A 3D anthropometric frame was constructed (Figure 4.10 and Figure 4.11). The entire frame was made from non-magnetic materials (aluminium, wood and plastic) as any distortion to the earth‟s magnetic field reduces the accuracy of the IMU to body segment calibration. The frame was designed to hold an athlete in a repeatable reference position while the measurement arms were used to measure the location of bony landmarks in 3D space. To our knowledge this was the first (and maybe the last) time such a frame has been constructed.
Figure 4.11: Mapping the local IMU coordinate systems to the local body segments
In the first version of the anthropometric frame (Figure 4.10) the athlete‟s reference position was determined by locating the feet in a self-selected position. Fixation of the PSIS and base of skull with measurement arms reduced the postural sway during measurements. Instructing the athlete to rest a bar on the upper thighs with fixed hand positions further reduced the superior limb degrees of freedom. Postural sway of 1-5cm was however still present, especially in the extremities, which could reduce measurement accuracy.
As a consequence improvements were made to the anthropometric frame to reduce postural sway (Figure 4.11). A calibration frame with seat was constructed to hold the athlete in place. Cross bars prevented movement of the athlete‟s feet, knees and hands, while measurement arms provided support for the sacrum, C7 and the base of skull.
In addition, the front section of the calibration frame could be removed and placed on the snow so the athlete could replicate the calibration position. This could provide additional reference positions at the start and finish of each run to improve the accuracy of FMC.
Special measurement arms were designed for specific bony landmarks. Each measurement arm had a scale fixed to it; the smallest division of the scale was 2mm. Because neither the
frame nor the measurement arms were perfectly machined the scale for each arm, in a typical measurement position, was calibrated against the 1mm square scale attached to the platform base. A plumb line was dropped from the end of the measurement arm onto the base grid; height was measured with a two metre ruler. Calibration was limited to the mean X, Y and Z offset from three typical measurements.
Frombonylandmar ks to‘ TheBiom echanical Man’
The body model was constructed from 42 measurements of the athlete and 11 measurements of the ski equipment (Table 4.1). A spreadsheet containing the full calculations is available on the accompanying CD (Appendix
A). In the spreadsheet the 42 raw measurements are used to produce 52 anatomical landmarks, including the estimated joint centres. These were required to produce the body model named; „The Biomechanical Man‟ (Figure 4.12). The name was coined because the resulting avatar consists of the points of interest to a biomechanist. In Figure 4.12 the proximal joint centre and two distal landmarks are generally used to visualise each segment. The calculations of joint centre locations and body segment inertial parameters were completed using the method proposed by Dumas and Reed (Dumas, et al., 2007; Reed, et al., 1999).First the athlete with skis and helmet was positioned in the frame so that all measurement arms could reach their targets. The seat height was adjusted for comfort (Figure 4.11). Thirty posterior measurement arms were adjusted to their target bony landmarks, starting with the helmet vertex and ending with the left calcaneus (the complete list is in the spreadsheet, Appendix
A). While the athlete remained in the frame the height of five anterior bony land marks were measured; sellion, suprasternale, the anterior-superior iliac spines (ASIS) and pubic symphysis.The athlete was then free to move out of the frame. Six additional calliper measurements were then taken to locate the anterior bony landmarks relative to the posterior bony landmarks including, base of skull to sellion, C7 to suprasternale, right posterior superior iliac spine (PSIS) to right ASIS, left PSIS to left ASIS, inter ASIS width, and right PSIS to pubic symphisis. The exact calculations are provided in the spreadsheet.
The 3D anthropometric frame measurements were adjusted to account for the measurement arm offset and then the 3D location of each joint centre was calculated. For the elbow this was simply the midpoint between the medial and lateral humeral epicondyles. For the hip and lumbar joint centres, the calculation required more steps. A local pelvic reference system was constructed using five bony landmarks; the left and right PSISs and ASISs and the pubic symphisis. The skin artefacts were removed, as suggested by Dumas, before a new pelvic coordinate system was constructed. The hip and lumbar joint centres were then located by scaling the characteristic pelvis width, depth and height to get an x, y and z offset from the pelvis origin. The pelvis origin was defined as the „new‟ ASIS midpoint.
Table 4.1: Measurements made in the 3D anthropometric frame
Number Anatomical land mark
1 Subject Mass 78 [kg] Measurements [cm] Posterior Measurements Z Y X 2 Vertex 180.2 43.8 24.3 3 Base of Skull 168.3 43.5 15 4 Right Acromion 152.7 63.6 21.8 5 C7 152.7 28 6 6 Left Acromion 152.7 23 20
7 Right Lateral Humeral Epicondyle 113.6 69.8 20.7
8 Right Medial Humeral Epicondyle 113.6 61.1 19.8
9 Left Medial Humeral Epicondyle 113.6 22.4 22.2
10 Left Lateral Humeral Epicondyle 113.6 15 22.9
11 Right Posterior-Superior Iliac Spine 103.8 32.2 9.1
12 Left Posterior-Superior Iliac Spine 103.8 22.6 9.6
13 Right Radial Styloid 92.5 74.7 37.1
14 Right Ulna Styloid 92.5 69.5 32.8
15 Left Ulna Styloid 92.5 13.3 32.5
16 Left Radial Styloid 92.5 9.8 36.8
17 Right 3rd Metacarpal Head 81 76 36.1
18 Left 3rd Metacarpal Head 81 8.4 34.2
19 Right Lateral Femoral Epicondyle 52.4 53.2 33.4
20 Right Medial Femoral Epicondyle 52.4 40.9 32.6
21 Left Medial Femoral Epicondyle 52.4 15.5 32
22 Left Lateral Femoral Epicondyle 52.4 3.2 32.8
23 Right Lateral Malleolus 15.4 52.9 16.2
24 Right Medial Sphyrion 15.4 41 16.4
25 Left Medial Sphyrion 15.4 14.4 16.5
26 Left Lateral Malleolus 15.4 2.7 15.6
27 Right Calcaneus 0 5.9 -2.1
28 Right 2nd Toe Tip 0 6.1 29
29 Left Calcaneus 0 -31.7 -1.7
30 Left 2nd Toe Tip 0 -33.3 29.4
Anterior Measurements
31 Vertex 180.2 43.8 29
32 Sellion 168.3 44.4 16.6
33 Suprasternale 146 44.2 27.2
34 Right Anterior-Superior Iliac Spine 102.5 33.2 24.5
35 Left Anterior-Superior Iliac Spine 102.5 55.4 23.2
36 Pubic Symphysis 89.5 44.3 25.2
Caliper Measurements length [cm]
37 Base of Skull to Sellion 18.7
38 C7 to Suprasternal 11.7
39 Right PSIS to Right ASIS 15.8
40 Right PSIS to Left ASIS 20.6
41 Left PSIS to Left ASIS 15.6
Figure 4.12: Visualisation of measurements from the 3D anthropometric frame
The Biomechanical Man in MATLAB
The previous calculations produced 52 biomechanical landmarks including joint centres that were then used to construct the biomechanical man body model in MATLAB (Figure 4.12). The biomechanical man visualisation was constructed entirely from triangles because triangles can be drawn efficiently in MATLAB. At each epoch an array of global points that described the subject‟s posture in the global coordinate system was calculated from the FMC data. Each triangular face was then rendered by connecting three specific global points with a planar surface. Details of the rendering process are provided in Appendix G and on the accompanying CD (Appendices\Example Code\Biomechanical_Man\More about the Biomechanical Man.pdf).