Introduction to Force Measurement
AANT 304 Biomechanics
This lab is designed to introduce you to kinetic measurement (i.e., the measurement of forces)
that are applied to the body. We will use two tools for doing this: (i) a Force platform and (ii)
handheld dynamometer.
Force plate:
x
y
z
FIGURE 1. An AMTI force platform (different model that the one in the lab). Note that
the positive x direction points away from the end of the plate with the output cable.
Inside the forceplate are strain-gauge sensors that measure the forces and moments applied to the
plate in 3 orthogonal directions Fx, Fy, and Fz are the force components and act along the axes
of an orthogonal x, y, z-coordinate system. In the diagram, the arrows point in the direction of
positive force along each of the axes, following the right-hand rule. Fx and Fy are the horizontal
or shear force components, and Fz is the vertical force component.
Mx, My and Mz are the three moment components. Moments are rotations around the
corresponding x, y and z axes. Positive moments are determined according to the right-hand rule.
When looking down an axis (in its positive direction) positive moments have a clockwise
rotation. We will not concern ourselves with moments.
Strain gauges sensors measure very precise changes in electrical resistance that result from very
∆𝑙
tiny deformations (strain = 𝑙 ) on precisely-milled metal elements inside the plate.
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In this lab your goals are to understand: 1. how the forceplate records forces and 2. What
causes positive and negative changes in force.
Basics:
1. Data are recorded at a pre-set sampling rate (# of measurements/second) or Hertz, for
a specified duration of time (seconds). Since you will be applying relatively static loads,
a low sampling rate is adequate (100Hz), and length of time of 20s for each trial should
be adequate. The total number of samples collected will be duration x sampling rate so
in our case 2000 samples.
2. Data files: The data files are saved as comma-delimited text files with columns ordered
as ForceX, ForceY, ForceZ, MomentX, MomentY, MomentZ. With sampling frequency at
100 Hz, each row represents 1/100s or 0.01s.
3. Direction of forces. You will need to pay attention to the direction (sign) of the forces
and to understand the difference between applied force and measured reaction force.
Following Newton’s third law these forces will be equal but opposite in direction.
The expected positive direction of forces is indicated in FIGURE 1 above. You will need
to pay attention to the direction of the applied force and the sign of the resulting output
for each trial
4. You will collect data for XX trials that will give you information about how the forceplate
works and how you can use this to interpret behavior from force traces.
Your TA will direct the collection of data and the naming, storage and sharing of files.
a. Remember to tare (or zero) the plate before starting each data collection
b. Start with the plate completely untouched and unloaded.
c. Follow the instructions for each of the 3 activities described for each session.
TIME MATTERS so please follow the times as precisely as possible.
d. Name each file according to the measured behavior (i.e., Vertical, HorizontalX,
HorizontalY)
Data collection trials:
1. Vertical (Z) forces.
a. Measure one person’s mass (kg) using a standard scale.
b. Start force collection trial.
i. Wait 5 seconds then have person
ii. stand quietly in the center of the plate for 5
seconds, then
iii. squat down by flexing knees and hips. Wait 2
seconds and then
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iv. return to standing position
c. Stop force collection trial and save as Vertical Force Trial
2. Horizontal (X) forces.
a. Start force collection trial
i. Wait 2 seconds then
ii. Have person stand in center of force plate
HOLDING 10lb weights at the sides of their body.
Stand quietly for 3 seconds, then
iii. Rapidly abduct the right arm to 90deg. so that
weight is held out to the side, wait 3s and return to
starting position. Wait 3 seconds and then
iv. Rapidly abduct the left arm to 90deg. Wait 3s then
return to starting postion
v. Wait 3 s then stop trial.
b. save as Horizontal X
3. Horizontal (Y) forces
a. Start force collection trial
i. Wait 2 seconds then
ii. Have person stand in center of force plate
HOLDING 10lb weights their sides. Stand quietly for
3 seconds, then
iii. Flex both shoulders to 90deg (arms stretched out
in front) Hold for 3 seconds
iv. Return to starting position, wait 3 seconds then
v. Extend both arms behind the body, wait 3 seconds
vi. Return to starting position, wait 3 seconds
b. Stop force trial and save as Horizontal Y
5. Data analysis: Use the excel spreadsheet “Forceplate introduction worksheet” to
analyze your data.
a. This spreadsheet has 3 pages. One for each of the trials described above.
b. In Excel, import each of the 3 .csv files.
A. FileOpenselect ‘All Files’ (not just excel files)  select file (e.g.,
Vertical)  Import select ‘Delimited’ select comma  Finish.
B. Select, Copy and Paste first 3 columns (X,Y,Z force data) to the
appropriate page of the ‘Forceplate Introduction worksheet’
c. For your interpretation of the results remember Newton’s laws:
1. An object at rest remains at rest and a moving object maintains the same
velocity until acted on by a net external force and
2. F=ma.
3. Every force has an equal and opposite reaction force.
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6. Vertical force analysis:
a. Enter the subject’s body mass in the appropriate box, notice that the weight
force is 9.81*body mass
b. The vertical offset is a measure of the fact that the plate was not perfectly tared
prior to data collection.
c. Average quiet standing force should be a measure of body mass.
A. Is this an accurate measure? Do you need to take the vertical offset into
account?
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Yes, the average quiet standing force should be a measure of body mass because
without any movement, the only force acting on the body is gravity so it should
remain the same.
The subjects body mass was ~81.6 kg (800N) on a weight measuring scale, however
the average quiet standing reading from the force plate was ~950N. It is certainly
applicable that vertical offset, or the measure of the fact that the plate was not
perfectly tared prior to data collection may be a reason for the discrepancy between
the measured and actual body masses.
d. Make a scatter plot: Select Column A, Hold Control, Select Column D;
InsertChartScatter (choose the option that just has lines). This is a ForceTime plot. Use this plot to answer the following questions:
A. How did the force change when the subject squatted down? Did it
increase or decrease relative to body weight? Why?
As the subject squatted down there was a decrease relative to body weight due to a
negative change in force. In other words, during the descent phase of a squat your
upper body is essentially falling towards the ground in a controlled fashion. During
this descent the force that began as the total body mass on the plate will be less
because less force is acting on the force plate during the squat.
B. How did the force change when the subject stood up? How did this
compare to when the subject squatted down? Why is it different?
As the subject stood up, there was an increase relative to body weight due to a
positive change in force. In other words, as the subject stood up they exerted their
entire body weight, as well as additional force onto the force plate in order to propel
their body back to standing. Compared to when the subject squatted down, the force
relative to bodyweight was substantially higher because during the ascent, positive
change in force was applied while during the descent, negative change in force was
applied.
7. Horizontal (X) force analysis
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a. Make a scatter plot of time vs. X
b. Find the events abduction and adduction events.
c. How are the forces underlying these events similar and different from one
another? Can you determine the direction of the motion from the force?
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The forces underlying the events of adduction and abduction seem to be different
based on the plot of time vs X. The direction of the motion from the force from the
plot showed that abduction had a high negative change in energy while adduction had
a low positive change in energy. While the forces underlying the events of adduction
and abduction were different per arm, the actions remained consistent from right to
left arm.
d. Make a scatter plot of X vs Y. This plot shows movement of the center of mass.
e. Select the data points in the chart. Restrict the range to avoid the beginning and
ending events. (i.e., change the row references to plot only approx. rows
300:1700).
f. Which axis shows greater motion?
Based on the X vs Y plot that shows movement of the center of mass, the Y axis
8. Horizontal Y force analysis
a. Make a scatter plot of time vs. X
b. Find the events abduction and adduction events.
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c. How are the forces underlying these events similar and different from one
another? Can you determine the direction of the motion from the force?
The forces underlying the events of the front arm raise (flexion of shoulder) and the
rear arm raise (extension of shoulder) are different based on the plot of time vs Y. The
direction of the motion from the force from the plot showed that the front raise had a
negative change in energy followed by a positive change in energy when returning to
the standing position while the rear raise had a positive change in energy followed by
a negative change in energy before returning to standing.
d. Make a scatter plot of X vs Y. This plot shows movement of the center of mass.
e. Select the data points in the chart. Restrict the range to avoid the beginning and
ending events. (i.e., change the row references to plot only approx. rows
300:1700).
f. Which axis shows greater motion? Compare this to 7f above.
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