PEX-02-05

Exercise 2: Skeletal Muscle Physiology: Activity 5: Fatigue in Isolated Skeletal Muscle Lab Report Pre-lab Quiz Results You scored 50% by answering 2 out of 4 questions correctly. 1. When skeletal muscle twitches fuse so that the peaks and valleys of each twitch become indistinguishable from each other, the muscle is in a state known as You correctly answered: d. complete (fused) tetanus. 2. When the stimulus frequency reaches a value beyond which no further increase of skeletal muscle force can occur, the muscle has reached its You correctly answered: c. maximal tetanic tension. 3. A decline in a muscle's ability to maintain a constant level of force, or tension, after prolonged, repetitive stimulation is called Your answer : d. rigor mortis. Correct answer: c. fatigue. 4. Which of the following is not thought to be a contributing factor to the development of fatigue? Your answer : c. buildup of ADP in the muscle fibers Correct answer: a. buildup of Ca2+ in the muscle fibers

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Experiment Results Predict Question: Predict Question: If the stimulator is briefly turned off for defined periods of time, what will happen to the length of time that the muscle is able to sustain maximal developed tension when the stimulator is turned on again? Your answer : a. The length of the rest period will not substantially impact the length of time for sustained muscle tension. Stop & Think Questions: Why does the stimulated muscle force begin to decrease over time despite the maintained stimuli? (Note that a decrease in maximal force indicates muscle fatigue is developing.) Your answer : b. Intracellular ADP and Pi are accumulating. Correct answer: d. More than one of these answers could be correct. Why did the length of the intervening rest period affect the length of time the skeletal muscle can maintain maximum tension once the stimulator is turned on again? Your answer : b. Intracellular concentrations of acetylcholine increased during the rest period. Correct answer: c. Intracellular concentrations of ADP and Pi declined during the rest period. Experiment Data: Voltage

Stimuli/sec

Rest Period (sec)

Active Force (g)

8.5 8.5 8.5 8.5

120 120 120 120

0 0 12 22

5.86 5.86 5.86 5.86

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Sustained Maximal Force (sec) 10 10 1.80 5.80

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Post-lab Quiz Results You scored 80% by answering 4 out of 5 questions correctly. 1. During cross bridge cycling in skeletal muscle, force is created by the You correctly answered: c. power stroke of the myosin heads. 2. The term tetanus refers to You correctly answered: b. sustained muscle tension due to repetitive stimuli. 3. A decline in a muscle's ability to maintain a constant level of force, or tension, after prolonged, repetitive stimulation is called You correctly answered: c. fatigue. 4. During fatigue Your answer: d. intracellular calcium becomes depleted; therefore, cross bridges cannot cycle any longer. Correct answer: c. the number of active cross bridges begins to decline although the rate of stimulus delivery (frequency) remains constant. 5. If an intervening rest period is imposed on active skeletal muscle You correctly answered: c. the development of fatigue will be delayed.

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Review Sheet Results 1. When a skeletal muscle fatigues, what happens to the contractile force over time? Your answer: When a skeletal muscle fatigues the contractile force declines due to previous contractile activity. Once the muscle has reached maximum tetanic tension, there is no longer an increase in force generated by the muscle. At this stage, the muscle becomes fatigued and contractile force starts to decrease over time.

2. What are some proposed causes of skeletal muscle fatigue? Your answer: The buildup of lactic acid, ADP, and Pi in the muscle fibers. One other reason comes from Calcium levels decreasing from muscle contractions.

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? Your answer: By turning the stimulator off, a period of rest is created; thus allowing the concentrations of intercellular concentrations of Lactic Acid, ADP, and Pi to decrease. With these concentrations being at a decreased level, the length of time that the muscle is able to maintain maximum tension increases. If the stimulator was not turned off for a short time, the muscle would not be allowed to reach a rest period and further fatigue would continue. The results proved my prediction to be incorrect. 4. List a few ways that humans could delay the onset of fatigue when they are vigorously using their skeletal muscles. Your answer: Doing multiple sets of low repetition exercise -Allowing for multiple times of rest periods - Healthy Diet, along with adequate exercise. (Frequent exercise creates endurance, and endurance resists fatigue.)

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