Center of Gravity Lab

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KIN 330

Center of Gravity Lab

Walton’s Template

Purpose: The purpose of this exercise is to acquaint the student with a manual technique for locating the center of gravity of the body. The specific method to be employed uses Walton’s Template with Dempster’s weighting.

Equipment/Materials: Pencil, cm ruler, a hand calculator, and a picture of a person in which all body segments can be seen.

Procedures:

1. On your picture, identify joint centers by placing a dot at the center of each joint, tip of toes and hands, top of the head and the seventh cervical vertebrae. The location of the joint center is as you view the joint, literally one needs to take the joint diameter from your perspective (camera view) and find the center. Accuracy in placement if these small dots is critical to the accuracy of the results.

2. Using a straight edge, draw a stick figure by connecting the head to C7 and C7 to the midpoint of the distance between the two hip joints. Then for each side connect: the tip of fingers to wrist, wrist to elbow, elbow to shoulder, (shoulder to shoulder is optional), tip of the toes to ankle, ankle to the knee, and knee to the hip.

3. Using Walton’s Template, identify the center of gravity of each body segment with a short dash mark in the following manner:

- Locate the correct triangle for the body segment to be measured,

- The “D” and “P” on each triangle indicate distal and proximal ends of the segment.

- Align the template so that the shorter sides of the triangle are exactly over each small dot that represents the respective joint center.

- The segment line (which was drawn) must be parallel to the lines in the triangle

- Mark the location of the cog by placing a sharp pencil in the appropriate slit on the template and make a short dash mark.

4. Draw and arbitrary X-Y axis on you picture. The placement of this axis relative to the figure will determine whether your X-Y measurements are positive, negative, or mixed.

5. Measure the X and Y distances of each center of gravity from the axes and place them in their respective column and row on the chart provided with this lab. The right and left referred to on the chart are relative to the right and left of the figure, not as the figure is viewed.

6. After step 5 is completed, multiply the measured x-distance of each segment with the corresponding segmental weight percentage given. Do the same for the Y-distances. The products obtained are the X-moments and Y-moments, respectively.

7. Sum the X and Y moments and use these moments as the coordinates for the center of gravity location of the figure, from that camera’s perspective. Locate the body’s center of gravity on the figure.

8. “Eyeball” your results. Is this location of the body’s center of gravity in a realistic position? (i.e. Does it look correct?)

9. Now, repeat steps 5 through 7, but place a new axis through the calculated center of gravity of the body. All measurements will be taken from the new axis.

10. Answer the following questions:

a.What are the sums of the new X and Y moments? Explain your answer in light of the first cog calculation

b.What should these sums be if the first cog calculation was correct and its location was accurate? Why?

c. How is knowledge of the center of gravity of the body important in biomechanics? (i.e. how is the cog location used in biomechanical analyses?

Name ______

PID ______

Data Table to be used with Walton’s Template

Center of Gravity Lab

Name ______

PID ______

Data Table to be used with Walton’s Template

Center of Gravity Lab

Ulibarri / KIN330