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ADVANCED BIOMECHANICS OF PHYSICAL ACTIVITY
Laboratory Experiments: Measurement and Interpretation of Electromyographical
Signals of Various Types of Muscle Contractions
Dr. Eugene W. Brown
Department of Kinesiology
Michigan State University
A.
Purposes:
This laboratory experiment has several purposes. They include the following:
1.
Introduce electromyography (EMG) as a tool in biomechanics research
2.
Develop an understanding of the EMG signal
3.
Provide an understanding of recording techniques associated with:
a.
analog to digital conversion
b.
electronic amplification of the EMG signal
c.
input impedance of an electronic amplifier
d.
frequency response of an electronic amplifier
e.
common mode rejection
4.
Learn techniques of processing EMG signals in the time domain:
a. rectification (half-wave, full-wave or absolute value)
b.
smoothing (linear envelope – low pass filter applied to a full-wave
rectified signal)
c.
integration (IEMG)
d.
root mean square(RMS)
5.
Learn techniques of processing EMG signals in the frequency domain
a.
power density spectrum
b.
mean frequency
c.
median frequency
d.
band width
e.
peak power frequency
6.
Develop an understanding of the EMG signal as it relates to:
a.
force of muscular contraction
b.
muscular fatigue
c.
velocity of muscular contraction
d.
type of muscular contraction (concentric, eccentric, isometric, isokinetic,
and dynamic)
7.
Learn how to calibrate the relationships between a variable analog voltage
signal and the digital representation of its parameter
a.
voltage (battery voltage, temporal pulse, EMG signal)
b.
torque
c.
angle
8.
Learn techniques for skin preparation for surface electrode
placement
9.
Understand the reasons supporting proper electrode placement
B.
Definitions of Terms
1.
Bandwidth
2.
Common mode rejection
3.
Concentric contraction
4.
Cross-talk
5.
Curve smoothing
6.
Eccentric contraction
7.
Electromechanical delay (EMD)
8.
Frequency spectrum
9.
Impedance
10. Indwelling electrodes
11. Integrated EMG (IEMG)
12. Integration
13. Isokinetic contraction
14. Isometric contraction
15 Mean frequency
16. Median frequency
17. Motor end plate
18. Motor unit
19. Motor unit action potential (muap)
20. Power density spectrum
21. Raw EMG
22. Rectification
23. Resting potential
24. Root mean square (RMS)
25. Signal processing
26. Surface electrodes
27.
C.
Equipment and Supplies
1.
Adhesive tape pre-wrap
2.
Anatomical chart showing muscle location
3.
APAS or MYOPAC system and EMG software module
4.
Elastic tape, surgical tape
5.
Electrode gel
6.
Isokinetic dynamometer or comparable resistance set-up
7.
Level
8.
Metronome
9.
Muscle stimulation device
10. Preamplified or other types of surface electrodes
11. Sand paper, rubbing alcohol, electrode gel, razor
12. Subject
13. Voltmeter
14. Weight training and resistance equipment
15. 9V battery (various other volt batteries)
D.
Calibrating Analog Signals from Electronic Measurement Devices
What is Calibration?
Electronic measurement devices produce an output signal voltage in response to a particular quantity of
some property being measured (e.g., force, position, temperature). Calibration is a procedure to relate the
amount of a given property to its corresponding output voltage. For example, if our device is an electronic
thermometer, then a given temperature will result in a particular output voltage. Let us say that the
temperature being measured is 37C, and this results in an output voltage of 3.7 V. Assuming that the
output of the device and the input parameter are linear, meaning that every increase of x degrees results in a
change in voltage of y volts, then the relationship between temperature and voltage for this thermometer is
0.1 V for every 1C, or alternatively, 1 V for every 10C. Thus for this device we can say that the
calibration coefficient is 10C per volt. The goal of the calibration procedure is to determine exactly what
the relationship is between the output voltage and the amount of the property measured, and if that
relationship is linear, to calculate the calibration coefficient.
Determining the Calibration Curve
The following discussion assumes the use of the APAS system, although it is generally enough to be
applied to other systems. It also assumes that the user will have a basic working knowledge of the APAS
system.
As an example of how to calibrate an instrument, let us consider an electronic scale. This scale provides an
output voltage in response to weight, with increasing weight producing increasing voltage. In order to
calibrate our scale, we need to do the following:
1)
2)
3)
4)
5)
Plug the output cable from our scale into the analog-to-digital converter board associated with the
APAS system.
Within the APAS Analog Module, select Options and then Channels; under the channel being used
for the scale, set the Units/Volt (this will ultimately be our calibration coefficient) to 1 (i.e., 1 unit per
volt). This means that every change of 1 V will result in a change of 1 unit. In other words, we will
get a recording of actual voltage.
Choose a range of known weights to calibrate the scale; let us say we chose weights ranging from 10
to 100 lbs in 10 lb increments.
Beginning with no weight, record the output voltage for each weight.
Plot the results on a graph with weight on the x-axis and voltage on the y-axis. The resulting curve is
our calibration curve:
Calibration Curve
6
Voltage (V)
5
4
3
2
1
0
0
10
20
30
40
50
60
70
80
90
100
Weight (lbs)
6)
Examine the calibration curve for linearity. In our example, we have a perfectly linear relationship.
In reality things are usually not quite so perfect, but we need to be certain that the relationship
between weight and output voltage is basically linear.
7) Determine the equation of the curve. Because the curve is a straight line, we use the equation y = mx
+ b, where y is the output voltage, x is the weight, m is the slope of the line and b is the y-intercept.
For most, if not all, measurement devices, including our scale, the y-intercept will be 0, meaning 0
weight equals 0 volts. The slope of the line is the change in y divided by the change in x, or in our
case, the change in voltage divided by the change in weight. For our scale, a change in weight of 20
lbs results in a change in voltage of 1 V, so we divide 1 V by 20 lbs to arrive at a slope of 0.05 V/lb.
Our equation therefore is
y = 0.05x.
8) The calibration coefficient (Units/V), which we will enter into the APAS system, is simply the
inverse of the slope of the line, or 20 lbs/V.
9) In the APAS system, enter 20 as the value for Units/V.
10) Test the calibration by loading several known weights onto the scale, recording the weight, and
checking to see if the actual weight matches that recorded by the APAS system. If it does, our
calibration was successful!
This procedure can be followed for any electronic measurement device by substituting for the weight
several known quantities of the property being measured by the device. For example, if the device is a
thermometer, we would use several known temperatures. If the device measures volume, we could use
several known volumes of water.
The preceding procedure is a relatively straightforward way to calibrate analog signals from electronic
measurement devices, and allows virtually any device that has an output voltage to be used with the APAS
system.
E.
General Methods and Procedures
There will be several laboratory experiments conducted to attempt highlight
differences in the EMG signal under different muscle contraction conditions. The
students share the responsibility of carrying out the experiments. The general
methods and procedures for each of these experiments is as follows:
1.
Calibration and equipment set-up:
The APAS or MYOPAC recording system and software, and electrodes
must be prepared to properly record the isokinetic torque and angle
output, metronome output, and signals from the EMG electrodes.
a.
To calibrate the torque output from the isokinetic dynamometer,
estimate the maximum torque the subject is likely to produce in
forceful knee extension. (Note that even though the subject will
perform isokinetic knee flexion, the maximum magnitude of knee
flexion will not likely be as great as knee extension. Also note that
maximum knee extension torque is likely to be associated with
isometric contraction (i.e., angular velocity = 0 or with the
isokinetic dynamometer set at a very slow angular velocity.) Place
loads on the arm of the isokinetic dynamometer that will
approximate the estimated maximum torque likely from the subject
to establish an upper limit for calibration of torque. In determining
the torque from the loads placed on the arm of the isokinetic
dynamometer, use the following calculations:
Force = (mass) x (acceleration)  F = (mass load in kg) x (acceleration due to gravity)
F = (mass load in kg) x (9.81m/sec2)
units in Newtons
Torque = (Force) x (perpendicular distance from point of rotation) 
T = (F) x (d)
units in Newton-meters
When the loaded arm of the isokinetic dynamometer is determined
to be horizontal by use of a level, the torque can be determined by
measuring the distance of the load from the point of rotation (d)
and multiplying this value times the calculated force. It should be
noted that this calculated torque does not take into account the
torque associated with the weight of the arm of the isokinetic
dynamometer acting at its center of gravity. To take this into
account the following formula is used:
Ttotal = (Fload) x (d) + (weight of arm) x (arm center of gravity distance from point of
rotation)
units in Newton-meters
The voltage output sampled by the recording system will be related
to the measured torque. Now it is essential to obtain a second
measure of torque to establish a linear ratio between changes in
changes in torque and changes in voltage output from the
isokinetic dynamometer. Use zero for the lower end of the torque
range. This value can be obtained with the arm of the isokinetic
dynamometer in a vertical position. When the arm is in this
position, the perpendicular distance of the load from the point of
rotation is zero (i.e., d = 0). Thus, torque is equal to zero.
b.
c.
d.
To calibrate the output of the electrogoniometer (electronic angular
measurement device) of the isokinetic dynamometer, establish the
working range of the knee joint for the experiments. This is likely
to be a range from a position of complete extension, which will be
defined as zero degrees, to approximately a right angle, defined as
ninety degrees. One problem exists with the establishment of these
positions, namely the orientation of the attached arm of the
isokinetic dynamometer is offset from the position of the shank.
Once this offset is determined, the position of the arm of the
isokinetic dynamometer can be oriented for a corresponding knee
angle of zero and output to the recording system. Similarly the
arm of the isokinetic dynamometer can be positioned for the
corresponding ninety degree orientation of the knee and output to
the recording system. These two voltage outputs and
corresponding knee angles establishes a linear ratio for calibration
of the electrogoniometer.
Calibration of the output of the metronome is not needed because it
is only used to indicate a temporal event.
Calibration of the voltage signals from the surface electrodes is not
needed because these signals are not used to correspond to another
parameter (e.g., one of the voltage outputs in the isokinetic
dynamometer corresponds to torque and the other corresponds to
angle). Also the exact voltage output from the sum of the motor
unit action potentials (maup’s) received at the electrodes and its
frequency is not known. All that is known is that the larger output
signals from surface electrodes are approximately 5mV and these
signals have negligible power beyond 1000Hz. Thus the
voltage received at the recording system should range from zero to
approximately 5mV. Caution should be taken in noting the
amplification of these signals by either preamplified electrodes
that may be used or multiplication of the signals through the
recording system. When using Delsys preamplified electrodes, the
signals are amplified by 1000. Therefore, the largest signals input
into the recording system will be approximately 5V. Use a
voltmeter to measure the voltages of various batteries in the range
of the expected amplified signals (0 – 5V). Input these signals into
the recording system to confirm their readings. If additional
amplification is be performed by the recording system, this should
be noted. Since the signals have little power beyond 1000Hz and
in accord with the Nyquist frequency (signals should be sampled at
2.
3.
4.
5.
no less than twice the maximum expected frequency), it is
recommended that the digital sampling rate of the records of the
EMG signals be set at 2000Hz.
Subject preparation:
a.
The central contractile regions (bellies) of the gastrocnemius, long
head of the biceps femoris (one of the hamstring muscles), and
rectus femoris (one of the quadriceps muscles) must be located by
palpation. Muscle stimulation should be used to locate their
motor points for electrode placement on the bellies of the muscles
at a point midway between the motor points and tendons.
b.
The electrical impedance of the surface of the skin (area
approximately 5cm x 5cm) over the central contractile of the three
muscles must be reduced in preparation for electrode placement by
the following sequence of activities: cleansing with rubbing
alcohol, shaving, cleansing with rubbing alcohol, abrading with
sand paper, and cleansing with rubbing alcohol.
Electrode placement:
a.
Apply electrode gel over the prepared skin surface areas.
b.
Compress the electrodes into the gel and against the skin at the
prepared sites.
c.
Hold the electrodes in place by first wrapping around the body
segments and electrodes with adhesive tape pre-wrap and then
elastic tape.
d.
Make a loop in the cables of the electrodes and tape them to the
adhesive tape. [Note that this is done to prevent the cables from
being dislodged from the electrodes. If too much tension is applied
to the cables, they should pull out from the adhesive tape as a
warning.]
e.
Using the procedure described in 3. a.-d., place surface electrodes
on the rectus femoris, long head of the biceps femoris, and
gastrocnemius muscles and test. Input the signals to the recording
system and test the signals by having the subject contract these
muscles.
Familiarization of subject to setting and activity:
Before collecting data in each of the experiments, the subject should be
familiarized with the setting and tasks to be performed. It is appropriate
to provide a warm-up and a few practice trials. This may reduce the use
of antagonistic muscle contraction, typical of the early learning phase of
a motor skill, and increase the reproducibility of the contraction/skill
Labeling and saving records:
Recorded signals will need to be saved. Each recording should be
uniquely distinguishable by a file name to eliminate confusion from the
capture of several records in carrying out this experiment. The
following chart is provided as a suggested format for file names in
keeping records of each of the data sets. An example of the use of this
chart for EMG of the rectus femoris (recorded on channel 3 and
amplified by 1000), torque (recorded on channel 5), and angle (recorded
on channel 6) records for intermediate isokinetic knee extension at 90
degrees per second sampled at 2000Hz would result in the following file
name:
RF3_2K_1K_K_E_C_I_9_T5A6
FILE INFO.
1. Muscle/channel
File Record Chart
FILE CODE
FILE INFO.
Rectus femoris/#
6. Type of
(RF#)
contraction
Gastrocnemius/# (G#)
Long head of biceps
femoris/# (BF#)
where # indicated
channel number where
parameter was
recorded
1000Hz (1K)
2. Sampling
2000Hz (2K)
frequency
etc.
3. Amplification – 1000/channel 1 (1K1),
magnitude/channel etc.
Ankle (A)
4. Joint
Knee (K)
Flexion (F)
5. Movement
Extension (E)
Skilled sequence (S)
7. Force/load
8. Angular
velocity
(deg/sec)
9. Other
parameters
recorded/chan
nel
FILE CODE
Isometric (I)
Concentric (C)
Eccentric (E)
Fatigue (F)
Mild/light (M)
Intermediate (I)
Forceful/heavy (F)
Isometric; zero (0)
30 (3)
90 (9)
120 (2)
Torque (T#)
Knee Angle (A#)
Metronome (M#)
where # indicated
channel number
where parameter
was recorded
Specific Methods and Procedures:
In addition to the general methods and procedures, each experiment will have its
own specific methods and procedures.
Experiment 1 – Variation of Force of Isometric Contraction
Action and muscle to be recorded – attempt isokinetic knee extension
(0 deg/sec), recording EMG signal of rectus femoris muscle, torque, and
joint angle (approximately 45 degrees)
Subject performance - three different isometric contractions at relatively
different forces (mild, intermediate, and forceful); forces should be held
relatively constant
Signal recording– record raw EMG signal, torque, and joint angle at 2000Hz
or higher; three files
Experiment 2 – Electromechanical Delay (EMD) in Isometric Contraction
Action and muscle to be recorded – attempt isokinetic knee flexion
(0 deg/sec), recording EMG signal of biceps femoris muscle, torque, and
joint angle; start from muscle rest with knee joint at approximately 45
degrees, contract and hold contraction for approximately 3 seconds, and
return to muscle rest for approximately 2 seconds
Subject performance – from a relaxed condition of the biceps femoris the
subject attempts a fast turn on (contraction) of the muscle, holds the
isometric contraction, and then a quick shutoff (relaxation) of the muscle
Signal recording – record raw EMG signal, torque, joint angle at 2000Hz or
higher; one file [Note that recording must begin prior to start of
muscular contraction and end after relaxation starts.]
Experiment 3 – Concentric Contractions Under Different Loads and Angular
Velocities
Actions and muscles to be recorded – concentric isokinetic knee extensions
and flexions recording EMG signal of rectus femoris and long head of
the biceps femoris muscles, torque, and joint angle; record, at minimum,
two complete cycles of the knee extension-flexion movement over a
complete range of the knee joint
Subject performance – concentric isokinetic knee extensions and flexions at
three angular velocities (30, 90, 120 deg/sec), each performed at three
relatively different forces (mild, intermediate, and forceful)
Signal recording – record raw EMG signals, torque, and joint angle at 2000Hz
or higher; 9 files
Experiment 4 – Eccentric Contractions Under Different Loads and Angular
Velocities
Actions and muscles to be recorded – eccentric isokinetic knee flexions
recording EMG signal of the rectus femoris muscle and knee joint angle;
knee should subtend complete range of movement
Subject performance – beginning with a straight knee (knee angle = 0 degrees)
eccentric isokinetic knee flexions at three angular velocities (30, 90, 120
degrees/sec), each performed at three relatively different forces (mild,
intermediate, and forceful); knee is held at zero degrees by aid and then
a load is applied by the aid; 9 files
Signal recording – record raw EMG signal and joint angle at 2000Hz or
higher, 9 files
Experiment 5 – Concentric Versus Eccentric Contraction Under Similar and
Different Loads and Angular Velocities
[Note that data for this experiment are collected in Experiments 3 and 4.]
Experiment 6 – Skilled Movement Pattern (and Pattern Development) Under
Similar and Different Loads and Velocities
Actions and muscles to be recorded – metronome is to be used to set the
time intervals for repetitions of the “skilled” motor performance
sequence (stand from a seated position – hold –toe up – hold – toe down
– hold – sit –hold) while EMG signals from the rectus femoris, long
head of the biceps femoris, and gastrocnemius muscles are recorded
Subject performance – the “skilled” motor performance sequence is
completed at three velocities (slow, intermediate, and fast), each
performed under three load conditions (body weight only, intermediate
weight bar on shoulders, and heavy weight bar on shoulders)
Signal recording – record raw EMG signals and metronome for a minimum
of one complete pattern for all load conditions; also record raw EMG
signals and metronome for 1st, 20th, and 40th performance of the slow
velocity sequence in the body weight only condition; 2000 Hz or higher;
12 files
Experiment 7 – Fatigue Under Maximum Isometric Contraction
Action and muscle to be recorded – attempt to hold full knee extension under
heavy load over prolonged time period recording knee joint angle and
EMG signal of rectus femoris during early, middle, and late phases of
fatigue of muscle
Subject performance – prolonged heavy load knee extension
Signal recording – knee joint angle and raw EMG signal of early, middle, and
late phases of fatigue of the rectus femoris muscle; 2000Hz or higher; 3
files
Processing EMG Signals:
(See handout on processing EMG signals.)
Results and Conclusions:
Raw and processed EMG signals will be provided following the laboratory
experiments.
The Results and Conclusions are the responses to the questions that follow. They
are to be written in a scientific format using references (e.g., required and supplemental
readings, EMG research studies) to support and/or refute the results of the laboratory
experiments. Note that answers to some questions require the use of data from more than
one of the experiments. Figures and tables must be labeled and included in the responses.
Responses must be written in an order that follows the order of the questions and must be
labeled so that they can easily be matched with the questions and experiments.
Questions:
Experiment 1
1.
What are the reasons for the differences between the raw EMG, IEMG,
and RMS curves of the isometric contractions? In answering this
question, explain the nature of how the raw EMG data is processed.
2.
What does the power spectrum tell you about the nature of the raw data?
3.
What did you observe when comparing signals from the three different
isometric contraction forces? Relate your answer to the electrical
activity of muscular contraction. Are your observations compatible with
what is expected? Explain.
Experiment 2
1.
2.
What is electromachanical delay (EMD)?
Did you observe EMD in the fast isometric
contraction? Why or why not?
3.
According to the EMG literature, what should have been seen and why?.
Experiment 3
1.
What differences did you observe in the IEMG signals of the concentric
contractions under the same relative force conditions, but different
angular velocities? Relate your answer to the electrical activity of
muscular contraction, muscle force-length variation, muscle force
velocity variation, and changes in moments of muscle contraction.
2.
What differences did you observe in the IEMG signals of the concentric
contractions under the same angular velocity, but different relative force
conditions? Relate your answer to the electrical activity of muscular
contraction, muscle force-length variation, and changes in moments of
muscle contraction.
Experiment 4 (and possibly Experiment 6)
1.
What differences did you observe in the IEMG signals of the eccentric
contractions under the same relative force conditions, but different
angular velocities? Relate your answer to the electrical activity of
muscular contraction, muscle force-length variation, muscle force
velocity variation, and changes in moments of muscle contraction.
2.
What differences did you observe in the IEMG signals of the eccentric
contractions under the same angular velocity, but different relative force
conditions? Relate your answer to the electrical activity of muscular
contraction, muscle force-length variation, and changes in moments of
muscle contraction.
Experiment 5
1.
What differences did you observe between the IEMG signals of the
concentric and eccentric contractions under the same angular velocity
and same relative force conditions? Base your explanation for
differences on the differences between concentric and eccentric
contraction.
2.
Are the trends in IEMG signals for concentric and eccentric contraction
the same as relative force and velocity changes? Explain.
Experiment 6
1.
Is there a correspondence between the observed movement and the
known actions of the rectus femoris, long head of the biceps femoris,
and gastrocnemius muscles? Explain.
2.
For the “skilled” movement pattern, is there evidence of the existence of
Lombard’s Paradox in the raw EMG signals from the rectus femoris and
long head of the biceps femoris muscles? Explain.
3.
Is there evidence of skill development from the 1st to the 40th
performance of the slow velocity sequence in the body weight only
condition? Explain.
4.
As load increases, is there evidence of additional reliance on the use of
muscles as stabilizers? Explain.
5.
What differences did you observe in the temporal pattern of muscle
contraction and relaxation in the performance of the “skilled” movement
under different velocity and load conditions? Explain.
6.
Do you think EMG could be effectively used to differentiate between
levels of skill in selected movement patterns? Explain.
Experiment 7
1.
What differences did you observe in the RMS curves and median
frequency among the early, middle, and late phases of fatigue of the
rectus femoris muscle? Explain.
2.
Are observed differences compatible with previous research findings?
3.
What is the neuromuscular basis for the observed differences? Explain
Experiments 1-7
1.
How can EMG be used as a tool to support study in your academic
interest area?