Student Protocol
Muscle Stimulation & Fatigue
In this experiment, you will explore muscle function through stimulation and fatigue. You will electrically
stimulate the nerves in the forearm to demonstrate recruitment, summation, and tetanus.
Written by staff of ADInstruments and modified by Drs. Davis and Moeller. Revised 1 Nov 2011
Background
The skeleton provides support and articulation for the body. Bones act as support structures, and joints
function as pivot points. Skeletal, or striated, muscles are connected to the bones either directly or by
tendons, strong bundles of collagen fibers. Skeletal muscle is composed of long, multinucleate cells
called fibers grouped into fascicles (Figure 1). Two or more muscles usually work antagonistically. In
this arrangement, a contraction of one muscle stretches, or elongates, the other.
Figure 1. Skeletal Muscle Organization
A single motor neuron, and all the muscle fibers that it innervates, is known as a motor unit. An action
potential in a motor neuron induces an action potential in the muscle fibers it innervates by releasing the
neurotransmitter acetylcholine into the neuromuscular junction. This muscle action potential causes a
brief increase in the intracellular concentration of calcium ions, [Ca2+], and activates the contractile
molecular machinery inside the fiber. This requires the use of intracellular supplies of adenosine
triphosphate (ATP) as the energy source. The result is a brief contraction called a twitch. A whole
muscle is controlled by the firing of up to hundreds of motor axons. These motor nerves control
movement in a variety of ways. One way in which the nervous system controls a muscle is by adjusting
the number of motor axons firing, thus controlling the number of twitching muscle fibers. This process is
called recruitment.
A second way the nervous system controls a muscle contraction is to vary the frequency of action
potentials in the motor axons. At stimulation intervals greater than 200 ms, intracellular [Ca 2+] is
restored to baseline levels between action potentials, and the contraction consists of separate twitches.
At stimulation intervals between 200 and 75 ms, [Ca 2+] in the muscle is still above baseline levels when
the next action potential arrives. The muscle fiber, therefore, has not completely relaxed and the next
contraction is stronger than normal. This additive effect is called summation. At even higher stimulation
frequencies, the muscle has no time to relax between successive stimuli. The result is a smooth
contraction many times stronger than a single twitch, called a tetanic contraction. The muscle is now in a
state of tetanus.
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Muscle Stimulation & Fatigue
Student Protocol
Required Equipment
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LabChart software
PowerLab Data Acquisition Unit
Finger Pulse Transducer
Hand Dynamometer
Stimulating Bar Electrode
Electrode Cream or Paste
Medical tape
Procedure
This experiment involves application of electrical shocks to muscle through
electrodes placed on the skin. Students who have cardiac pacemakers or who suffer
from neurological or cardiac disorders should not volunteer for this exercise. If the
volunteer feels major discomfort during the exercise, discontinue the exercise and
consult your instructor.
Equipment Setup
1. Make sure the PowerLab is turned off and the USB cable is connected to the computer.
2. Connect the Finger Pulse Transducer to Input 1 on the front panel of the PowerLab and the
Stimulating Bar Electrode to the Isolated Stimulator output on the front panel (Figure 2). Make sure
the red (positive) connector is in the red output and the black (negative) connector is in the black
output. The hardware needs to be connected before you open the settings file.
Figure 2. Equipment Setup for PowerLab 26T
3. Place the pressure pad of the Finger Pulse Transducer on the top of the table; tape the transducer in
place along the Velcro strap. The Finger Pulse Transducer needs to be close to the edge of the table
(Figure 2). If that position is too awkward, you might try positioning your hand palm-side down on
the table surface with your thumb resting over the Transducer (see Picture # 1 on Page 6.)
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Student Protocol
4. Place a small amount of Electrode Cream or Electrode Paste on the two silver pads of the Stimulating
Bar Electrode and place it over the volunteer’s ulnar nerve at the wrist (Figures 2 and 3). The
Stimulating Bar Electrode should lie along the axis of the arm, with the leads pointing toward the
hand – a red (positive) dot on the back of the bar should be placed away from the hand. Hold the
Stimulating Bar Electrode in place.
5. Check that all connections are correct, and turn on the PowerLab.
Figure 3. Position of the Ulnar Nerve
Exercise 1: The Effects of Nerve Stimulation
In this exercise, you will explore the motor and sensory effects of electrical stimuli on the nerves of the
forearm in a resting volunteer. In this exercise, the PowerLab acts as a stimulator, instead of a recorder.
Muscular responses will be observed by watching the hand of the volunteer (Figure 4). Some motor
effects that may be observed include:
 Bending of the wrist (due to the flexor carpi radialis and flexor carpi ulnaris muscles)
 Bending of the last segments of the fingers (due to the long finger flexor muscles)
 Movement of all fingers, combined with the pulling of the thumb towards the index
finger (due to the intrinsic muscles of the hand innervated by the ulnar nerve)
 Lifting of the thumb (due to the muscles at the base of the thumb innervated by the
median nerve)
Figure 4. Muscles of the Hand and Forearm
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Student Protocol
1. Launch LabChart and use the following settings:
Sampling Rate: 200/s
View: 1:1 compression
Number of Channels: Channel 1 should be “on.” Check the box to do this, if necessary.
Channel 1: Title: Force do this under Setup; Channel Settings; Title; Highlight Ch 1 and enter Force.
Range: 10 V
Input Settings: Input Amplifier
Calculation: Integral
Source: Channel 1 Integral type: Standard Integral. Reset Type: No Reset; Select Autoscale.
Channel 2: on
Range: 10 V
Stimulator:
Stimulator Mode: Pulse
Isolated Stimulator: Check the box
Output: Continuously
Marker Channel: Channel 2 (Very Important so the stimulus appears in Channel 2.)
Start: When recording starts
Range: 20 Hz
Frequency: 1.0 Hz
Pulse Duration: 200 µS
Amplitude: 10.0 mA
2. Have the volunteer sit in a relaxed position. Make sure the volunteer is still holding the Stimulating
Bar Electrode in place over the median nerve. To set up nerve stimulation, select Stimulator from
the Setup menu. Change the settings to match those in the dialog below (Figure 5).
Select Channel 2
Here.
Figure 5. Stimulator Dialog
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Student Protocol
3. Turn on the Isolated Stimulator by flipping the switch on the PowerLab. Note that the Isolated
Stimulator only becomes active during sampling.
4. Start recording. Note the twitch contractions affecting the thumb and fingers. Examine the effect of
small adjustments to the placing of the electrode, and locate the position giving the largest twitches.
Note: If nothing happens, go back to the Stimulator dialog, and make sure On is selected. Also, you
may need to increase the stimulus amplitude (in the Setup Stimulus box) to observe a twitch.
5. Explore other results of stimulating at other places in the forearm. Each time you move the electrode
to another location wipe away the residual Electrode Cream from the skin to prevent short-circuiting.
Remember the two pads need to be aligned along the arm’s length.
Note: Stimulation in most places gives rise to little discomfort. In some places, there is substantial
sensory effect; there may be painful sensation in the forearm or hand away from the site of stimulation
toward the fingers. At these places, a cutaneous sensory nerve is being stimulated.
6. Stop recording. You do not need to save your data as nothing was recorded. You’ve simply been
searching for stimulus locations that lie over nerves in the forearm.
7. Wipe the paste off your skin and reapply a small amount to the metal buttons of the stimulating
electrode.
Exercise 2: Twitch Response and Recruitment
In this exercise, you will measure the muscular twitch response to nerve stimulation and show
recruitment in the twitch response as the stimulus strength increases.
1. Once you can reliably locate the stimulating electrodes over the median nerve at the wrist and
produce twitches in muscles that move the thumb, you’ll now use the Finger Pulse Transducer to
detect those twitches.
2. Have the volunteer place their hand as shown in Figure 6, with the fingers under the edge of the
table, and the edge of the thumb resting lightly on the Finger Pulse Transducer.
Figure 6. Position of the Hand
3. Select Input Amplifier from the Channel 1 Channel Function pop-up menu. The dialog should show
a stable baseline reading in its display. A deflection of the trace should be seen when pressing lightly
on the Finger Pulse Transducer. If you have difficulty obtaining good twitches in this configuration,
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Student Protocol
try placing your hand on the tabletop with the thumb resting lightly on the Finger Pulse Transducer
as shown in Photo 1.
Photo 1.
4. Wipe the Electrode Cream from the volunteer’s wrist. Apply a small amount of the cream to the pads
of the Stimulating Bar Electrode, as done in the Equipment Setup. Hold the electrode at the site of
stimulation for the median nerve (Figure 6). Make sure the volunteer’s thumb is resting lightly on the
Finger Pulse Transducer.
5. To set up the Stimulator miniwindow, select Stimulator Panel from the Setup menu. This allows
you to change the stimulus amplitude without having to open the menu each time. Click-and-drag
on the miniwindow to move it to a convenient position on the screen.
6. Start recording. Move the stimulating electrode and reposition the thumb as necessary to see
twitches in Channel One. Change the Range of Channel 1 as needed to optimize your view of the
twitches, and use the Window Zoom Window to look carefully at a selected portion of the recording.
Once you’re recording good twitches, Stop recording. Once you’ve obtained good recordings, do not
reposition the stimulating electrode nor your hand and thumb until you’ve completed this entire set of
recordings. It is important that you keep the stimulating electrode in the same location and that your
hand and thumb remain in the same position over the transducer. Make the following changes under
Setup, select Trigger, then Stop: Fixed duration = 0.5 seconds.
7. Begin with a stimulus amplitude of 10.0 mA, and press Start. . Click somewhere in the recording
and add a comment (Apple key + k) with the stimulus amplitude used. For each subsequent
recording, increase the amplitude in 1.0 mA increments, pressing Start after each one until the
twitches no longer increase in size. Add a comment to each recording. For most volunteers, this
maximal stimulus is in the range of 6-15 mA.
8. Reset to 10 mA and make a recording. Decrease the stimulus intensity in increments of 1 mA and
make a recording, adding a comment each time with the stimulus value. Repeat this process until
you no longer can detect a twitch in Channel 1. At this point, you’ve passed below threshold for
stimulating action potentials in the ulnar nerve. For most volunteers, the threshold stimulus is in the
range of 3-8 mA. Once you’ve gotten all records, the subject can remove the stimulating electrode
and take their hand away from the transducer.
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Student Protocol
9. Your results should look similar to those in Figure 7. Have you produced evidence of recruitment?
Figure 7. Sample Data Showing an Increase in Stimulus Strength.
Note: We will use 1 mA intervals instead of the 0.5 mA intervals shown here.
Exercise 3: Summation and Tetanus
In this exercise, you will demonstrate the effects of changing the interval between paired stimulus pulses
and will observe a short tetanic contraction.
1. Make the following changes to the LabChart settings:
Stimulator:
Output: Set number of pulses, number of pulses: 2
Start: When recording starts, delay: 500 ms
Amplitude: 1.0 mA
Trigger
Stop: Fixed Duration, 3.0 seconds
2. Select Stimulator Panel from the Setup menu. Move the miniwindow to a convenient position.
3. Make sure the volunteer’s hand is in the same position before, with the thumb resting on the Finger
Pulse Transducer and the Stimulating Bar Electrode on the ulnar nerve. (It may be necessary to
make a longer recording to determine the best position for the stimulating electrode and thumb over
the transducer. If you do, remember to reset the duration of the recording back to 3 seconds.)
Once you’ve gotten good recordings, do not reposition the electrode nor the hand and thumb until all
recordings for this section have been completed.
4. In the Stimulator Panel, set the pulse current to 5.0 mA greater than the maximal stimulus value you
determined in Exercise 2. If that value would be greater than 20mA, use a 20 mA stim which is the
largest the PowerLab can produce. Add a comment with “1 Hz” in the new block of data to note the
stimulus frequency used.
5. Start recording. LabChart will record for a fixed duration of three seconds and will stop
automatically. Adjust the Range as needed to clearly see the twitches.
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Student Protocol
6. Increase the stimulus frequency to 2 Hz in the miniwindow, and press Start. Add a comment with
“2 Hz.” Repeat the stimulation for the frequencies 5, 10, and 20 Hz, adding a comment with the
frequency each time. Notice when you observe summation.
7. Select Stimulator from the Setup menu. Change the number of pulses from two to three. Start
recording. The volunteer should receive a burst of three stimuli at 20 Hz. Add a comment with
“tetanic stimuli 3” to the new block of data. If three pulses did not cause the volunteer too much
discomfort, use four pulses. Add a comment with “tetanic stimuli 4.” Can you see wave
summation?
8. Once you have a complete set of recordings, the subject can remove the stimulating electrode and
take their hand away from the transducer.
9.
Save your data to the Desktop in a file named “yournamelabday” when you are finished recording.
10. Review your recordings, noting the peak force produced by each stimulus. Fill in the datasheets by
making the measurements necessary on the appropriate recordings. Set the Marker at the baseline
and measure to the peak of the twitch in millivolts. Normally we would calibrate the transducer to
record tension in grams, but for brevity we’ve skipped this step. The important thing is the relative
amplitudes of the twitches regardless of whether they’re measure in millivolts or grams.
11. Before moving on to the next experiment on Muscle Fatigue, completely exit the LabChart program
so that all recording and stimulator settings will be eliminated. Under Trigger, reset the Trigger to
Stop = User. Remove the Finger Pulse Transducer cable from Channel One and remove the
Stimulating Electrode wires from their sockets.
Table 1. Effects of Varying Stimulus Strength on Twitch Force
Stimulus
Response
mV
Stimulus
Response
mV
Stimulus
0.0 mA
7.0 mA
14.0 mA
1.0 mA
8.0 mA
15.0 mA
2.0 mA
9.0 mA
16.0 mA
3.0 mA
10.0 mA
17.0 mA
4.0 mA
11.0 mA
18.0 mA
5.0 mA
12.0 mA
19.0 mA
6.0 mA
13.0 mA
20.0 mA
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Response
mV
Muscle Stimulation & Fatigue
Student Protocol
Table 2. Summation
Stimulus
Frequency
(Hz)
1
Stimulus
Interval
(s) =1/stim freq
Amplitude of
First Response
(mV)
Amplitude of
Second Response
(mV)
Stimulus
Interval
(s)
Number of
Pulses
Amplitude of
Response
(mV)
2
5
10
20
Table 3. Tetanus
Stimulus
Frequency
(Hz)
20
20
3
4
Study Questions
1.
What was the threshold stimulus of the volunteer? What was the maximal stimulus?
2.
What can you conclude regarding the number of fibers contracting as the current was
raised from threshold to that required for a maximal contraction?
3.
Why does varying the stimulus strength affect the twitch force?
4.
What are the two ways by which the nervous system can control the force generated
by a muscle?
5.
During weak contractions, the firing frequency of muscle fibers is low, so that each
fiber produces distinct twitches. The force produced by the whole muscle, however, is
relatively smooth. How do you think this occurs?
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