuracy is within 1.0 mm (Yu et al., 2008, 2010). Each subject was measured in 9 postures: 1 standing posture and 8 different postures shown as Fig. 2. The standing posture was used as the base model for the construction of digital models and the rest 8 postures were used for evaluation purpose.

For each stratum, a representative subject was selected as the base model, with the criterion that the representative subject had overall height and body weight the most close to 50%.

Fig. 1(a-b):	Gemini 3D body scanner (a) and a high resolution hand-foot scanner, (b) (Chang et al., 2005)

Fig. 2:	Nine postures scanned of subject

Table 1:	The anthropometric distribution of the 3D bodybank is stratified on 5 stature heights and 3 body weights (BMI) of the male, and the female
Male; Male sub.-total, N = 135, Height range = 149.0-~185.5 cm, Avg. = 167.1 cm, SD = 6.26 cm, BMI range = 17.3~29.6, Avg. = 25.9, SD = 4.75. Female; female sub.-total N=135, Height range = 143.0-~168.5 cm, Avg. = 155.4 cm, SD = 5.22 cm, BMI range = 17.2~29.4, Avg. = 25.3, SD = 4.62

Fig. 3:	One female base models of the M-medium size

Here, for simplicity only 1 female model of the M-Medium size are illustrated here (Fig. 3). It is noted that the base models has no holes on them, they had been carefully hole-filling by template-matching technique (Wu, 2005).

The matching of stick-skeleton into the base model: The stick-skeleton consists of a total of 35 links (segments) which are organized in multi-linkage system with 29 nodes, 7 vertex (end nodes) and in addition 11 rigid bodies, shown as Fig. 4a. The rigid bodies are the segments which are not deformed during body movement, for example, the head and the pelvis.

Like human skeleton, the stick-skeleton is organized in 5 paths: one axial path attached with 2 leg paths and 2 arm paths (Fig. 4b). The axial path originates at pelvis, progresses upward and ends at the cortex of the head. The pelvis is designated as mother node, because it is both the base of the axial and near the mass center of the body. The leg path also originates at pelvis, progresses downward to the foot. The arm path originates at T1 node of the axial path and progresses downward to the hand.

This multi-linkage system is defined in spherical coordinate system. The spherical coordinate system consists of 4 variables (L, θ, ψ, α): L designated as link-length, θ as the including angle between the projection of the link with X-axis, ψ the azimuth angle of the link (to XY plan) and α the rotation angle between the distal and the proximal coordinates (as axial rotation of the link). Then the stick-skeleton was superimposed on the base model and fine tuned by dragging its nodes to register with corresponding rotation center of the base model (Fig. 5).

Fig. 4(a-b):	(a) The stick skeletal system of digital model and (b) The matched model

Fig. 5:	The surface points in a segment are assigned as the function of the 2 nodes (elbow and wrist) of its correspondent link and/or two other nearby nodes (shoulder and hand)

First, several key joint rotation centers of the base model, such as pelvis, shoulder, elbow, wrist, hip, knee and ankle joints were visually identified on monitor. These joints serve as control points for scaling the stick-skeleton.

The surface points as a function of the stick-skeleton: Next, the relationship between the surface points of the base model and the stick-skeleton was established. The 3D coordinates of each surface point in a segment are assigned as the function of the 2 nodes of its correspondent link and/or two other near by nodes. For example, the 3D coordinates of any surface point Pi on the lower arm are related to the coordinates of the elbow node and wrist node of the lower arm link and in addition to the shoulder node (proximal to elbow node) and the hand node (distal to the wrist node) (Fig. 5). The purpose of these two nearby nodes are used to secure the deformation of surface in the joint region is as smooth and real as possible. Therefore, when the lower arm link was moved in space would bring about the movement of the surface points of the lower arm segment.

Evaluation: Upon the completion of the digital model, the goodness of these digital models was evaluated. The digital model was manipulated into the postures to match the other 8 scanned postures as illustrated in Fig. 2. The manipulated posture was then superimposed onto the scanned postures. The mean and maximal of mismatching error between the two was computed for each posture.

The results of mean and maximal mismatching errors of the 8 postures of the M-Medium male and female were tabulated first and the summary statistics of the minimum, mean and maximum were also calculated. Then likewise, the summary statistics of the 15 males and 15 females were tabulated individually and the overall statistics were also calculated.

RESULTS

The superimposed figures of the digital models and the scan data of the female are presented in Fig. 6. The digital models are colored with purple, while the scan data gold. The purple areas mean that the surface of the digital models are systematically protruded out of the scan data, while the gold areas, on the other hand, are sunk below the scan data. The areas randomly dotted with both colors are perfect match areas.

The results of mean and maximal mismatching error of the 8 postures of the M-Medium male and female are tabulated in the left half of Table 2 and the summary statistics in the right half.

Table 2:	The least square mismatching error (mm) between the digital models and the scanned data of the other 8 postures of male and female

For the male, the average mismatching errors range between 2.25 and 14.86 mm (mean 6.74 mm), the maximal mismatching errors range between 16.54 and 29.75 mm (mean 21.84 mm). For the female, the average mismatching errors range between 3.49 and 14.90 mm (mean 7.54 mm), the maximal mismatching errors range between 17.76 and 28.46 mm (mean 21.51 mm).

Like Table 2, the summary and overall statistics of the 15 male digital models are summarized in Table 3 and the females in Table 4. For the males, the overall mean mismatching errors range between 4.32 and 8.61 mm (mean 6.40 mm), while the maximum mismatching errors range between 25.2 to 33.1 mm (mean 28.72 mm). For the female, the overall mean mismatching errors range between 6.74 and 10.82 mm (mean 7.54 mm), while the maximum errors range between 26.3 to 37.7 mm (mean 30.6 mm).

As a summary to these tables, the results of these 15 male and 15 female digital models seem to be promising in general application, in fact, these 8 scanned postures are extreme postures with maximal torso flexion, arm reach and leg squatting.

Table 3:	The mean and maximum mismatching errors (mm) of 15 male digital models
ME: Mismatching error, Max.: Maximum, Min.: Minimum, SD: Standard deviation

Table 4:	The mean and maximum mismatching error (mm) of 15 female digital models
ME: Mismatching error, Max.: Maximum, Min.: Minimum, SD: Standard deviation

Fig. 6:	The superimposed figures of the digital models and the scan data of the female

DISCUSSION

This study is an initial attempt to develop a more anatomically realistic and motion correct digital human model. The base human model used in this study has great body form fidelity, as shown in Fig. 4. The stick-skeleton was designed with enough links (35) for simulating complex joint motion. From results of the supposition figures and mismatch error analyses presented in the results section, these digital models could be thought of much better than Pseudo 3D digital model in terms of realistic body shape and functional posture accuracy, it should be highlighted that the maximal mismatch error for these digital model is less than 37.7 and in average the mismatch error is 6.40 and 7.54 mm.

Fig. 7:	Digital human models as design guideline for the design and innovation of workplaces

Fig. 8:	The incorporation of digital human models with CAD/CAE

This would be quite promising in product and workplace design application. These models could address safety concerns on the digital mock-ups in computer, long before physical prototypes are constructed. For example, the contour of the seat-pan and backrest can be designed with great fit. The hand reach envelope can be estimated with great accuracy. These digital human models could also be applied in evaluating human postures as in virtual simulation.

To date, this set of digital human models has been used to simulate many types of working postures for workplace design (Azadeh et al., 2007). A booklet entitled “A collection of workplace design” which consists of 100 representative workplaces, was compiled for use as design guideline for the design and innovation of workplaces (Chang et al., 2005) (Fig. 7). It is postulated that these digital human models can be further incorporated with Computer-aided Design (CAD) and Computer-aided Engineering (CAE) in the future (Fig. 8).