Download Exercise 5: Muscle Physiology I

Exercise 5: Muscle Physiology I - Electromyography
Readings: Silverthorn, 6th ed. pg. 410 – 420
Your brain communicates with your muscles through action potentials on the motor
neurons, which are then transmitted across the neuromuscular junction to the surface of the
muscle fiber. The muscle fiber action potential triggers the release of calcium into the cytoplasm,
which in turn results in the initiation of the actin-myosin sliding filament mechanism responsible
for muscle contraction. The coordination of these electrical and mechanical events is called
excitation-contraction coupling (see Fig.1). A single contraction-relaxation cycle in a skeletal
muscle fiber that results from a single action potential is known as a twitch.
Figure 1. Excitation-Contraction Coupling (Fig. 12-10, from Silverthorn textbook)
There are two ways that the force of a muscle contraction can be increased. One way is
called summation. During normal voluntary contraction, the motor neuron releases a volley of
action potentials, rather than one at a time. Increasing the frequency, or rate, of these action
potentials from a single motor neuron results in an increase in the force of muscle contraction, or
summation.
The second way to increase the force of muscle contraction is called recruitment and
involves increasing the number of motor units that contract simultaneously in a whole muscle.
Each motor unit is composed of many muscle fibers that are innervated by a common motor
neuron. More motor neurons firing simultaneously will cause more motor units to contract
together, which produces a greater force, or recruitment. We will be learning more about both
summation and recruitment in next week's lab exercise, but this week we will focus only on
recruitment.
Each motor unit is made up of one of three muscle fiber types: slow twitch fibers,
intermediate twitch fibers, and fast twitch fibers. Each fiber type has its own advantages and
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disadvantages. Slow twitch fibers are resistant to fatigue, but develop tension the slowest (hence
the name) and are the weakest. Fast twitch fibers fatigue more easily, but develop tension more
quickly and produce more force. Slow twitch fibers are able to sustain a lower tension for longer
periods of time, while fast twitch fibers produce stronger contractions for shorter periods of time.
If a voluntary muscle contraction is sustained for a long enough period of time, however,
even slow twitch muscle fibers will begin to tire. As the tension produced by these motor units
decreases with fatigue, stimulus from the brain will increase and result in the recruitment of more
and more motor units, until all the motor units in a muscle are being used.
Recruitment of motor units always occurs in the same sequence: slow twitch motor units
are recruited first, then intermediate twitch motor units, then fast twitch motor units. The motor
neurons of slow twitch motor units have the lowest threshold and so are the first to respond to a
weak stimulus. As the stimulus increases (ie. the brain tells the muscle to produce more force),
threshold is reached next for the intermediate twitch motor units, then finally the fast twitch
motor units.
This increase in motor unit activity and the corresponding electrical activity on the
sarcolemma of the muscle fibers can be detected by electrodes placed on the skin. A recording of
this electrical activity is called an electromyogram or EMG. In an unfatigued muscle, the
electrical activity is proportional to force production. In other words, the recorded EMG signal
reflects motor unit recruitment and summation taking place within the muscle tissue; greater
force is reflected by an increase in the EMG activity. An EMG is a quantitative measure of
muscle activity for a given workload. As fatigue sets in, however, the relationship between force
and electrical activity changes. An action potential on the muscle fiber will not produce greater
force if the muscle fiber is fatigued.
Electromyography can be used to diagnose the cause of muscle weakness.
Electromyography can help to differentiate primary muscle conditions, such as muscular
dystrophy, from muscle weakness caused by neurological disorders, such as multiple sclerosis.
EMG is also used in studies of kinesiology and exercise physiology.
In today's lab, we will be using the EMG to observe electrical activity of your arm
muscles and the resulting motor unit recruitment. Keep in mind that the electrical activity we are
detecting is primarily the result of action potentials generated on the sarcolemma of the muscle
fibers, not the electrical activity of the motor neurons. Consider the difference in surface area
between a motor neuron and the muscle it innervates.
Today’s Objectives
1.
Investigate the EMG response in the forearm flexors to increasing workloads.
2.
Investigate the EMG response to fatigue when the muscle is fully recruited.
3.
Investigate the EMG response to fatigue when the muscle is partially recruited.
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Getting Started
1.
The equipment for today’s experiment is listed below. You should be able to explain the
purpose of the dynamometer, electrode leads, and electrodes.
•
Computer
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Biopac MP35 Acquisition unit
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BIOPAC electrode lead set (SS2L) plugged into Ch. 1
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BIOPAC Hand Dynamometer (SS25LA) plugged into Ch. 2
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Disposable paste-on electrodes
•
Biopac Student Lesson L02 - Electromyography (EMG) II
A dynamometer is a device that measures force. In this case, squeezing the dynamometer
generates the force that is converted to electrical activity and displayed on the computer. The
unit of force is kilograms.
2.
The subject should remove all jewelry from wrists and ankles.
Clean and scrub three regions on the forearm for electrode attachment using an alcohol pad.
The first area is about 2 inches below the anterior cubital fossa of the elbow over the mass of
forearm muscles that originate on the medial epicondyle of the humerus (Remember your
anatomy?). If you aren't sure where this is, clench your fist to make the muscle bulge. The
second and third are distal, over the lateral and medial anterior surface of the wrist. Let the
areas dry after scrubbing with the alcohol pad. Refer to Fig. 2 for placement of the
electrodes.
Figure 2. Electrode and Lead Placement
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3.
Snap the ends of the three color-coded electrode cables onto the disposable electrodes
attached to the subject's arm:
•
the white “VIN-” lead is attached to the electrode near the elbow.
•
the black “GND” (ground)lead is attached to the electrode on the medial (little finger)
side of the wrist.
•
the red “VIN+” lead is attached to the electrode on the lateral wrist.
The pinch connectors work like a small clothespin and latch onto the nipple of the electrode.
Be sure the metal backing of the connectors is in contact with the metal nipple.
4.
5.
Use the checklist below to ensure that the basic set-up is working:
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Turn the computer on.
•
Check that the MP35 unit (blue box) is plugged in to a working electrical outlet.
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Check that the MP35 unit is connected to the computer via a black USB cable.
•
Turn on the MP35 unit. Wait until the "Busy" light goes off before launching the
Bioipac Student Lab application.
•
Launch the Biopac software application, click on the Biopac Student Lab application
icon in the Mac dock, or BSL Student Lessons 3.7.7 application on the PC desktop.
Choose lesson "L02 - Electromyography (EMG) II".
Type your file name in the box that opens. Choose a name that identifies and distinguishes
you from others, e.g. janedoe EMG. Write down your file name for future reference.
File Name: ____________________________________________
6.
Once the BioPac data collection window opens, follow the prompts to calibrate the
dynamometer.
a.
Place the dynamometer on the table and click the Calibrate button in the left upper
corner of the screen. A window will open to remind you to put down the dynamometer.
Click OK. This sets the zero baseline. Pick up the dynamometer with the short grip bar
against the palm of your hand. See Figure 3.
Figure 3. Grip position for the dynamometer.
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Another window will open up instructing you to pick up the dynamometer. The calibration
will begin recording as soon as you click OK, so be ready. Once you click OK, wait 2
seconds, then squeeze the dynamometer as hard as you are able for 2 seconds, then release
the dynamometer and allow the calibration to finish. Monitor the passing of time on the Xaxis to determine how long 2 seconds lasts. This sets the highest possible value. The
Calibration recording should look something like Figure 4. If it doesn’t, then repeat the
calibration. If it does, click the Continue button. A new screen will appear with a Record
button in the top left corner. Note that all command buttons will appear in the upper left
corner as you progress through the different screens.
Figure 4. Calibration Screen.
The software sets the scale of the display based on your calibration grip force so that the
recording fits nicely in the window.
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Muscle Fiber Recruitment With Increasing Force;
Muscle Fiber Recruitment and Fatigue
At this point, you will no longer be following the prompts and instructions displayed on the
computer screen. Follow only these written instructions. You will collect all the data for both
the increasing force demonstration and fatigue experiment before you analyze them:
1.
•
First segment: Demonstrates motor unit recruitment as the dynamometer is squeezed or
clenched with increasing force on the dominant forearm.
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Second segment: Demonstrates fatigue in dominant forearm.
Read all of the following instructions in this section before proceeding. When you click
Record (upper left corner of screen), the recording will begin and an append marker labeled
“Dominant arm: increasing clench force” will appear in the marker window at the top of the
screen. The screen will change to display only the dynamometer channel, and a grid will
appear so that you can visually estimate the force you are exerting. You can watch the
screen and adjust the force of your clench as needed. Be sure to hold your arm still while
you are recording and move only the muscles you need to squeeze the dynamometer.
Remember that EMG is recording all muscle activity in your arm, even if you are not
squeezing the dynamometer and no force appears on the screen.
You are going to make a series of five clenches of increasing strength. To determine how
hard you should squeeze each time, first determine the maximum force of your handgrip:
•
Click Record, wait 2 seconds, squeeze as hard as possible for 2 seconds, release your
grip, wait another second or two, then click Suspend.
•
Divide your maximum force by five. This is how much you will increase the strength of
your clenches with each squeeze. For example, if your maximum was 15 kg, your first
clench will be 3 kg, the second clench will be 6 kg, the third will 9 kg, and so on.
•
Take a practice run first: Click Redo, then Yes when the computer asks if you are sure
you want to record over your previous data. To begin recording, click Resume. Watch
the Y-axis as you squeeze. You may initially squeeze too hard or not hard enough. The
important thing is to sustain the desired force for at least 2 seconds. Your clench may
last a bit more than 2 seconds as you adjust your grip. Pause two seconds before your
first clench, between each succeeding clench, and before you click Suspend after your
final clench.
3.
Now you are ready to actually record your data. If all goes well, your recording should look
something like Figure 5. If it doesn’t, you can redo your recording as many times as you
need by clicking the Redo button. But don't tire out your muscles - save some strength for
the next segment!
4.
When you are satisfied with the recording of your five clenches, click Continue.
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Figure 5. EMG recording for clenches of increasing strength.
5.
In Segment 2, you will hold a single sustained clench at HALF maximum force for as long
as you can stand it. This experiment will demonstrate how additional muscle fibers are
recruited as fatigue sets in. Ignore the instructions in the window at the bottom of the screen,
which tell you to clench the dynamometer with your maximum force. Keep some muscle
fibers in reserve for later.
•
This experiment can only be done once, so make sure you understand the instructions
before beginning.
•
You will be using a clench force that is half of what you determined to be your
maximum in the previous segment. For example, if you maximum force was 15 kg,
squeeze the dynamometer to produce a continuous force of about 7 to 8 kg.
•
Remember that the EMG records all muscle activity, so be sure to start out in a
comfortable position. If you change your arm position in the middle of your recording,
this motion will be recorded and make your data difficult to interpret.
•
Click Resume to begin recording. Squeeze at half force until your muscles fatigue to
the point of quivering. Click Suspend to stop recording when you have had enough.
Ideally, your muscles should be fatigued to the point of quivering.
It is very important that you maintain the same force throughout and that you do not
stop until your arm is too fatigued to continue squeezing.
6.
Click Stop to complete your recording and Yes when asked if you are sure. A new screen
will open with Listen and Done buttons at the top. We won't be listening to the EMG with
headphones, so click Done. When the dialog box asks if you are sure, click Yes, then choose
"Analyze Current Data File", then OK.
If you want to analyze previously recorded data, you can choose “Analyze previously
recorded data” instead. This should open up a finder window where you can find your saved
file buried under L02-EMG-2 > Libraries > Documents > BSL lessons > Data Files > File
Name (folder) > File name. You can also get to a saved file from any data screen by going to
the Lessons pull-down menu and choosing “Review Saved Data” at the bottom of the menu.
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Analysis of the Data
Refer to the “Physiological Instrumentation Tutorial" from last week that introduced you to the
Biopac equipment to refresh your memory about how to navigate the analysis functions. Set up
your display for analysis:
•
Close the journal window at the bottom of the screen by dragging the top of the Journal box
to the bottom of the screen. Your display should look something like Fig. 6.
Figure 6. Force and EMG Integral channels displayed.
•
Select the channels and functions you want to analyze in the first three measurement regions
(refer to pg. 44 of the Tutorial for a description of the channels and analysis functions):
1st: Ch 41 (blue), function: Mean
2nd: Ch 40 (green), function: Mean
3rd: Ch 41 (blue), function: Delta T
Increasing Clench Force
1.
For the first part of the analysis, use the data labeled “Dominant arm: increasing clench
force” recorded in Segment 1.
Click on the zoom tool, located at the right lower corner of the screen. Select the clench data
from segment 1 by clicking and dragging a box around all five force peaks.
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You may need to adjust the trace so that the peaks and valleys are visible by going to the
Display menu and selecting Autoscale Waveforms to enhance the visibility of the data. See
Figure 7.
Figure 7. Analysis window zoomed into the five clenches.
2.
Use the I-Beam cursor to select a flat area on the top of the plateau phase of the first clench.
Record the mean Force and mean EMG Integral values for each clench in Table 1 below.
These values will be found in the measurement function boxes above the marker region in
the data window. “Mean” displays the average value in the selected area. You will be using
the mean of the force exerted on the dynamometer by the clench (kg) and the mean of the
integrated EMG (mV-sec).
Table 1: Increasing Force of Separate Clenches
Dominant Arm (Mean)
Clench
FORCE (kg)
EMG Integral (mV-sec)
1
2
3
4
5
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3.
Using the data recorded in Table 1, make a graph of the relationship between the force
production and the electrical activity by plotting the Force on the X-axis and the EMG
Integral on the Y-axis. This arrangement may seem counterintuitive, but in this particular
experimental design, your brain is determining the force. This becomes the independent
variable. The dependent variable is the response being recorded and measured, which is the
EMG Integral. Don't forget to include the units in your labels.
Questions
1. Describe the molecular events that occur along the path of electrical
stimulation from the motor neuron to the sarcolemma of the muscle fiber. In
other words, describe the mechanism of excitation–contraction coupling that
links the electrical activity of the action potential on the sarcolemma to the
molecular events of contraction.
2. What are the names and functions of the two pieces of equipment the subject
was connected to in this exercise?
3. What is the source of the electrical signal picked up by electrodes on the
subject's skin? What is an electromyogram? Why is the EMG Integral used,
rather than the actual EMG?
4. Based on the graph you made, what conclusions can you draw about the
relationship between Force and electrical activity (EMG Integral) for each of
the five increasing clenches?
5. Define recruitment in muscle. Discuss the relationship between recruitment,
motor units and your data. In other words, how did the data you collected
demonstrate the concept of recruitment?
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Analysis of EMG Activity in Response to Fatigue
Since excitation of additional muscle fibers is required for recruitment, an increased EMG
recording indicates recruitment is occurring even if additional force cannot be generated. For the
last set of data analysis, we’ll be comparing the first two seconds of the recording to the last two
seconds and compare the amount of electrical activity generated on the muscle per kilogram of
forced produced.
1.
Use Delta T displayed in the third measurement results box to select the first 2 seconds of
the recording after the clench has reached a plateau (see Fig. 8). Record the mean Force and
mean EMG Integral in Table 2.
Note: If your recording has a peak at the very beginning, avoid that region when measuring
the first two seconds of data. Instead, use the section that immediately follows the peak.
Figure 8. Measuring force and EMG integral in the first 2 seconds of the fatigue experiment.
2.
Move forward through your recording to the last two seconds of the half-force clench. Again
using the Delta T display, highlight the 2 seconds of the recording that occurs right before
you released the squeeze (see Figure 9). Record the mean Force and mean EMG Integral in
Table 2.
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Figure 9. Measuring the last 2 seconds of the fatigue experiment.
3.
It is impossible for anyone to maintain the exact same half-maximal force throughout this
experiment. To distinguish between changes in the EMG integral due to simple changes in
force and changes in EMG due to increased recruitment, we will calculate the electrical
activity of the EMG per 1 kg of force. We will call this the Unit Activity, or mV-sec/Kg.
This represents the amount of excitation on the muscle needed to generate 1 Kg of force.
Table 2: EMG Activity During Continuous Sub-Maximal Force
Recruitment
Step 1 & 2: Measure the Mean
First 2 seconds
Last 2 seconds
EMG Integral (mV-sec)
Force (Kg)
Step 3: Calculate Unit Activity
(mV-sec/Kg)
Step 4: Percent Change (%)
•
Calculate the Unit Activity by dividing the EMG Integral by Force:
EMG Int = Unit Activity in mV-sec
Force
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4.
Calculate the change in electrical activity as the muscle fatigues by determining the percent
change in EMG activity/Kg force. Use a "+" sign to indicate an increase in activity and a "–"
sign to indicate a decrease in activity.
Unit Activity for last 2 seconds – Unit Activity for first 2 seconds X 100 = % change
Unit Activity for first 2 second
QUESTIONS:
6. What was the purpose of holding the clench at half your maximum strength
rather than at full strength?
7. Did the EMG Integral increase or decrease as your arm fatigued? How do you
explain this?
8. How does the concept of recruitment apply to muscle as it fatigues?
9. Motor units are recruited by motor unit type: slow-twitch first, then
intermediate, then fast-twitch motor units. How do these motor unit types
differ? What is the cause for the order of recruitment?
10. The cause of muscle fatigue is no longer as clear as it was once believed.
What are some of the possible causes of muscle fatigue?
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