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2 EXERCISE
Skeletal Muscle Physiology
Advance Preparation/Comments
1. Prior to the lab, suggest to the students that they become familiar with the exercise before
coming to class. If students have a home computer, or access to a computer on campus, they
can become familiar with the general operation of the simulations before coming to class.
2. You might do a short introductory presentation with the following elements:
• Describe the basics of muscle contraction at the cellular level, focusing on the sarcomere.
This explanation is especially important for the isometric part of the simulation.
• Students often have problems distinguishing between in vivo stimulation via the nervous
system versus the electrical stimulation we apply to whole skeletal muscle in an experiment.
Mention that increasing the intensity of an electrical stimulus to the surface of whole muscle
is not the same as stimulation via the nervous system, but that the outcome of increased force
production is similar in both methods.
• Encourage students to try to apply the concepts from the simulation to human skeletal
muscles as they work through the program.
• If a demonstration computer screen is available, briefly show students the basic equipment
parts.
3. Keep in mind that many students in an introductory science course are deficient in their
graphing skills. Reviewing the principles of plotting before the class begins may prove helpful.
4. Be prepared to help the students answer the more difficult “What if . . . ” questions.
Answers to Questions/Experimental Data
Pre-lab Quiz in the Lab Manual
1. Tendons
2. b. A motor neuron and all of the muscle cells it innervates
3. a. Latent
4. acetylcholine
5. True
6. False
7. d. Threshold voltage
8. b. Isometric
Activity 1: The Muscle Twitch and the Latent Period (pp. 18–20)
Predict Question 1: No, changes to the stimulus intensity will not change the duration of the latent
period. The latent period is a chemical event initiated by the stimulus regardless of its intensity.
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Chart 1: Latent Period Results
Voltage
Active force (g)
Latent period (msec)
0.0
0.00
3.0
1.04
4.0
1.32
3.20*
6.0
1.65
3.20*
8.0
1.81
3.20*
10.0
1.82
3.20*
*- -Students use a visual ruler to determine the latent period. A student who
enters 2.80 msec as the latent period likely understood how to correctly
measure the latent period (the data points at 2.80 msec and 3.20 msec look
very similar in the software).
Activity Questions:
1. A graph similar to the tracing generated in the simulation. See figure 2.3 in the simulation for
comparison.
2. The events of the latent period include the events of excitation contraction coupling, most
notably the release of calcium from the sarcoplasmic reticulum.
Activity 2: The Effect of Stimulus Voltage on Skeletal Muscle Contraction (pp. 20–22)
6. 0.8 volts
Predict Question 1: The active force will first increase and then plateau at some maximal value as the stimulus
voltage increases.
12. 8.5 volts
Chart 2: Effect of Stimulus Voltage on Skeletal Muscle Contraction
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Voltage
Active force (g)
Voltage
Active force (g)
0.0
0.00
5.0
1.51
0.2
0.00
5.5
1.59
0.8
0.02
6.0
1.65
1.0
0.15
6.5
1.70
1.5
0.43
7.0
1.74
2.0
0.66
7.5
1.78
2.5
0.87
8.0
1.81
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3.0
1.04
8.5
1.82
3.5
1.19
9.0
1.82
4.0
1.32
9.5
1.82
4.5
1.42
10.0
1.82
Activity Questions:
1. The active force produced by the muscle increased as the stimulus voltage was increased.
2. In the body this is achieved by motor unit recruitment. More muscle fibers are recruited to
increase the force generated.
Activity 3: The Effect of Stimulus Frequency on Skeletal Muscle Contraction (pp. 22–23)
Predict Question 1: As the stimulus frequency increases, the muscle force generated by each
successive stimulus will increase. There will be a limit to this increase.
Predict Question 2: The stimulus frequency will need to increase.
Chart 3: Effect of Stimulus Frequency on Skeletal Muscle Contraction
Voltage
Stimulus
Active force (g)
8.5
Single
1.83
8.5
Multiple
Variable, ≤2.42
8.5
Multiple
Variable, ≥2.42
8.5
Multiple
Variable, >2.42 and
<5.20
10
Multiple
Variable, >2.42 and
<5.20
8.5
Multiple
Variable, ≥5.20
Activity Questions:
1. Treppe is known as the staircase effect because the tracing looks like a staircase, with each
subsequent wave higher than the previous wave.
2. More force is generated by the muscle with each successive twitch, thought to be due to
increased availability of calcium.
3. When you increase the frequency of stimulation, the amount of force generated increases.
4. Wave summation occurs in the body when muscle fibers are stimulated before they have had a
chance to completely relax.
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Activity 4: Tetanus in Isolated Skeletal Muscle (pp. 24–25)
Predict Question 1: As the stimulus frequency increases, the muscle force generated by each
successive stimulus will increase. There will be a limit to this increase.
Chart 4: Tetanus in Isolated Skeletal Muscle
Stimuli/second
Active force (g)
50
5.12
130
5.88
140
5.91
142
5.94
144
5.94
146
5.95
148
5.95
150
5.95
Activity Questions:
1. A summation of force is occurring at a high frequency of stimulation to produce smooth muscle
contraction.
2. “Lockjaw” is a pathological tetanus. Tetanus boosters are vaccines to prevent the development
of tetanus, the disease.
Activity 5: Fatigue in Isolated Skeletal Muscle (pp. 25–26)
Predict Question 1: The length of the rest period will proportionately increase the length of time for
sustained muscle tension.
Chart 5: Fatigue Results
Rest period (sec)
Active force (g)
Sustained maximal force (sec)
0
5.86
10
0
5.86
10
Variable, 8–12
5.86
Variable, 0.2–1.8
Variable, 8–12
5.86
Variable, 4.2–5.8
Activity Questions:
1. Fatigue is still being investigated, but it is thought to involve the buildup of lactic acid, ADP,
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and inorganic phosphate, and possibly oxygen debt.
2. They best way to delay the onset of fatigue with intense exercise is to schedule brief periods of
rest to allow muscle recovery.
Activity 6: The Skeletal Muscle Length-Tension Relationship (pp. 26–28)
Predict Question 1: Total force can increase or decrease depending upon the starting resting length.
Chart 6: Skeletal Muscle Length-Tension Relationship
Length (mm)
Active force (g)
Passive force (g)
Total force (g)
75
1.82
0.00
1.82
70
1.75
0.00
1.75
65
1.55
0.00
1.55
60
1.21
0.00
1.21
55
0.73
0.00
0.73
50
0.11
0.00
0.11
80
1.75
0.02
1.77
90
1.21
0.25
1.46
100
0.11
1.75
1.86
Activity Questions:
1. Changes in the resting length of the sarcomere directly affect the amount of passive, active and
total force that results as described by the length-tension relationship.
2. The dip in the total force curve is due to the fact that at a very short muscle length, there is too
much overlap to generate a significant amount of active force. Additionally, there is no passive
force at this muscle length.
Activity 7: Isotonic Contractions and the Load-Velocity Relationship (pp. 28–29)
Predict Question 1: The latent period will increase, the shortening velocity will decrease, the
distance will decrease, and the contraction duration will decrease.
Chart 7: Isotonic Contraction Results
Weight (g)
Velocity (cm/sec)
Twitch duration (msec)
Distance lifted (mm)
0.5
0.100
78.00
4.0
1.0
0.057
49.00
2.0
1.5
0.022
30.00
0.5
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2.0
0.00
0.00
0.0
Activity Questions:
1. As the weight of the load increases, the initial velocity to move the weight decreases.
2. This is because a heavier weight will have a slower velocity for the repetitions, so it will take
you longer to repeat the same number of repetitions of a heavier weight.
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REVIEW SHEET
Exercise
Skeletal Muscle Physiology
NAME _______________________
LAB TIME/DATE _____________
ACTIVITY 1 The Muscle Twitch and the Latent Period
1. Define the terms skeletal muscle fiber, motor unit, skeletal muscle twitch, electrical stimulus, and latent
period. See definitions provided in the Introduction.
2. What is the role of acetylcholine in a skeletal muscle contraction? Acetylcholine binds to receptors in the
motor end plate, initiating a change in ion permeability that results in the end-plate potential.
3. Describe the process of excitation-contraction coupling in skeletal muscle fibers. Excitation-contraction
coupling is the release of calcium which binds to troponin, removing the blocking action of tropomyosin so
that myosin can bind to actin.
4. Describe the three phases of a skeletal muscle twitch. Latent period is the time preparing for contraction.
Contraction is when muscle tension peaks. The relaxation period is at the end of muscle contraction.
5. Does the duration of the latent period change with different stimulus voltages? How well did the results
compare with your prediction? The latent period did not change with changes in stimulus voltage.
6. At the threshold stimulus, do sodium ions start to move into or out of the cell to bring about the membrane
depolarization? Sodium would move into the cell to bring about membrane depolarization.
ACTIVITY 2 The Effect of Stimulus Voltage on Skeletal Muscle Contraction
1. Describe the effect of increasing stimulus voltage on isolated skeletal muscle. Specifically, what happened
to the muscle force generated with stronger electrical stimulations and why did this change occur? How well
did the results compare with your prediction? The active force increased as predicted to the point in which it
reached a plateau and was no longer able to increase.
2. How is this change in whole-muscle force achieved in vivo? This is achieved by the recruitment of more
muscle fibers over time.
3. What happened in the isolated skeletal muscle when the maximal voltage was applied? All of the muscle
fibers have been recruited and so the maximal force has been achieved.
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ACTIVITY 3 The Effect of Stimulus Frequency on Skeletal Muscle Contraction
1. What is the difference between stimulus intensity and stimulus frequency? The stimulus intensity is the
electrical changes that relate to the action potential. The frequency is the number of action potentials per
minute.
2. In this experiment you observed the effect of stimulating the isolated skeletal muscle multiple times in a
short period with complete relaxation between the stimuli. Describe the force of contraction with each
subsequent stimulus. Are these results called treppe or wave summation? With complete relaxation, it would
be treppe. This is the staircase effect, where you see an increase in the force/tension produced.
3. How did the frequency of stimulation affect the amount of force generated by the isolated skeletal muscle
when the frequency of stimulation was increased such that the muscle twitches did not fully relax between
subsequent stimuli? Are these results called treppe or wave summation? How well did the results compare
with your prediction? The voltage needed to increase because the tension wasn’t great enough at the lower
voltage. This is consistent with wave summation.
4. To achieve an active force of 5.2 g, did you have to increase the stimulus voltage above 8.5 volts? If not,
how did you achieve an active force of 5.2 g? How well did the results compare with your prediction? Yes, it
was necessary to increase the voltage above 8.5 volts to achieve the active force of 5.2 grams.
5. Compare and contrast frequency-dependent wave summation with motor unit recruitment (previously
observed by increasing the stimulus voltage). How are they similar? How was each achieved in the
experiment? Explain how each is achieved in vivo. Frequency-dependent wave summation is dependent
upon stimulation by the nervous system. The motor recruitment depends upon the number of motor fibers
available.
ACTIVITY 4 Tetanus in Isolated Skeletal Muscle
1. Describe how increasing the stimulus frequency affected the force developed by the isolated whole skeletal
muscle in this activity. How well did the results compare with your prediction? The force developed
increases as the stimulus frequency increases – to a point.
2. Indicate what type of force was developed by the isolated skeletal muscle in this activity at the following
stimulus frequencies: at 50 stimuli/sec, at 140 stimuli/sec, and above 146 stimuli/sec. At 50 stimuli/sec:
5.12g. At 140 stimuli/sec: 5.91g. Above 146 stimuli/sec: 5.95g
3. Beyond what stimulus frequency is there no further increase in the peak force? What is the muscle tension
called at this frequency? After 146 stimuli/sec there is no further increase in force. This is the maximal
tetanic tension.
ACTIVITY 5 Fatigue in Isolated Skeletal Muscle
1. When a skeletal muscle fatigues, what happens to the contractile force over time? When skeletal muscle
fatigues, the contractile force decreases over time.
2. What are some proposed causes of skeletal muscle fatigue? The buildup of lactic acid, ADP and inorganic
phosphate are thought to be involved in muscle fatigue.
3. Turning the stimulator off allows a small measure of muscle recovery. Thus, the muscle will produce more
force for a longer time period if the stimulator is briefly turned off than if the stimuli were allowed to
continue without interruption. Explain why this might occur. How well did the results compare with your
prediction? When you increase the rest periods, you see an increase in the muscle tension produced.
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4. List a few ways that humans could delay the onset of fatigue when they are vigorously using their skeletal
muscles. They could periodically rest during vigorous exercise.
ACTIVITY 6 The Skeletal Muscle Length-Tension Relationship
1. What happens to the amount of total force the muscle generates during the stimulated twitch? How well did
the results compare with your prediction? Total force can increase or decrease depending upon the starting
resting length. This is due to the length-tension relationship of the sarcomere.
2. What is the key variable in an isometric contraction of a skeletal muscle? The length-tension relationship.
The passive force is important in determining the active force produced.
3. Based on the unique arrangement of myosin and actin in skeletal muscle sarcomeres, explain why active
force varies with changes in the muscle’s resting length. The active forces vary with the number of
crossbridges formed, which changes with the resting length of the muscle.
4. What skeletal muscle lengths generated passive force? (Provide a range.) The muscle lengths from 80-100
mm generated passive force.
5. If you were curling a 7-kg dumbbell, when would your bicep muscles be contracting isometrically? No, it
would be changing in length, so this would not be isometric contraction.
ACTIVITY 7 Isotonic Contractions and the Load-Velocity Relationship
1. If you were using your bicep muscles to curl a 7-kg dumbbell, when would your muscles be contracting
isotonically? Yes, because your muscle is changing in length.
2. Explain why the latent period became longer as the load became heavier in the experiment. How well did
the results compare with your prediction? The latent period became longer because it takes more time to
generate the force required.
3. Explain why the shortening velocity became slower as the load became heavier in this experiment. How
well did the results compare with your prediction? It takes more time to generate the force required to lift
the heavier load.
4. Describe how the shortening distance changed as the load became heavier in this experiment. How well did
the results compare with your prediction? The shortening distance decreased with the heavier load.
5. Explain why it would take you longer to perform 10 repetitions lifting a 10-kg weight than it would to
perform the same number of repetitions with a 5-kg weight. The velocity of shortening decreases with a
heavier load, so the repetitions will take longer with a 10 kg weight.
6. Describe what would happen in the following experiment: A 2.5-g weight is attached to the end of the
isolated whole skeletal muscle used in these experiments. Simultaneously, the muscle is maximally
stimulated by 8.5 volts and the platform supporting the weight is removed. Will the muscle generate force?
Will the muscle change length? What is the name for this type of contraction? The muscle will still generate
force and change length. The type of contraction is isotonic.
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