TEXAS WOMAN'S UNIVERSITY
KINS 3591

Segmental Method

This lab was adapted from one developed by
Young-Hoo Kwon, Ph.D., of Texas Woman's University.

The segmental method involves computation of the segmental CMs (Fig 1). The whole body CM is then computed based on the segmental CMs.

Figure 1. The segmental method (Adapted from Hamill & Knutzen, 1995, Biomechanical Basis of Human Movement, Baltimore, MD: Williams & Wilkins.) Another example.

Coordinates - The first step in the segmental method is to quantify the body posture of the subject. The best way to do this is to record a subject's motion and to read off the coordinates of selected body points, such as joints, from the recorded images (photo, film, or video). In your video image, you will find the X & Y coordinates of body joints with a ruler. Use the lower left corner of the image as the origin and measure vertically and horizontally from that point to determine the X & Y coordinates in millimeters.

Body Segment Parameters - Since the human body consists of several segments, such as hands, forearms and upper arms, the overall mass distribution within the body is a function of the mass distribution within the individual segments and the body posture. As explained above, body posture can be quantified by determining coordinate data.

Mass distribution within the segments is known in the form of body segment parameters (BSPs). BSPs include body segment masses and the locations of their centers of mass. These parameters were obtained mainly from cadavers in the 50s, 60s, and 70s 1,2,4.

Research conducted in the former Soviet Union in the 1980's 5,6,7,8 provided an alternative source of BSP's. This research differed from previous research in that it was performed on young, fit, living subjects. As such, the BSP's developed by Zatsiorsky are considered by some to be superior to those developed from cadaver research. Zatsiorsky's BSP's have not found wide use, however, due to his selection of non-standard segment endpoints. Adjustments to Zatsiorsky's data were published in 1996,3 making them more usable. Still, old habits die hard, and many scientists still use cadaver based data despite the availability of Zatsiorsky's BSP's.

Example: The CM of the forearm (female) is located at 45.59% of the forearm length from the elbow, the proximal end of the forearm; the forearm mass is 1.38% of whole body mass.

If the location of the end points of a segment is known, one can compute the location of the segment CM using the CM location data shown in the BSP table (see figure 2):

XCM = (XD)(%cm) + (XP)(1 - %cm)  =  (XP) + (%cm)(XD - XP)] [1]

YCM = (YD)(%cm) + (YP)(1 - %cm)   =  (YP) + (%cm)(YD - YP)] [2]

Figure 2. Segmental CM

where (XCM, YCM) = X & Y coordinates of the segmental CM, (XD, YD) = coordinates of the distal end of the segment, (XP, YP) = coordinates of the proximal end, and %cm = CM location ratio shown in the BSP table divided by 100.

Computation of the Body CM - The body CM can be computed from the CMs and the masses of the segments using the following equations:

    [3]     [4]

 

where (X,Y) = coordinates of the body CM, i = segment number, (Xi, Yi) = the X & Y coordinates of the CM of segment i, and mi = mass of segment i. In other words, the body CM coordinates are equal to the sum of the products of segmental mass and segmental CM coordinates divided by the body mass (Smi).

Locate the Center of Mass of one of the following images. BSP's and segment equations are already provided in Excel worksheets available for your use. 

Make sure you use the correct worksheet (male or female).

Images displayed on a computer monitor are composed of very small dots which are referred to as "pixels." These are the units of measure in the video images in this lab. Links to the images below will bring up a new page with the image included. When you pass the cursor over the image, you will notice a pair of numbers at the bottom of the page (the web page MUST be maximized for these numbers to appear). These are the image coordinates (x, y) at the cursor. Place the cursor arrow over where you visualize the locations of the joint centers to be. Record the coordinates in the Excel worksheet for each frame.

Next, find a [whole body] picture of an individual doing something (dancing, participating in a sport, or performing a task) in a magazine or newspaper article. Draw x- and y-axes along the top and left sides of the picture. Place dots on the locations of the athlete's joints (wrist, elbow, shoulder, etc). Using a ruler marked in millimeters, measure the x- and y- coordinates of each point. Enter these values into the spreadsheet. When the spreadsheet is filled and a center-of-mass location determined, measure and draw this point [X marks the spot ;-)] on your picture. Turn in your picture with your spreadsheet printout.

References:

1.    Chandler RF, Clauser CE, McConville JT, Reynolds HM, & Young JW (1975). Investigation of inertial properties of the human body (AMRL Technical Report 74-137). Wright-Patterson Air Force Base, OH: Aerospace Medical Research Laboratories.

2.    Clauser CE, McConville JT, & Young JW (1969). Weight, volume, and center of mass of segments of the human body (AMRL Technical Report 69-70). Wright-Patterson Air Force Base, OH: Aerospace Medical Research Laboratories.

3.    de Leva, P (1996). Adjustments to Zatsiorsky-Seluyanov's segment inertia parameters. Journal of Biomechanics, 29(9), 1223-1230.

4.    Dempster, WT (1955). Space requirements for the seated operator (WADC Technical Report 55-159). Wright-Patterson Air Force Base, OH: Wright Air Development Center.

5.    Zatsiorsky V & Seluyanov V (1983). The mass and inertia characteristics of the main segments of the human body. In H. Matsui & K. Kobayashi (Eds.), Biomechanics VIII-B (pp. 1152-1159). Champaign, IL: Human Kinetics.

6.    Zatsiorsky V & Seluyanov V (1985). Estimation of the mass and inertia characteristics of the human body by means of the best predictive regression equations. In DA Winter, RW Norman, RP Wells, KC Hayes & AE Patlaa (Eds.), Biomechanics IX-B (pp. 233-239). Champaign, IL: Human Kinetics.

7.    Zatsiorsky V, Seluyanov V, Chugunova L (1990). In vivo body segment inertial parameters determination using a gamma-scanner method. In N. Berme & A. Cappozzo (Eds.), Biomechanics of Human Movement: Applications in Rehabilitation, Sports and Ergonomics (pp. 186-202). Worthington, OH: Bertec Corp.

8.    Zatsiorsky VM, Seluyanov VN, & Chugunova LG (1991). Methods of determining mass-inertial characteristics of human body segments. In GG Chernyi & SA Regirer (Eds.), Contemporary Problems of Biomechanics (pp. 272-291). Moscow: Mir