Date post: | 25-Jun-2015 |
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Education |
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Lesson Objectives
Demonstrate ability to calculate Centre of mass using the Segmental method
Identify the strengths and weaknesses.
Segmental method
The segmental method is the estimation of the location of the body’s total centre of gravity
It is based on the concept that each individual segment has its own centre of gravity
A standard set of values for mass ratio, and centre of gravity for each body segment is used.
Cartesian coordinates are obtained for each segment
Segmental method A study on cadavers by Dempster (1955)
helps us quantify body segment parameters
Dempster collected data from 8 complete cadavers to determine centre of rotation at each joint, segment lengths masses and volumes.
Segment Centre of Gravity Location Relative Mass(% of length) (%)
Head 59.8% from Vertex 6.94Trunk 44.9% from supersternale 43.46
Upper Arms 57.7% from shoulder 5.42Forearms 45.7% from elbow 3.24
Hands 79.0% from wrist 1.22Thighs 41.0% from hip 28.32Shanks 44.6% from knee 8.66
Feet 44.2% from heel 2.74
Step 2Carefully Mark the following segment endpoints on your drawing:1. Vertex2. Chin-neck Intersect3. Suprasternal Notch4. Shoulder axis5. Elbow axis6. Wrist axis7. Knuckle8. Hip axis9. Knee axis10.Ankle axis11.Heel12.toe
Step 1Make an educated guess as to the location of the centre of gravity
Step 3Construct a stick figure, by drawing straight lines between appropriate end points
Step 4Measure the length of each segment and record in table 1
Step 5Using the lengths, and the data expressing each body CG as a percentage of segment length, calculate the CG of each body segment.
Segment Length (mm) Centre of Gravity Location Centre of Gravity Location(% of length) (mm)
e.g. Head length x 0.598
Head 59.8% from Vertex
Trunk 44.9% from supersternale
Upper Arm 57.7% from shoulder
Forearm 45.7% from elbow
Hand 79.0% from wrist
Thigh 41.0% from hip
Shank 44.6% from knee
Foot 44.2% from heel
Step 7Plot horizontal and vertical axis. Then for each segment, measure the horizontal (x) and vertical (y) distance from each CG location to each axis
x
y Step 6Mark the segment CG locations on your diagram
Segment Relative Mass Horizontal CG Distance (X) Horizontal Moment Vertical CG Distance (Y) Vertical Moment
(%) (mm) (mass % x X) (mm) (mass % x Y)
E.g. 6.94 x X E.g. 6.94 x Y
Head 6.94
Trunk 43.46
Upper Arm 5.42
Forearm 3.24
Hand 1.22
Thigh 28.32
Shank 8.66
Foot 2.74
∑= ∑=
Centre of Mass Location X = E.G ∑ horizontal moment 100 Y =
E.G ∑ Vertical moment 100
X = Y =
Step 8To find each segment moments about each axis. Multiply the relative mass of each segment by its distance from the axis
Step 9Calculate the X and Y co-ordinate for the TOTAL BODY CENTRE OF GRAVITY by dividing the sum of segment moments about each axis by the total relative body weight (100, which represents 100% of body weight)
Step 10Plot the Centre of gravity location (x,y) on your diagram
Check against your estimated guess.
x
y
Weaknesses / Strengths.....
Data based on 8 cadavers, old ones at that. Is this data relevant to athletes? (muscle structure)
Human error in plotting points Time Consuming
Can be applied to actual sporting movements and techniques
Computer software could be used to improve accuracy/ speed.
Can be fully automated using computer system to get quantitative analysis.
Conclusion
Demonstrate ability to calculate Centre of mass using the Segmental method
Identify the strengths and weaknesses.
Segment Length (mm) Centre of Gravity Location Centre of Gravity Location(% of length) (mm)
e.g. Head length x 0.598
Head 59.8% from Vertex
Trunk 44.9% from supersternale
Upper Arm 57.7% from shoulder
Forearm 45.7% from elbow
Hand 79.0% from wrist
Thigh 41.0% from hip
Shank 44.6% from knee
Foot 44.2% from heel
Segment Relative Mass Horizontal CG Distance (X) Horizontal Moment Vertical CG Distance (Y) Vertical Moment(%) (mm) (mass % x X) (mm) (mass % x Y)
E.g. 6.94 x X E.g. 6.94 x YHead 6.94Trunk 43.46Upper Arm 5.42Forearm 3.24Hand 1.22Thigh 28.32Shank 8.66Foot 2.74
∑= ∑=
Centre of Mass Location X = E.G ∑ horizontal moment 100 Y =
E.G ∑ Vertical moment 100
X = Y =
Table 1
Table 2