Download Measurement and Interpretation of Body Segment and Landmark

BIOMECHANICAL ANALYSIS OF PHYSICAL ACTIVITY
Laboratory Experiment: Measurement and Interpretation of Body Segment and
Landmark Acceleration via Accelerometry
Dr. Eugene W. Brown
Purposes:
This laboratory experiment has several purposes for the students. They include:
1.
developing or reviewing the concepts of linear position, velocity, and
acceleration;
2.
understanding how to use the analog module of the APAS software and
hardware system;
3.
learning concepts of analog to digital conversion;
4.
developing an understanding that mass and acceleration are the bridge
between kinematics and kinetics (i.e., force = mass x acceleration and torque
= moment of inertia x angular acceleration);
5.
learning terminology associated with linear position, velocity, and
acceleration;
6.
learning how to make measurements of acceleration via accelerometer;
7.
learning about various instrumentation for measuring biomechanical
acceleration (e.g., accelerometer, photoinstrumentation, optical encoder,
multiple turn potentiometer);
8.
understanding the difficulty to calibration of an accelerometer;
9.
setting up experimental procedures in biomechanics;
10. understanding the problems associated with the attachment of an
accelerometer to a subject and factors that interfere with the interpretation of
the output signal;
11. learning about uni-dimensional and multidimensional accelerometers;
12. understanding the influence of orientation of the accelerometer with respect to
the output signal in axis systems with different orientation;
13. developing an understanding of appropriate sampling frequency for recording
acceleration parameters;
14. preparing subjects for participation in research experiments;
15. understanding the relationship of experimental error to measurements
that are recorded;
16. developing insight into the occurrence and reduction of measurement errors
and how these errors influence experimental results;
17. understanding how to set sampling frequency for biological signals;
18. being able to interpret and compare results of biomechanical experiments;
19. learning how to display numeric data in graphical and tabular forms;
20. developing analytical skills;
21. learning how to report the results of laboratory experiments; and
22. projecting the application of technology to future research.
List of Equipment and Supplies
1.
athletic tape
2.
athletic tape pre-wrap
3.
calculator
4.
computer and software for down loading, calibrating, and interpreting
5.
drop jump set-up
6.
electronic trigger
7.
floor matting
8.
force platform
9.
subjects
10. tape measure
11. uni-axial accelerometer (range = 10 g; note that g = acceleration due to
gravity; 1g = 9.8 meters/second2)
12. voltmeter
13. weight plates of known mass
14. 9 volt batteries
15.
16.
17.
18.
19.
Definition of Terms:
1.
acceleration – the time derivative of velocity or the second time derivative of
position
dv d 2 x
a

dt dt 2
2.
3.
accelerometer – a device, usually electronic, to measure acceleration
shock rating – highest input for which the transducer in not damaged
Warning!!! Be careful not to exceed this the shock rating. It could be an expensive
proposition.
4.
5.
6.
7.
analog module of the APSA system – computer module that receives voltage
variations from electronic devices and transforms them to digital signals
APAS – Ariel Performance Analysis System; computer software and
hardware designed for kinematic and kinetic analyses
calibration – the process of establishing an accurate relationship (mapping)
between the magnitude an input parameter (e.g., acceleration) and an output
signal (voltage)
data sample rate – the frequency with which analog data is sampled (e.g.,
1000 samples/second = 1000Hz) (Note that the data sample rate should be at
least two times the expected frequency of the signal. For accelerometry, high
sample frequencies are suggested (e.g., 1000Hz or higher))
8.
9.
10.
11.
12.
13.
14.
15.
16.
Review:
1.
2.
inductive accelerometer – consisting of a mass element positioned and
magnetically coupled between a pair of coils which changes inductive current
in proportion to acceleration of the mass element
linearity of electronic signal – proportional changes in a physical phenomena
(e.g., change in acceleration) correspond to proportional changes in electronic
phenomena (e.g., change in voltage); a one-to-one mapping of two variables;
(Note that we will assume linearity of the accelerometry system.)
piezoelectric accelerometer – often consisting of ceramic materials with
piezoelectric properties that produce an electrical signal in response to a stress
piezoresistive accelerometer – similar to a strain gauge accelerometer except
peizoresistive material (typically solid state, silicon crystals which change
their electric resistance in proportion to the applied mechanical stress) is
substituted for the strain sensitive wires of a strain gauge accelerometer
reference measure – an object or electronic signal of known magnitude (e.g.,
1 g) that is used to establish standards of measurement and changes of
measurement
strain gauge accelerometer – often consisting of four strain sensitive wires,
(Wheatstone bridge circuit) attached to a cantilevered mass element, whose
strain is electronically proportional to the acceleration of the mass
tri-axial accelerometer – accelerometer capable of detecting acceleration in
three Cartesian coordinate axes that are orthogonal to each other (Note that the
coordinate system is a local system of the accelerometer.)
trigger – an electronic device that provides sufficient input voltage to the
APAS system to activate a sampling of all active data channels
uni-axial accelerometer – accelerometer that is capable of detecting
acceleration in one axis (Note that the coordinate system is a local system of
the accelerometer.)
Read chapter 3.3 (pages 237-253) of Biomechanics of the Musculo-skeletal
System edited by B. M. Nigg and W. Herzog, 1994, John Wiley & Sons,
Chichester.
The use of an accelerometer to determine the acceleration of body segments
and landmarks is an indirect process.
a.
It is indirect because the readings obtained from the accelerometer are
measures of acceleration of the accelerometer and likely to differ from
the acceleration of the body segment or landmark to which the
accelerometer is attached. The acceleration values may greatly differ
from acceleration of deeper (underlying) tissues because of the
separation and dampening between the accelerometer and the
intervening tissues.
b.
Experimentation has shown the method of attachment of the
accelerometer to a simulated segment to cause differences in
accelerometer readings.
c.
According to past experimentation utilizing accelerometers, there are
differences in results obtained from accelerometry readings of simulated
3.
segments in association with the surface that the simulated segment
contacts (e.g., soft to hard surfaces) to cause acceleration.
d.
Experimentation has shown, that the farther the accelerometer
attachment is from the location of body perturbation, the greater the
difference in acceleration reading. Basically, intervening tissue and
joints, which are capable of absorbing shock, tend to reduce the
magnitude of the acceleration values.
Mass and acceleration are the bridge between kinematics and kinetics:
force = mass x acceleration (linear concept)
F = ma
torque = moment of inertia x angular acceleration (angular concept)
 = I
4.
Each accelerometers has its own inherent axis system. The following
example demonstrates how a linear axis system (uni-axial accelerometer) may
distort the actual reading that is sought.
Y
vertical
laboratory
orientation
axis system of
accelerometer
vertical
laboratory
acceleration
vector of
accelerometer
(av)

horizontal laboratory
orientation
acceleration vector of
accelerometer (aa)
X
horizontal laboratory
acceleration vector of
accelerometer (ah)
Note that the output of the accelerometer (aa) is not the same as the vertical
laboratory acceleration vector of the accelerometer (av).
av = aacos
However, the orientation of the accelerometer may not be known and may be
assumed to be collinear with the laboratory axis system. This will result in
erroneous conclusions.
Accelerometry for the Determination of Acceleration of Body Segments and
Landmarks
General Methods and Procedures:
There will be two experiments to highlight the use of accelerometry in determining the
dynamics of motion of body segments and landmarks. Students must share the
responsibility of carrying out this experiment!!! The general methods and procedures
for each of these experiments are as follows:
1.
Set Up of Trigger
a.
The electronic trigger must be attached to the APAS system and
appropriate fixed parameters (e.g., units/volt, off set) must be entered.
b.
A channel for data input must be selected that matches the electronic
trigger input location.
c.
The trigger must be tested to determine if it functions.
2.
Set Up of Accelerometer
a.
The accelerometer must be attached to the APAS system and
appropriate fixed parameters (e.g., units/volt, off set) must be entered.
b.
A channel for data input must be selected that matches the
accelerometer input location.
c.
The accelerometer must be calibrated on the basis of known parameters.
The problem with the calibration of the accelerometer is that we are
technically limited to 1g in the static mode. The output range of the
accelerometer, that is to be used in this laboratory experiment, is 10g,
but we do not have the ability to test this range either statically or
dynamically. Check the accelerometer data sheet provided by the
manufacturer. Linearity of output signal will be assumed.
d.
The accelerometer must be tested to determine if it functions.
e.
Warning!!! Be careful not to exceed this the shock rating of the
accelerometer. It could be an expensive proposition.
3.
Data Collection
a.
All data should be collected as accurately as possible.
b.
As data is being collected, note where inaccuracies occur. This
information can potentially be used to assist in the explanation of
results.
c.
Look at the data in graphical form to see if the output makes sense. If
not, make appropriate adjustments and repeat data collection until
meaningful data is obtained.
d.
Because acceleration patterns tend to be composed of high frequencies,
the sampling frequency should be set at a high rate (1000Hz of greater).
e.
An AMTI force platform will be used to collect vertical force data for
Experiment 4.
4.
Subject Preparation
a.
The subjects should be dressed in a manor to remove any interference of
their clothing with the attachment of the accelerometer and to permit
unencumbered movement relative to the task being studied.
b.
Before collecting data, each subject should be familiar with the setting
c.
d.
e.
f.
g.
and task requirements. The subject performance will consist of attempts
to replicate a rigid landing from a drop jump from a pre-established
height.
Each subject should not be exposed to any physical harm as a result of
performance and/or physical limitations.
For any strenuous activity, subjects should be provided with a warm up
and a few practice trials. They must also be apprised of the tasks they
are being asked to perform. This may reduce the chance of injury.
The accelerometer must be attached to the subject in a way that does
not cause any harm to the subject or to the accelerometer and does
not alter the normal or prescribed movement pattern.
The accelerometer must be attached to the subject in a manner that
minimizes experimental error.
The subject should be given practice trials in which the accelerometer
is determine to be operating properly.
Specific Methods and Procedures:
In addition to the general methods and procedures, the four experiments have their own
specific methods and procedures that must be followed. Basically, each experiment will
involve the collection of vertical accelerometry data. It is important to orient the
accelerometer axis with the vertical laboratory axis. The students are collectively
responsible for establishing procedures that will permit data collection and analysis
to fulfill the desired comparisons.
Experiment 1 – Influence of Firmness of Accelerometer Attachment
Past research has indicated that the degree of firmness of attachment of an accelerometer
to a simulated body segment results in variations in accelerometer output signals. To test
this research conclusion, we will attach an accelerometer to the lateral malleolus and
record the accelerometer output for three degrees of firmness (firm, moderately firm, and
loose). The subject will be asked to do three drop jumps, from approximately ½ meter
above the floor, for each of the three levels of attachment.
1. Contrast and compare the accelerometer patterns for each of the three levels of
attachment. Explain the results.
2. Contrast and compare the magnitude of the maximum acceleration signals for each
of the three levels of attachment. Explain the results.
Experiment 2 – Influence of Location of Accelerometer Attachment
Past research has shown that the location of attachment of an acceleration on a body
segment causes variations in accelerometer output signals. To test this research
conclusion, we will firmly attach an accelerometer to the lateral malleolus, lateral midshank, and lateral head of the fibula. The subject will be asked to do three drop jumps,
from approximately ½ meters above the floor, for each of the three points of attachment.
1. Contrast and compare the accelerometer patterns for each of the three points of
attachment. Explain the results.
2. Contrast and compare the magnitude of the maximum acceleration signals for each
of the three points of attachment. Explain the results.
Experiment 3 – Influence of Distance of Accelerometer from Perturbation
The intervention of tissues and joints of the body between the site of body perturbation
and site of attached accelerometer is likely to influence the output signal from the
accelerometer. To test this theory, we will firmly attach an accelerometer to the lateral
malleolus, greater trochanter of the femur, and skull. The subject will be asked to do
three drop jumps, from approximately ½ meters above the floor, for each of the three
points of attachment.
1. Contrast and compare the accelerometer patterns for each of the three points of
attachment. Explain the results.
2. Contrast and compare the magnitude of the maximum acceleration signals for each
of the three points of attachment. Explain the results.
Experiment 4 – Relationship of Vertical Acceleration from an Accelerometer and
from a Force Platform
Note: The vertical force reading from the force platform is the sum of the individual
body segment masses times their vertical accelerations.
F  i mi ai 
n
F  m1a1  m2 a2  m3a3 ...mn1an1  mn an
where mi is the mass of the ith segment, ai is the acceleration of the ith segment, and
there are i to n segments. Since we do not know, from our data collection process, what
the individual segment accelerations are, we will assume that the acceleration of the crest
of the ilium represents this value and the vertical force recorded at the force platform to
represent the force pattern of the center of gravity of the whole body.
Note: If we know the mass of the body, we can determine the overall vertical acceleration
of the center of mass of the body by dividing the force readings by the mass of the body.
F  i mi ai 
n
F
a
m
1.
2.
Contrast and compare the accelerometer patterns with those obtained from the
vertical force record of the force platform. Explain the results.
Contrast and compare the magnitude of the maximum acceleration signals with
those obtained from the vertical force record of the force platform. Explain the
results.
Results:
The results are the responses to the statements and questions posed for each experiment.
They are to be written in a scientific format. You should develop figures, graphs, and
spreadsheet tables and refer to these in your write-up to make the results easy to read.
Also, include and label graphs generated as output. Your format should differ from the
normal scientific format in that you must show your work (i.e., how you calculated your
results). If there are several iterations of the same calculation process, you only need to
show the first to demonstrate your understanding.
Experiment
#
1
2
Data Collection Guideline Table
Experiment Name
Experiment 1 – Influence of Firmness of
Accelerometer Attachment
Experiment 2 – Influence of Location of
Accelerometer Attachment
Data to be Collected
Three degrees of firmness
of attachment to lateral
malleolus (firm, moderately
frim, and loose)
Three readings at each
firmness
Firm attachment to lateral
malleolus*, lateral midshank, and lateral proximal
enc of fibula
Three readings at each site
3
4
Experiment 3 – Influence of Distance of
Accelerometer from Perturbation
Experiment 4 – Relationship of Vertical
Acceleration from an Accelerometer and
from a Force Platform
*Note, firm attachment is
collected in Experiment 1.
Firm attachment to lateral
malleolus, greater
trochanter, and top of skull
Three readings from each of
the three sites
Firm attachment to crest of
ilium
Three readings from
accelerometer and force
platform
BIOMECHANICAL ANALYSIS OF PHYSICAL ACTIVITY
Laboratory Experiment: Measurement and Interpretation of Body Segment and
Landmark Acceleration via Accelerometry
Grade Report
Student: ______________________________
Write-up Area/Comments
Experiment 1 – Influence of Firmness of Accelerometer
Attachment
1. Contrast and compare the accelerometer patterns for each
of the three levels of attachment. Explain the results.
2. Contrast and compare the magnitude of the maximum
acceleration signals for each of the three levels of
attachment. Explain the results.
Experiment 2 – Influence of Location of Accelerometer
Attachment
1. Contrast and compare the accelerometer patterns for each
of the three points of attachment. Explain the results.
2. Contrast and compare the magnitude of the maximum
acceleration signals for each of the three points of
attachment. Explain the results.
Experiment 3 – Influence of Distance of Accelerometer from
Perturbation
1. Contrast and compare the accelerometer patterns for each
of the three points of attachment. Explain the results.
2. Contrast and compare the magnitude of the maximum
acceleration signals for each of the three points of
attachment. Explain the results.
Experiment 4 – Relationship of Vertical Acceleration from an
Accelerometer and from a Force Platform
1. Contrast and compare the accelerometer patterns with
those obtained from the vertical force record of the force
platform. Explain the results.
2.
Contrast and compare the magnitude of the maximum
acceleration signals with those obtained from the vertical
force record of the force platform. Explain the results.
Overall Results
Assess the various ramifications of using an accelerometer
in the study of human movement.
Grand Total:
Points
Received
Points
Possible
5
5
5
5
5
5
5
5
10
50