Introduction to Bioengineering
Skeletal Muscle Physiology – Recruitment and Fatigue
I. OBJECTIVES
1. To understand skeletal muscle and electromyography.
2. To acquire and analyze data with BIOPAC system.
3. To compare the maximum clench force and fatigue time of different subjects.
4. To study motor unit recruitment with increased power of muscle contraction.
5. To study the force produced by contraction, EMG and integrated EMG during fatigue.
II. BACKGROUND
The introduction sections from Lessons 1 and 2 of BIOPAC Student Lab V3.0 manual
are provided below to introduce electromyography.
The human body contains three kinds of muscle tissue and each performs specific tasks to
maintain homeostasis: Cardiac muscle, Smooth muscle, and Skeletal muscle.
Cardiac muscle is found only in the heart. When it contracts, blood
circulates, delivering nutrients to cells and removing waste.
Smooth muscle is located in the walls of hollow organs, such as the
intestines, blood vessels or lungs. Contraction of smooth muscle changes
the internal diameter of hollow organs, and is thereby used to regulate the
passage of material through the digestive tract, control blood pressure and
flow, or regulate airflow during the respiratory cycle.
Skeletal muscle derives its name from the fact that it is usually attached to
the skeleton. Contraction of skeletal muscle moves one part of the body
with respect to another part, as in flexing the forearm. Contraction of
several skeletal muscles in a coordinated manner moves the entire body in
its environment, as in walking or swimming.
The primary function of muscle, regardless of the kind, is to convert chemical energy to
mechanical work, and in so doing, the muscle shortens or contracts.
Human skeletal muscle consists of hundreds of individual cylindrically shaped cells
(called fibers) bound together by connective tissue. In the body, skeletal muscles are
stimulated to contract by somatic motor nerves that carry signals in the form of nerve
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impulses from the brain or spinal cord to the skeletal muscles (Fig. 1.1). Axons (or nerve
fibers) are long cylindrical extensions of the neurons. Axons leave the spinal cord via
spinal nerves and the brain via cranial nerves, and are distributed to appropriate skeletal
muscles in the form of a peripheral nerve, which is a cable-like collection of individual
nerve fibers. Upon reaching the muscle, each nerve fiber branches and innervates several
individual muscle fibers.
Figure 1 Example of Motor Units
Although a single motor neuron can innervate several muscle fibers, each muscle fiber is
innervated by only one motor neuron. The combination of a single motor neuron and all
of the muscle fibers it controls is called a motor unit (Figure 1). When a somatic motor
neuron is activated, all of the muscle fibers it innervates respond to the neuron’s impulses
by generating their own electrical signals that lead to contraction of the activated muscle
fibers.
Note: The size of the motor unit arrangement of a skeletal muscle (e.g., 1:10, 1:50, or 1:3000) is
determined by its function (flexion, extension, etc.) and the location in the body. The smaller the size of a
muscle’s motor units, the greater the number of neurons needed for control of the muscle, and the greater
the degree of control the brain has over the extent of shortening. For example, muscles which move the
fingers have very small motor units to allow for precise control, as when operating a computer keyboard.
Muscles that maintain posture of the spine have very large motor units, since precise control over the extent
of shortening is not necessary.
Physiologically, the degree of skeletal muscle contraction is controlled by:
1. Activating a desired number of motor units within the muscle, and
2. Controlling the frequency of motor neuron impulses in each motor unit.
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When an increase in the strength of a muscle’s contraction is necessary to perform a task,
the brain increases the number of simultaneously active motor units within the muscle.
This process is known as motor unit recruitment.
Resting skeletal muscles in vivo exhibit a phenomenon knows as tonus, a constant state
of slight tension that serves to maintain the muscle in a state of readiness. Tonus is due
to alternate periodic activation of a small number of motor units within the muscle by
motor centers in the brain and spinal cord. Smooth controlled movements of the body
(such as walking, swimming or jogging) are produced by graded contractions of skeletal
muscle. Grading means changing the strength of muscle contraction or the extent of
shortening in proportion to the load placed on the muscle. Skeletal muscles are thus able
to react to different loads accordingly. For example, the effort of muscles used in
walking on level ground is less than the effort those same muscles expend in climbing
stairs.
When a motor unit is activated, the component muscle fibers generate and conduct their
own electrical impulses that ultimately result in contraction of the fibers. Although the
electrical impulse generated and conducted by each fiber is very weak (less than 100
μvolts), many fibers conducting simultaneously induce voltage differences in the
overlying skin that are large enough to be detected by a pair of surface electrodes. The
detection, amplification, and recording of changes in skin voltage produced by
underlying skeletal muscle contraction is called electromyography. The recording thus
obtained is called an electromyogram (EMG).
Mechanical work, in the physical sense, refers to the application of a force which results
in the movement of an object. Skeletal muscle performs mechanical work when the
muscle contracts and an object is moved, as in lifting a weight. To lift a weight, your
muscles must exert a force great enough to overcome the weight. If you exert less force,
then the weight does not move (Figure 2).
Figure 2. Lifting a weight with a force great enough to overcome
the weight or with a force not great enough to overcome the weight.
Physiologically, skeletal muscle is stimulated to contract when the brain or spinal cord
activates motor units of the muscle.
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Note: Motor units are defined as a motor neuron and all of the muscle fibers that the motor neuron
innervates. An action potential (AP) in a human motor always causes an action potential in all of the
muscle fibers of the motor unit. As a matter of fact, humans generally do not send just one AP at a time
down a motor neuron. Instead, a train of APs is sent – enough to induce tetany (the sustained fusion of
individual muscle twitches) in the muscle fibers of the motor unit.
Most human skeletal muscles are composed of hundreds of motor units (see Figure 1
again). When a skeletal muscle is called on to perform mechanical work, the number of
motor units in the muscle activated by the brain is proportional to the amount of work to
be done by the muscle: the greater the amount of work to be done, the greater the number
of motor units activated. Thus, more motor units are simultaneously active when a
skeletal muscle lifts 20 kilograms than when the same muscle lifts 5 kilograms.
The brain determines the number of active motor units required for a muscle to perform a
given task by utilizing sensory information from stretch receptors in the muscle and
associated tendons and joint capsules. For example, when lifting a bucket of water from
the ground, the brain first activates several motor units in the requisite skeletal muscles.
If sensory information returning from the muscles indicates the muscles are contracting
but not developing adequate power to lift the bucket, the brain activates additional motor
units until the sensory information indicates the bucket is being lifted. The sequential
activation of motor units to perform a designated task is called motor unit recruitment.
Once you have lifted a light object, the brain recruits approximately the same number of
motor units to keep the object up, but cycles between different motor units. The muscle
fibers consume stored energy available in the muscle, and generate a force by
contracting. As the muscle fibers deplete this fuel source, more energy must be created
in order to continue contracting. By recruiting different motor units, motor units can
relax and replenish their fuel sources.
Skeletal muscles performing acute maximum work or chronic sub-maximum work of a
repetitive nature will eventually fatigue. Fatigue is defined as a decrease in the muscles’
ability to generate force. Fatigue is caused by a reversible depletion of the muscle’s fuel
supply. If the muscle uses its energy sources faster than it can be generated by cellular
metabolism, fatigue occurs. During contraction, skeletal muscle cells convert chemical
energy into thermal and mechanical energy, and, in the process, produce chemical waste
products.
Normally the waste products are removed from the muscle by the circulatory system as
the blood brings nutrients to the muscle for energy transformation. If certain waste
products (metabolites) are not removed at an adequate rate, they will accumulate and
chemically interfere with the contractile process, thereby hastening the onset of fatigue.
Some accumulated waste products also stimulate pain receptors in surrounding
connective tissue and induce cramping of skeletal muscle, a general sign of inadequate
blood flow to the muscle.
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Power is defined as the amount of work done per unit of time. Dynamometry means the
measurement of power (dyno = power, meter = measure), and the graphic record derived
from the use of a dynamometer is called a dynagram.
III. MATERIALS
In this lesson, you will examine motor unit recruitment and skeletal muscle fatigue by
combining electromyography with dynamometry.
BIOPAC SS25 Hand Dynamometer
BIOPAC SS2L Electrode Lead Set
BIOPAC disposable electrodes, 3 per subject
BIOPAC MP35 acquisition unit
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IV. EXPERIMENTAL METHODS
The experimental methods are presented in a two-column format. The left column gives
a brief explanation of each step. The right column contains more detailed information
about the steps or concepts in the Fast Track column.
A. Set-up
FAST TRACK STEPS
Detailed Explanation of Steps
1. Attach three electrodes to the dominant
forearm of the subject on the
Brachioradialis muscle.
Use the dominant forearm of the subject.
Generally, if the subject is right-handed,
select the right forearm or if left-handed,
select the left forearm. You will monitor
the Brachioradialis muscle of the forearm.
Please refer to the muscle diagram in the
lab for proper electrode placement and
attach the three electrodes as shown in Fig.
3.
Figure 3
2. Attach the electrode lead set to the
electrodes, following the diagram shown in
Figure 4.
Each of the pinch connectors on the end of
the electrode cable needs to be attached to a
specific electrode. The electrode cables are
a different color, and you should follow
Fig. 4 to ensure that you connect each cable
to the proper electrode.
The pinch connectors work like a small
clothespin, but will only latch onto the
nipple of the electrode from one side of the
connector.
CONTINUED ON NEXT PAGE
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3. Measure the arm circumference near the
white lead.
Figure 4
Also measure the arm circumference near
the white lead for each subject.
END OF SET-UP
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B. Recording Lesson Data
FAST TRACK RECORDING
Detailed Explanation for Recording Lesson Data
1. Prepare for the recording.
You will record two experiments.
a. Experiment 1 records Motor Unit Recruitment.
b. Experiments 2 and 3 record Fatigue.
In order to work efficiently, read this entire section
before beginning.
EXPERIMENT 1
2. Following instructions are for
PCs in M204
3. Check for Bioengineering
template on the PC desktop
4. Plug in the electrode
connectors, SS2L, into channel 1
of the MP35 data acquisition
system.
Double click on the BE Lab (Biopac) template:
The template shows three graphs on the top half of
the screen, the first is the EMG readings in mV, the
second is the force exerted on the dynamometer,
measured in Kg, and the third represents a
continuous integral of the first EMG graph,
measured in Volts.
5. Plug in the hand dynamometer,
SS25L, to channel 2.
6. Make sure the switch on the
back of the MP35 box is turned
on.
7. Calibrate Channel 2 for the
hand dynamometer.
In the calibration procedure, you select the sensors
and set hardware internal parameters such as gain,
offset and scaling which optimize the signal.
Follow the on-screen directions to calibrate the hand
dynamometer.
CONTINUED ON NEXT PAGE
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8. Click on Start.
The start button is located in the lower right hand
corner of the screen.
9. Clench, release and wait and
Keep the wrist and arm position the same for all
repeat at 4-5 different levels to
clenches. Grip the dynamometer close to the
see your maximum possible force. crossbar but do not actually touch the crossbar.
Figure 5
10. Click on Stop.
11. If data looks correct, repeat
this 2 more times, for a total of
THREE runs for this subject.
Then go to step 13.
Your data should resemble the following figure.
Figure 6
12. If data looks incorrect, go to
step 8.
To reset graphs, go to EDIT ->
Select All and EDIT -> Clear
CONTINUED ON NEXT PAGE
Note: If you can’t see the maximum values of your
data, single-click on the right-hand axis. Increase
the scale value until you can see all of your data.
Alternatively, you can select the vertical autoscale
button under the menus. Doing this will not
interrupt your recording.
Go back to step 8 and repeat if your data is not
correct.
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EXPERIMENT 2
13. Click on Start to append to
Experiment 1 data.
14. Clench the hand dynamometer Note the maximum clench force so you can
with your maximum force. Note
determine when the force has decreased by 50%.
this force and try to maintain it.
Try to maintain the maximum force even though the
muscle will fatigue and the force will decrease. The
maximum force and fatigue time will vary greatly
among subjects.
15. When the maximum clench
force has decreased more than
50%, click on Stop.
16. Review the data on the screen. Your data should look like the following figure.
Figure 7
You may need to use the horizontal scroll bar to
view the data. You can also select the time range
displayed on the x-axis.
17. If data is correct, save the data
on the C drive as a BSL Profile
(*ACQ). Each subject will need a
unique file name. Then repeat
Experiments 1 and 2 for all
members of your group.
END OF FIRST RECORDING
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EXPERIMENT 3
1. In ONE subject, perform
Experiment 2 (clench and hold)
on the dominant arm THREE
times, allowing a 1 minute rest
period between runs.
2. Attach leads to the nondominant arm of that subject, and
measure the arm circumference,
as described in Step 3 of SET UP.
3. Click Start to append to the
Experiment 3 file, and perform
Experiment 2 (clench and hold)
on the non-dominant arm THREE
times, allowing a 1 minute rest
period between runs
4. Review the data on the screen.
5. If data is correct, use “Save
as…” to save the data on the C
drive as a BSL Profile (*ACQ)
file using REPEATED in the
filename.
You may need to use the horizontal scroll bar to
view the data. You can also select the time range
displayed on the x-axis.
DO NOT USE “Save..” as it will overwrite previous
file.
END OF LAST RECORDING
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V. DATA ANALYSIS
The Data Analysis should be done in a M204 or any computer lab in school of
engineering computer labs.
FAST TRACK RECORDING
Detailed Explanation for Analyzing Lesson Data
1. Open BIOPAC Pro and then
open your saved data file.
For the first part of the analysis, choose the data for
the first subject.
2. Set up your display for
optimal viewing of the first data
segment.
The following tools help you adjust the data window.
Autoscale horizontal
Autoscale waveform
Zoom Tool
Horizontal Scroll Bar
Vertical Scroll Bar
Zoom Preview
3. Using the I-Beam cursor,
select an area of the plateau of
the first clench and determine the
force and integrated EMG.
Figure 8
4. Repeat step 3 on the plateau of
each successive clench and
extract force and integrate EMG
data for all 3 runs from each
subject.
5. Go to Experiment 2 of the
data. Using the I-Beam cursor,
select a point in the region of
maximum clench force and note
the value.
The point selected should represent the maximal
clench force at the start of Experiment 2.
Figure 9
CONTINUED ON NEXT PAGE
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6. Calculate 50% of the
maximum clench force.
7. Find the segment when the
force is 50% of the maximum
clench force with the I-Beam
cursor.
Make an eyeball approximation of the point that is
50% of the maximal clench force. Then use the Ibeam cursor in this region and note the value
displayed in the measurement box. Move the cursor
until you are within 5% of the desired value and leave
the cursor there.
8. Select the area from the point
of 50% clench force back to the
maximum clench force using the
I-beam cursor. Note this will
yield the time to fatigue and
extract that value.
The cursor should be flashing on the point of 50% of
the maximal clench force. Hold down the mouse
button and drag to the left of this point until you
reach the point of maximal clench force. Then
release the mouse button.
Figure 10
9. Save the data file.
10. Repeat analysis for the next
subject data file.
11. Repeat analysis steps 5-9 for
all runs in Experiment 3. Be sure
to clearly document which
values came from the dominant
arm and which from the nondominant arm.
12. Exit the program.
END OF ANALYSIS
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VI. Laboratory Report
Check guidelines at http://www.seas.upenn.edu/~mtsi/mtsireport.pdf
Your individual laboratory report should be 3-4 pages of text plus tables and figures.
You may work together with your group members on the Results table and plot, but all of
the other work should be done on an individual basis.
1. Items to include in procedure
A brief description of the methods and procedures used in the experiment is given
here.
2. Items to include in the Results section
The results are presented quantitatively in tables and/or graphs. When plotting
experimental data points, plot individual data points and do not connect them with
a line unless you have a specific reason for doing so. Be sure to paste your
figures from Excel properly so that the entire spreadsheet is not included with the
figure. You should comment on significant aspects of the results and trends in the
data but not go into a detailed discussion of the analysis in this section.
a. Description of subjects (male, female, dominant arm circumference, level of
regular physical activity, etc.) and the maximum clench force and fatigue time
(the time for the maximum clench force to decay 50%) from Experiment 2 in a
table.
b. Plot the plateau of the clench force and integrated EMG values from
Experiment 1 for each subject on one graph. Show all 3 runs for each subject.
Clearly indicate which values are from which run from a single subject, and
distinguish one subject’s data from another.
3. Questions to address in the Discussion section
Here you analyze your results and what they mean. The data discussed here
should have already appeared elsewhere in the Results section.
a. Explain the source of the signals detected from the EMG electrodes.
b. Is there a difference in the maximum clench force generated by males and
females in your group? (if you have female members in your group) What
might explain the difference? What other factors may influence the maximum
clench strength?
c. As you fatigue, the force exerted by your muscles decreases. What
physiological processes explain the decline in strength? How did fatigue time
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vary across subjects and why? Between the dominant arm and non-dominant
arm? Between the first and third run in Experiment 3?
d. On your graph of all subjects, relate the integrated EMG signal to the
corresponding plateau of the clench force measured in Experiment 1. What is
the meaning of the slope? Determine the slope between EMG plateau value (y)
and clench force (x). How does the slope vary with subject? Why?
e. Are there experimental factors that might influence the integrated EMG value
or clench force?
f. What do your results indicate about the usefulness of EMG in the monitoring of
athletic training?
4. Conclusion Section
The Conclusion is a short summary of what you have already said. There is no
new information here, just a brief presentation of the most significant points
already mentioned. It should summarize the work concisely and completely.
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