The Centre of Mass Locator Biomechanics Tool allows the importing of sagittal plane images and digitises them to provide x-y coordinates of landmarks. Both specific segments and whole body centre of mass (CoM) locations can be determined. Two additional external resistances can be added and their effect of the whole body CoM determined.
Images that can be imported can be as large as 720 by 720 pixels. Best results are achieved by images with one dimension of 720 pixels magnitude. Exported images from video are typically of the following sizes:
Multiplying the width and height pixels gives an image size, thus 720 x 480 = 345, 600 pixels or 0.345 megapixels. Still cameras are typically of much higher resolution, but this higher resolution is not that useful on a computer monitor. Given that images can be in portrait or landscape modes, and area of 720 by 720 pixels is available to display an image. Thus HD images must be saved down to an image size where one dimension is 720 pixels.
A 2-point calibration of the space can be performed to enable real space data to be generated from the pixel-based data. The origin, which is typically located in the bottom left corner of the image space, can be reset to a particular landmark. This is particularly useful in problems where only specific segments are of interest.
The landmarks that are included in the digitising sequence are:
| Top of Head | Left Shoulder | Left Hip | External Mass 1 |
| Left Ear | Right Shoulder | Right Hip | External Mass 2 |
| Right Ear | Left Elbow | Left Knee | |
| Right Elbow | Right Knee | ||
| Left Wrist | Left Ankle | ||
| Right Wrist | Right Ankle | ||
| Left Heel | |||
| Right Heel | |||
| Left Metatarsal Head | |||
| Right Metatarsal Head |
A starting landmark is selected and digitising of the landmarks then occurs in sequence. If there is a need to re-digitise a landmark it can be selected at any time. A Reset Landmark button allows all digitised landmarks to be reset to zero values.
A Reset Origin button allows a specific point or landmark to be set as the origin (0, 0).
Scrolling down the page displays a table in which the calculated data is displayed. Data is provided in sections for segments, the centre of mass, and the mass. Fourteen segments, and two points for the external masses are created, these being:
| Head and Neck | Left Arm | Left Thigh | External Mass 1 |
| Trunk | Right Arm | Right Thigh | External Mass 2 |
| Left Forearm | Left Leg | ||
| Right Forearm | Right Leg | ||
| Left Hand | Left Foot | ||
| Right Hand | Right Foot |
The segments have a segment orientation (or segment angle) that is defined as the direction of the segment from its distal landmark to its proximal landmark. They also have a segment length displayed in both pixels and as scaled units.
The Centre of Mass lists the location of the CoM as a fraction (proportion) of the distance from the proximal landmark towards the distal landmark. The x-y coordinates of the CoM in both pixels and in scaled units are also displayed.
The Mass section lists the fraction (proportion) of the total body mass that the segment contains. If a value is entered into the Mass box at the top of the page then a value for the mass in kg is also displayed. Lastly, values of formula that multiply the mass fraction by the distance from the origin in both the x- and y-directions are displayed.
The bottom part of the table displays the x-y coordinates of the centre of mass of the segments, the whole body and whole body plus external masses, depending on the input. The fraction of the masses should sum to 1.000 if the whole body is digitised, and the total mass in kg should equal the entered mass. The fractions of the centre of masses relative to body weight are also displayed if they have been included in the analysis.
Geoff and James Strauss February 2011.