MUSCLE PHYSIOLOGY SIMULATIONS
USING PHYSIOEX 8.0
Required Materials
 computer
 PhysioEx CD
 lab instructions
 your lab notebook
Purpose
In this week’s lab you will perform a series of exercises demonstrating basic principles of muscle
contraction. In completing these exercises, you will review some important concepts of muscle contraction
described in lecture but also learn new concepts that will increase your understanding of muscle physiology.
Background
In this lab you will perform computer simulations that demonstrate important physiological principles
governing skeletal muscle contraction. The computer program provides all the equipment and materials
necessary for the experimental set up and to observe results. Because you will be working with a simulated
muscle, you need to watch carefully during the experiments. Think about what is happening in each situation.
You need to understand how you are experimentally manipulating the muscle in order to understand and
interpret your results.
Protocol: Muscle Recordings and Simulations using PhysioEx 8.0
(Instructions are adapted from Stabler, Smith, Peterson, Lokuta (2009). PhysioEx 8.0 for Human Physiology
Laboratory simulations in Physiology. Pearson, Benjamin, Cummings. San Francisco.)
1. Sign out a computer and insert the PhysioEx 8.0 disk. Double-click the “My Computer” icon. Then
choose the PhysioEx 8.0 (D:) icon. In the window that opens double-click the “StartHere.exe.” icon. In
the PhysioEx window that opens click “ENTER”. This will open the PhysioEx program.
2. There is a drop-down menu at the top of the window that allows you to select specific PhysioEx
exercises. Select “Exercise 2: Skeletal Muscle Physiology” and click “Go”.
3. Skip step #1. There is no need to download the lab instruction worksheet. All the required instructions
are provided here.
4. Watch the “Skeletal Muscle” video by clicking the link in step #2. The video illustrates the dissection of
a skeletal muscle from the mouse. Based on information provided in the video, answer the following
question in your lab notebook:
a. What muscle was dissected?
5. Proceed to step #3 and click on the “Single Stimulus” link.
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The opening screen should appear in a few seconds. The oscilloscope display (the grid at the top of the
screen) is the most important part of the screen because it graphically displays the contraction data for analysis.
Time is displayed on the horizontal axis. The force produced by muscle contraction (in grams) is displayed on
the vertical axis. You can click the “Clear Tracings” button as needed to clean up the oscilloscope display. To
retain your data, click the “Record Data” button at the end of each stimulus. In some of the following exercises
you will be asked to record your data in your lab notebook so that you can generate graphs for your lab report
(due Week 7).
When a skeletal muscle from an experimental animal is electrically stimulated (like the one dissected form
the mouse), it behaves in the same way as a stimulated muscle in the intact body. Thus, such an experiment
gives us valuable insight into muscle behavior.
A contracting skeletal muscle will produce force when nervous or electrical stimulation is applied. A single
contraction of skeletal muscle is called a MUSCLE TWITCH. In the first exercise, you will simulate an
ISOMETRIC, or fixed length, contraction of an isolated skeletal muscle. This activity allows you to investigate
how the strength and frequency of an electrical stimulus affect whole muscle function.
SINGLE STIMULUS
Activity 1: Practicing Generating a Tracing
1. Click the “Stimulate” button once.
 you will see a blue line moving across the bottom of the oscilloscope display. Because the
beginning voltage is set to zero, no muscle activity should result.
2. Click and hold the “+” button beneath the “Stimulate” button until the voltage window reads 3.0 volts.
Click “Stimulate” once.
 you will see the muscle react and a contraction tracing will appear on the screen.
3. Change the voltage to 5.0 volts, and click “Stimulate” again.
 notice how the force of contraction also changes
The force generated by a whole muscle reflects the number of MOTOR UNITS (i.e. a neuron and all the
muscle fibers it innervates) firing at a given time. Strong muscle contraction implies that many motor units are
activated and each unit has maximally contracted. Weak contraction means that few motor units are active;
however, the activated units are still maximally contracted. Increasing the intensity of the electrical stimulation
(i.e. voltage) mimics how the nervous system increases the number of motor units activated.
a. record the effect of voltage on contraction in your notebook. What is increasing the voltage
meant to simulate?
Activity 2: Determining the Latent Period
The latent period is a short period between the time of stimulation and the beginning of contraction.
Although no force is generated during this interval, chemical changes occur intracellularly in preparation for
contraction.
a. name some possible chemical changes you think might occur during the latent period in
your notebook
1. Click “Clear Tracings” to erase the oscilloscope display and set voltage to 5.0 volts
2. Click the “Simulate” button once and allow the tracing to complete
3. Measure the latent period.
To measure the length of the latent period using the computer, click the Measure button. Then click the right
arrow button next to the Time window repeatedly until you notice the first increase in the Active Force window.
This takes you beyond the actual length of the latent period. Now click the left arrow button next to the Time
window until the Active Force window again reads zero. At this point the computer is measuring the time
between the application of the stimulus and the last point where the active force is zero
a. record the latent period (msec) in your lab notebook
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Activity 3: Investigating Graded Muscle Response to Increased Stimulus Intensity
As the stimulus to a muscle is increased, the amount of force produced by the muscle also increases. As
more voltage is delivered to the whole muscle, more muscle fibers are activated and the total force produced by
the muscle is increased. MAXIMAL CONTRACTION occurs when all the muscle cells have been activated.
Any stimulation beyond this voltage will not increase the force of contraction. This experiment mimics muscle
activity where in the recruitment of additional motor units increases the total force produced. This phenomenon
is called MULTIPLE MOTOR UNIT SUMMATION or RECRUITMENT.
1. Erase any tracings on your screen. Set the voltage to 0.0 and click “Stimulate”
2. Click “Record Data”
a. copy each value for Voltage, Length, Active Force, Passive Force and Total force into your
lab notebook
3. Repeat step 1, increasing the voltage by 0.5 each time until you reach the maximum voltage of 10.0.
a. be sure to select the “Record Data” after each stimulation and to record the resulting
values in your lab notebook. You will need these values to generate graphs for your lab
report.
4. Record your answers to the following questions in your lab notebook:
a. What is the minimal or threshold stimulus?
b. What is the maximal stimulus?
c. How can you explain the increase in force you observed?
MULTIPLE STIMULUS
Go to the “Experiment” tab at the top of the screen and select “Multiple Stimulus”. The addition of the
“Multiple Stimulus” button allows you to alternately start and stop the electrical stimulator. When “Multiple
Stimulus” is first clicked, its name changes to “Stop Stimulus” and electrical stimuli are delivered to the muscle
at the rate specified in the “Stimuli/sec” window until the muscle completely fatigues or the stimulator is turned
off. The stimulator is turned off by clicking the “Stop Stimulus” button. The stimulus rate is adjusted by
clicking the (+) or (-) buttons next to the “Stimuli/sec” window.
Activity 4: Investigating Treppe
When a muscle first contracts, the force it is able to produce is less than the force it is able to produce in
subsequent contractions within a relatively narrow time span. This is referred to as TREPPE. For the first few
twitches, each successive stimulation produces slightly more force than the previous contraction as long as the
muscle is allowed to fully relax between stimuli and the stimuli are delivered relatively close together.
Treppe is thought to be caused by increased efficiency of the enzyme systems within the cell and increased
availability of intracellular calcium.
a. record these potential causes of treppe in your lab notebook
1. Set voltage to 8.2 volts and muscle length to 75mm. Drag the 200 msec button to the far left of the Xaxis.
2. Click the “Single Stimulus” button once. As soon as the force falls, click the “Single Stimulus” button
again. When force falls click the “Single Stimulus” button a third time.
a. in your notebook, record what happens to force production with each subsequent stimulus
Activity 5: Investigating Wave Summation
WAVE SUMMATION is achieved by increasing the stimulus frequency, or rate of stimulus delivery to the
muscle. Wave summation occurs because the muscle is already in a partially contracted state when subsequent
stimuli are delivered. TETANUS can be considered an extreme form of wave summation that results in a
steady, sustained contraction (and thus different from treppe). In effect, the muscle does not have any chance to
relax because it is being stimulated at such a high frequency. This fuses the force peaks so that we observe a
smooth tracing.
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1.
2.
3.
4.
5.
Erase any tracings on your screen. Set the voltage to 8.2 and muscle length to 75mm.
Drag the 200 msec button to the right edge of the oscilloscope display.
Click “Single Stimulus” and note the Active Force (g) generated
Underneath the “Multiple Stimulus” button, set the “Stimuli/sec display to 50 by clicking “+” button
Click the “Multiple Stimulus” button. When you do so, it will change to a “Stop Stimulus” button. You
can click the “Stop Stimulus” button when force begins to decline.
a. in your notebook, record what happens to force production at approximately 80msec
6. Clear the tracing. Increase stimuli/sec to 145 and click “Multiple Stimulus”. Click “Stop Stimulus” after
the trace has swept one full screen. The click “Record Data”.
a. in your notebook, record what happens to the shape of the curve as stimuli/sec was
increased
7. Repeat step 6, increasing stimuli/sec to 146, 147, 148.
a. in your notebook, record the frequency where force no longer increases
Activity 6: Investigating Muscle Fatigue
A prolonged period of sustained contraction will result in muscle FATIGUE, a condition in which the
tissue has lost its ability to contract. Fatigue results when a muscle cell’s ATP consumption is faster than its
production. Consequently, increasingly fewer ATP molecules are available for the contractile parts within the
muscle cell.
1. Erase any tracings on your screen. Set the voltage to 8.2 and muscle length to 75mm.
2. Adjust the stimulus rate to 120 stimuli/per sec.
3. Click “Multiple Stimulus”, allow the tracing to sweep through three screens, and then click “Stop
Stimulus” to stop the stimulator.
a. in your notebook, record what happens to force production over time
ISOMETRIC AND ISOTONIC CONTRACTIONS
When a muscle attempts to move a load that is greater than the force generated by the muscle, the muscle
contracts ISOMETRICALLY. In this type of contraction, the muscle stays at a fixed length (isometric means
“same length”). An example of isometric muscle contraction is when you stand in a doorway and push on the
doorframe. The load that you are attempting to move (the doorframe) is greater than the force generated by your
muscle, and so your muscle does not shorten.
Activity 7: Investigating Isometric Contractions
1. Click on the “Experiment” tab at the top of the screen and select “Isometric Contractions”
 Notice that there are now two oscilloscope screens. The screen on the left is basically identical to
the one you worked with in the previous activities. The screen on the right is new. The Y axis is
still “Force,” but the X axis is now muscle length.
2. Set the voltage to 8.2 volts and click “Stimulate”
 You will see indicators of three forces in the right-hand screen. You will see the Passive Force
indicator near the bottom of the screen (in green), and the Active Force (in purple) and Total
Force (in yellow) indicators together higher up on the screen. Note that the Active Force
indicator is seen inside of the Total Force indicator.
3. Click the (+) and (-) buttons beneath Muscle Length on the left side of the screen and notice how the
muscle may be stretched or shortened.
4. On the lower left side of the screen, click the (-) button underneath Muscle Length and reduce the length
to 50 mm. Click “Stimulate” to observe the trace, then click “Record Data”
5. Repeat steps 1–5, increasing the muscle length by 10 mm each time (i.e., 60 mm, then 70 mm,\ then 80
mm, etc.) until you reach 100 mm.
a. In your notebook, record the information contained in the resulting data table (i.e. voltage,
length, active force, passive force and total force). Remember to click “Record Data” after
each simulation.
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b. Looking at the data table, answer the following questions in your notebook:
a. What muscle lengths generated the most active force? (give a range)
b. At what muscle length does passive force begin to play less of a role in the total force
generated by the muscle? (give a range)
c. At what muscle length does Passive Force begin to pay a role in the total force
generated by the muscle? (give a range)
d. At 90 mm there is a drop in the force generated. Why do you think this is?
e. What is the key variable in an isometric contraction
Activity 8: Investigating Isotonic Contractions
When a muscle attempts to move a load that is equal to or less than the force generated by the muscle, the
muscle contracts ISOTONICALLY. In this type of contraction, the muscle shortens during a period of time in
which the force generated by the muscle remains constant (isotonic means “same tension”). An example of
isotonic contraction is when you lift a book from a tabletop. The load that you are lifting (the book) is equal to
or less than the force generated by your muscle. Your muscle shortens when it contracts, allowing you to lift the
book.
1. Click on the “Experiment” tab at the top of the screen and select “Isotonic Contractions”
 The screen that appears is similar to the Single Stimulus screen you worked with in Activities 1–3.
Note that fields for “Muscle Length” and “Velocity” have been added to the display below the
oscilloscope screen, and that the muscle on the left side of the screen is now dangling freely at its
lower end. The weight cabinet below the muscle is open; inside are four weights, each of which may
be attached to the muscle. Above the weight cabinet is a moveable platform, which you may move
by clicking the (+) or (-) buttons under Platform Height.
2. The Voltage should already be set at 8.2, and the Platform Height at 75 mm. If not, adjust the settings
accordingly.
3. Click on the 0.5g weight in the weight cabinet and attach it to the dangling end of the muscle. The
weight will pull down on the muscle and come to rest on the platform.
4. Click “Stimulate” and observe the trace. Note the rise in force, followed by a short plateau, followed by
a relaxation phase. Note that the Active Force display is the same as the weight that was attached: 0.5
grams. Click the record “Data button” and answer the following questions in your lab notebook:
a. How much time does it take for the muscle to generate 0.5 grams of force?
b. You can observe from the trace that the muscle is rising in force before it reaches the
plateau phase. Why doesn’t the muscle shorten prior to the plateau phase?
5. Remove the .5g weight and attach the 1.0g weight. Leave your previous trace on the screen.
6. Click “Stimulate” and then “Record Data”.
a. How does this trace differ from the trace you generated with the .5g weight attached?
7. Leaving the two previous traces on the screen, repeat the experiment with the two remaining weights.
a. Click “Record Data” after each result and write the resulting values in your notebook.
b. Examine your numerical data. At what weight was the velocity of contraction the fastest?
c. What happened when you attached the 2.0g weight to the muscle and stimulated the
muscle? How did this last trace differ from the other traces?
d. What kind of contraction did you observe? What was the force of contraction?
To leave today:
 Ensure you have answered all of the required questions in your lab notebook
 Ensure you have the necessary data tables recorded in your notebook. You will need this information for
the Discussion Lab next week
Next week: Discussion-Membrane Physiology
 We will be discussing your data in our “Discussion” lab next week. You should come prepared to
discuss the results recorded in your notebook. Additional instructions will be posted on Blackboard
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prior to meeting. Each student will again generate a lab report complete with Title Page, Results with
figures and Discussion sections.
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