Biomechanics 2, lab notes and large question answer Flashcards
How to calculate total body centre of gravity using a reaction board?
Determine the body weight of subject
Reset the readings on the scales so it isn’t weighing the board
Lay the person on board
Use the equation xW = 2(R2 - R1) rearranged to x = (2(R2-R1)) / W
W = subject weight (mass x 9.81)
R1 = scale reading for the board (will be 0 if you zero the scale) but if just given the force do the weight x 9.81
R2 = scale reading for the board + subject (mass x 9.81)
2 has come from the board being 2 metres long
x will give you height in which centre of gravity is, can then find percentage of total height
Calculation of total body centre of gravity using the segmental approach?
Head - vertex (very top of head) to chin-neck intersect Trunk - suprasternal notch to hip (lower down than you think) Upper arm - shoulder to elbow Forearm - elbow to wrist Hand - wrist to knuckle Thigh - hip to knee Calf - knee to ankle Foot - heel to tip of longest toe
(ALL JOINTS NEED TO BE MARKED IN CENTRE ESPECIALLY THE KNEE)
Mark all of these on diagram and connect them with straight lines
Measure the length of each segment and determine the location of the centre of gravity using the percentages that will be given (provide calculations to 4 d.p) and mark it
Create OX and an OY axis on the diagram
Measure the distance in mm from each centre of gravity to each axis
Create a results table which for each segment has the ratio weight (multiplied by 2 for any body segment which is double), distance to OY in mm, moments about OY, distance to OX in mm, moments about OX
Moments are calculated via ratio weight multiplied by distance to the axis
At the bottom write down the sums of moments for OY and OX
Write down the relationship Sum of moments = moment of the resultant, (sum of (use symbol E) migxi = MgX
mig = ratio weight and Mg = 1
Then write down X coordinate and Y coordinate, then mark on diagram where it is (from origin go the amount of mm down it then combine them)
ALL CALCULATIONS TO 4DP
How to calculate whole body moment of inertia?
Head - vertex (very top of head) to chin-neck intersect Trunk - suprasternal notch to hip (lower down than you think) Upper arm - shoulder to elbow Forearm - elbow to wrist Hand - wrist to knuckle Thigh - hip to knee Calf - knee to ankle Foot - heel to tip of longest toe
(ALL JOINTS NEED TO BE MARKED IN CENTRE ESPECIALLY THE KNEE)
Mark all of these on diagram and connect them with straight lines
Measure the length of each segment and determine the location of the centre of gravity using the percentages that will be given (provide calculations to 3 d.p) and mark it
Measure each centre of gravities distance to the centre of gravity given in mm
Determine a scaling factor, so to convert mm values to metres just have to divide the mm by how many mm the metre scaling factor is
Create a table which has the headings Segment, Moment of inertia about segment centre of gravity (kg.m^2) (will be given), Mass ratio (will be given), Segment mass in kg (multiply ratio by mass of subject), measured distance from segment centre of gravity to whole body centre of gravity (mm), Real distance from segment centre of gravity to whole body centre of. gravity, md^2, Icg = Is + md^2, number of segments, moment of inertia about body centre of gravity (kg.m^2)]
So calculations are md^2 = mass x distance^2
Then find Icg by adding the MI about segment centre of gravity, then multiply by the amount of segments, then sum them all up to have answer moment of inertia in kg.m^2
