Download ELECTROMYOGRAPHY (EMG) Lab

ELECTROMYOGRAPHY (EMG) Lab
Overview:
Today you will use physiological recording equipment interfaced with a computer to investigate the activity of
different muscles. The lab consists of two parts:
1) You will design experiments in which your lab group will decide which muscles to record EMGs
from, and what activities those muscles will perform.
2) You will also be combining electromyography with dynamometry to examine motor unit
recruitment and skeletal muscle fatigue.
Introduction:
• Electromyography (EMG) is the recording of the electrical activity associated with skeletal muscle
contraction. When a nerve impulse travels down a motor neuron, it reaches all of the neuromuscular
junctions supplied by that motor neuron. This generates impulses in the muscle cell membranes of all
the muscle cells within the motor unit. The electrical activity from the muscle cells is conducted
through body fluids to the skin, where it can be picked up by surface electrodes, amplified and recorded.
The recordings thus obtained are called electromyograms.
• Dynamometry is the study of power output of muscles. As muscle fibers contract, they produce force or
tension. We will be using a hand dynamometer that is equipped with an electronic transducer to record
the force exerted by the muscles on the transducer.
• A single action potential produces a single muscle contraction known as a muscle twitch. Since action
potentials are an all-or-none event, muscle twitches are also all-or-none.
Muscle Twitch
Calibration of a 50g weight •
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A motor unit is a single motor neuron and the group of muscle fibers it innervates. For weak
contractions, only a few motor units need to be activated, while for stronger contractions, additional
motor units can be activated. The activation of increasing numbers of motor units is known as motor
unit recruitment. We can measure the relative amount of motor unit recruitment by the overall
amplitude of the measured EMG.
Wave summation – impulses are delivered in succession, the second twitch is stronger than the first
Complete tetanus – rapid stimulation results in sustained smooth contraction without relaxation
Summation Tetanus Fatigue Increasing frequency of stimulation Tension – force muscle exerts on an object when contracted
Isotonic contraction – muscle length changes during contraction while tension remains constant
o Concentric - muscle shortens and does work
o Eccentric – muscle contracts as it lengthens
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Isometric contraction– tension increase but muscle length remains the same
Tonus or muscle tone is the small amount of tension produced by a resting muscle. This results from
the periodic activation of a small number of motor units. This process cycles through all of the motor
units, a few at a time, and functions to maintain the muscles in a state of readiness.
Fatigue is the decline in muscle tension as a result of previous contractile activity. Muscles are
considered fatigued when contracting at 50% less than their maximum strength.
o Psychological fatigue results from reduced input from the central nervous system and usually
occurs before the physical capabilities of the muscle are fully depleted. In psychological fatigue,
we would expect that the EMG and muscle tension should both decline at equal rates
o Physiological fatigue results from the depletion of ATP at a rate faster than it can be replenished
through respiration as well as a buildup of lactic acid (from anaerobic metabolism). In contrast,
in physiological fatigue, the muscles are continuously being stimulated by the motor neurons so
we would expect the EMG to remain high even as the tension declines.
Experimental Protocol:
1a) EMG in agonist muscle
1.
Choose an agonist muscle to record from.
2.
Choose 4 activities that will alter muscle force from light, medium, heavy, and overloaded (isometric
contraction)
3.
Clean the skin area above the muscle with an alcohol swab and allow to dry.
4.
Remove all jewelry or other metal objects and have the subject sit or stand in a comfortable position that
will minimize extraneous movement.
5.
Firmly attach the 3 gel-filled electrode pads:
1) to the proximal belly of the muscle,
2) the distal belly of the muscle and ground
3) near the distal belly of a nearby muscle.
NOTE: Please do not waste electrode pads. Once they are attached, do not remove them until the end of the
lab. Plan to use each site as the ground or recording electrode for another experiment.
6.
Clip the electrode wires to the electrodes. The red and white leads should be attached to the muscle being
measured, and the black lead to the third site (this is the ground electrode). A clip only works on one
side; if it doesn't attach, flip it over and clip it in place.
NOTE:Jewelry or other metal objects, excessive extraneous movements, or poor connections may cause
interference in data collection.
7.
GENTLY Plug the 3-lead electrode cable transducer into Channel 1 on the BIOPAC data acquisition box.
Turn the box on using the rocker switch on the back of the box. When instructed to record an EMG from
a second muscle later in part 1b, you will plug another cable into Channel 2.
2 8.Your instructor will sign-in to the lab computer for you.
9.Start the software by clicking the “EMG” file, located on the desktop. This will launch the BIOPAC
Student Lab PRO software.
10. Check that Channel 1, and P-P are selected in the upper left hand boxes above the graph.
11. Record an EMG during the 4 activities you have selected, click START to begin recording your
experiment, and STOP to end it. It is okay to stop in between activities while your subject resets. The
data will not be lost.
12. Analyze your data for experiment 1a. Select the I beam tool ( ). Click and drag it through the first
EMG cluster. Note the p-p (peak-to-peak) value that appears in the window above the graph. This
millivolt value can be used as an indication of the number of muscle fibers stimulated. Record your
results on your data sheet and repeat for the remaining clusters.
1b) EMG in agonist and antagonist muscle simultaneously
1.Apply 3 electrodes to the antagonist muscle group opposite the muscle you used in part 1a.
2.Attach another set of positive, negative and ground clips to the second muscle group.
3.Plug the second cable into Channel 2 on your BioPac.
4.Check that Channel 1, and P-P are selected in the upper left hand boxes above the graph, and in the next
boxes over Select Channel 2 and P-P.
5.As a group select a muscle action for the subject to perform that will allow you to record an EMG of the
agonist and antagonist muscles simultaneously.
6.Click START to begin recording your experiment, and STOP to end it. It is okay to stop in between
activities while your subject resets. The data will not be lost
7.Analyze your data for experiment 1b. Select the I beam tool ( ). Click and drag it through the first EMG
cluster. Note the p-p (peak-to-peak) values that appear in the windows above the graph. This millivolt
value can be used as an indication of the number of muscle fibers stimulated in each muscle group.
Record your results on your data sheet and repeat for the remaining clusters.
1c) EMG in agonist during concentric and eccentric actions
1.Choose 2 actions that will demonstrate concentric contraction and eccentric contraction in the agonist
muscle only.
2.You will now only record from the agonist muscle. Remove the wires from the antagonist muscle used,
you may leave the electrodes in place for future experiments.
3.Check that Channel 1, and P-P are selected in the upper left hand boxes above the graph.
4.Record an EMG during the activities you have selected, click START to begin recording your
experiment, and STOP to end it. It is okay to stop in between activities while your subject resets. The
data will not be lost
5.Analyze your data for experiment 1c. Select the I beam tool ( ). Click and drag it through the first EMG
cluster. Note the p-p (peak-to-peak) value that appears in the window above the graph. This millivolt
value can be used as an indication of the number of muscle fibers stimulated. Record your results on
your data sheet and repeat for the remaining clusters.
When you are finished, quit the program. Quit is located in a pull-down menu under “Biopac Student Lab
PRO” in the menu bar. Do not save your changes.
3 Experimental Protocol For Part 2:
1. Plug the hand dynamometer into Ch 2 on the front of the data acquisition box - you do not need to
tighten the 2 screws.
2. Plug the 3-lead cable into Ch 1 of the data acquisition box.
3. For the first recording, select the subject’s dominant forearm to attach the electrodes to. The white lead
should be attached to the proximal anterior forearm, the red lead on the distal forearm, and the black
lead to a third site (this is the ground electrode). Before attaching the electrode pads, scrub the skin
areas gently with an alcohol pad to remove some dead cells. This will increase electrical conductivity.
Then firmly apply the gel-filled electrode pads to the three prepared areas.
4. Clip the electrode wires to the electrodes. See electrode placement below:
5. Open up the Bio Pac Student Lab program by moving the mouse to the bottom of the desktop and
clicking on the folder titled Bio Pac Student Lab on the pop up menu.
6. Choose lesson L02-EMG-2 and click OK.
7. Enter a name such as the subject’s name for your filename.
Calibration:
8. Click on Calibrate and follow the onscreen prompts:
a. Set the hand dynamometer flat in your palm and do not apply pressure, click OK.
b. When prompted wait about two seconds and then clench the dynamometer with maximum strength
using your dominant forearm (as shown in image below), then release.
c. Wait for calibration to stop automatically.
d. A screen with your calibration recording will pop up that should look similar to the image below.
If your recording does not begin with a zero baseline, click Redo calibration. If it does proceed
to Record.
4 Collecting Data:
9. The subject will perform three muscle contractions using increasing strengths with 15-seconds of rest
between each contraction. Have a group member instruct the subject to do the following:
a. Click record, wait 5 seconds
b. Hold a weak contraction for 5 seconds
c. Rest for 15 seconds
d. Medium contraction for 5 seconds
e. Rest for 15 seconds
f. Maximum contraction for 5 seconds
g. Click Suspend to pause recording
10. Your data should look something like this:
11. After resting for 2 minutes click Resume to record the following contraction pattern:
a. Record a maximal strength contraction and sustain your grip for as long as possible. Relax your
grip only when you reach fatigue. (This is usually when your force has decreased by 50% or
more of your original max clench so make sure to note your original max force when you start this
exercise).
12. Click Suspend to pause recording. Your data should look something like this:
13. Click Stop and then Done when all your experiments are successfully recorded.
14. A pop up window will appear, select Yes, then click Done.
15. Unclamp electrode cables from electrodes on the subject.
16. Peel off electrodes and throw them out. They are not reusable.
17. Wash electrode gel from subject’s skin with soap and water.
Analyzing Data:
18. Select Analyze Current Data File from the menu that appears in the window.
19. Three recording panels will be visible (CH 1) EMG, (CH 41) force, (CH 40) integrated EMG.
We will be analyzing data in the first two panels only.
20. Locate the measurement boxes near the top of the data window.
21. Each measurement has a set of three boxes to it: channel number, measurement type, and results / value.
The drop down menu for measurement type presents a number of options, you will likely utilize the
following:
5 a. p-p = the minimum to maximum spread of peak amplitude values within the selected range of
data
b. mean = the average value within the selected range of data.
c. value = the exact value of the datum at the specific cursor location.
d. delta T = the time span covered by the selected range of data.
22. Set up the following measurement tools by clicking on them to activate the drop down menu. The
menus should be set as follows, from left to right:
23. Click on the I-beam tool at the bottom right of the data window. Select a data segment for analysis, by
placing the I-beam at the left edge of the portion of the data you wish to analyze, hold down the left
mouse button, and drag the I-beam across to the end of the portion you wish to measure. The selected
area will be indicated in black (as shown below) and the shaded area’s value will be displayed in the
measurement boxes at the top of your screen.
Ch 41 mean Ch 40 mean
24. Measure the mean force and mV (EMG) for the first weak clench
by your dominant hand. Drag the I-beam from the left portion of
the contraction “plateau” to the right end of the “plateau” (see
example at right); do not include the rising and falling portions of
the clench. Repeat these measurements for remaining
contractions and record the following results in the table on your
data sheet:
a. Forearm 1 (dominant hand), weak contraction, medium
contraction, and maximum contraction
25. Select a 1-second wide area of the EMG while you were relaxed,
such as to the left of your first contraction. Do not include any
rising or falling portion of the recorded clenches. This
measurement will be used to determine the electrical activity
produced during tonus. Record this data on your data sheet.
26. To determine the time to fatigue, scroll over to the part of the
graph that contains this data and change your measurement boxes
to read Ch 41: max and Ch 40: delta-T from the drop down
menu.
a. To find the max force produced, move your I-beam to the start of the sustained clench. Drag it to
the right to highlight contraction. Note the max value gives the magnitude of the data at the exact
location of the I-beam cursor. Make note of the max value and record it on your data sheet.
b. Divide the above number by 2 to calculate 50% of max force.
c. Change Ch 41 to p-p. Drag the cursor from the start of the clench to the right until you see the
value drops to 50% of the max clench force that you just determined.
d. Stop dragging the cursor and record the data for time to fatigue (from the Ch 40 delta-T box).
27. Change the Ch 40 measurement box to mean.
28. Starting from the 50% maximum force value, scroll to your left to the maximum force value back at the
beginning of your contraction. Record the CH 40 mean value before fatigue on your data sheet.
29. Starting from the fatigue value again (50% of maximum force), now click and scroll to your right to
select the rest of the sustained clench. Record the Ch 40 mean value for after fatigue on
your data sheet. Clean up
30. Gently unplug the 3-lead cable from CH 1 and the dynamometer from CH 2 on the Biopac.
31. Quit the Biopac Student lab program. Do not save your data.
6 NAME_____________________________
EMG PART 1 – DATA SHEET
1. Varying Force of Contraction Data
Light Weight Object: ________________
Medium Weight Object: ______________
Heavy Weight Object: ________________
Overload Object: ____________________
Muscle in use: ______________________
Light
Weight (mV)
Medium
Weight (mV)
Heavy
Weight (mV)
Overload (mV)
1. What is the EMG measuring? _________________________________________________________
2. Was there a difference in your data? If so, what specific changes occurred in your data when it
contracted with different degrees of force? __________________________________________________
_____________________________________________________________________________________
__________________________________________________________________________________________________________________ 3. What is the difference between an isotonic and an isometric contraction? ________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
4. Which of the above loads resulted in an isometric contraction? _________________________________
2. Agonist/ Antagonist Data
Agonist muscle:_____________________
Agonist (mV)
Antagonist muscle: __________________
Antagonist (mV)
7 1. Define agonist (prime mover) and antagonist in the space below. ______________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
2. Describe and explain the differences that you observe in the EMGs recorded from the two muscles
during different actions._______________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
3. Concentric/ Eccentric Data
Muscle in use: _________________________
Action: ___________________________
Concentric (mV) Eccentric (mV)
1. Define concentric and eccentric contractions. ______________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
2. Explain any differences that you observe between the two types of contraction. ___________________
_____________________________________________________________________________________
_____________________________________________________________________________________
8 EMG PART 2- DATA SHEET
I. COLLECTION OF DATA:
Subject’s Profile:
Name:___________________
Dominant forearm:
Right / Left
Varying Clench Strength Data:
Forearm 1 (Dominant)
Mean Force (kg) EMG Data (mV)
(CH 41)
(CH 40)
Mean
Mean
Weak
Contraction
Medium
Contraction
Maximum
Contraction
Tonus Measurements:
EMG Data (mV) during relaxation: __________mV
Estimate the percentage of motor units active by dividing the EMG value for tonus
by the EMG when all motor units are active at maximum contraction. Express as a percentage by multiplying
by 100%. Show calculation below:
Fatigue Data:
Forearm 1 (Dominant)
Max Force (kg) 50% of
Time to
(CH 41)
max clench Fatigue
Max value
force (kg) (CH 40)
Delta T value
9 Forearm 1 (Dominant)
EMG Data (mV)
(CH 40) Mean
Before fatigue
After fatigue
II. ANALYSIS OF DATA
1. What specific changes occur in a muscle when it contracts with different degrees of force? Describe
and explain the differences that you observe in the EMGs recorded. What accounts for these
differences? ________________________________________________________________________
___________________________________________________________________________________
___________________________________________________________________________________
2. Why did the EMG show activity when the subject’s hand and forearm were completely relaxed? _____
____________________________________________________________________________________
3. When you are holding an object for a short period of time does the number of motor units remain the
same? Are the same motor units used throughout the duration of holding the object? Explain. _______
__________________________________________________________________________________
__________________________________________________________________________________
4. Using your fatigue data, compare the force and mean EMG values prior to fatigue and after fatigue. As
force declined, did the EMG values remain constant or did EMG values and force decline in
proportional amounts? ________________________________________________________________
___________________________________________________________________________________
5. Based on the results in question 4 was the decline in strength the result of psychological or
physiological fatigue? Explain. __________________________________________________
___________________________________________________________________________________
___________________________________________________________________________________
6. As you reach physiological fatigue, the force exerted by your muscles decreased. What physiological
process explains the decline in strength? __________________________________________________
___________________________________________________________________________________
___________________________________________________________________________________
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