Laborator y Exercise 13 Muscle Fatigue and Force Variance Purpose of the Exercise Pre-Lab To perform an investigation to determine causes and effects of force variance and communicate findings. Carefully read the introductory material and examine the entire lab. Be familiar with factors affecting the force of muscle contraction from lecture or the textbook. Answer the pre-lab questions. Materials Needed Pre-Lab Questions: Select the correct answer for each of the following questions: Stopwatch Tennis ball Clothes pin 1. The functional contractile unit within muscle fiber is the a. actin. b. nucleus. c. mitochondrion. d. sarcomere. 2. According to the sliding filament model, muscle contraction begins when a. calcium ion concentration increases exposing binding sites. b. myosin heads pull on actin filaments. c. ATP splits to form ADP and P. d. myosin heads release the actin filaments 3. Summation is a process that occurs when muscle fibers are stimulated repeatedly. Learning Outcomes After completing this exercise, you should be able to: 1 2 3 4 5 Describe the relationship between muscle structures and muscle contraction. Research and investigate causes of variance in the force of muscle contractions. Communicate findings about the causes of force variance. Perform an investigation to determine causes and effects of force variance. Communicate findings about the effects of force variance. True False 4. Muscle fatigue is a cause of force variance in muscle contraction. True False 5. During a muscle contraction, the thick and thin fibers both shorten. True False 6. During a muscle contraction, cross-bridges are formed between actin and myosin. True False 7. The force of a single muscle twitch stays the same through the period of contraction and period of relaxation. True 117 False slide past one another. This causes the sarcomere to shorten. The shortening of many sarcomeres shortens the muscle fiber, which pulls on the sites of attachment of the muscle. The sliding filament model of muscle contraction explains that the shortening of the sarcomeres occurs when the filaments slide past one another, with the thin filaments moving toward the center of the sarcomere. Figure 2 illustrates the sliding filament model of muscle contraction. The contraction cycle is dependent on the presence of ATP and calcium ions. A skeletal muscle is composed of muscle fibers, as shown in figure 1. Each muscle fiber is multinucleated, thin, and elongated. The cytoplasm of the muscle fiber (called the sarcoplasm) contains many mitochondria. The sarcoplasm also contains structures called myofibrils, which play a key role in muscle contraction. Myofibrils contain thick filaments made up of myosin (a protein) and thin filaments made up of actin (a protein). Myofibrils are composed of a repeating pattern of units called sarcomeres. Sarcomeres can be considered the functional unit of muscle contraction. During a contraction, the filaments FIGURE 1 Structure of a segment of a muscle fiber. Nucleus Triad Terminal cisternae Transverse (T) tubule Myofibrils Openings into transverse tubules Sarcoplasmic reticulum Sarcolemma Thick filament Mitochondria Thin filament 118 FIGURE 2 The sliding filament model of muscle contraction. (1) relaxed muscle. (2) and (3) When calcium ion concentration rises, binding sites on actin filaments open and myosin heads bind to the actin, forming cross-bridges. (4) upon binding to actin, myosin heads spring from the cocked position and pull on actin filaments. (5) ATP binds (but is not yet broken down), causing the myosin heads to release from the actin filament. (6) ATP breakdown provides energy to “cock” the unattached myosin heads. as long as ATP and calcium ions are present, the cycle continues. When the calcium ion concentration in the cytosol is low, the muscle is relaxed. Tropomyosin Thin filament Troponin Actin molecules Thick filament ADP + P 1 Relaxed muscle Muscle relaxation + ATP Active transport of Ca 2 into sarcoplasmic reticulum, which requires ATP, makes myosin binding sites unavailable. ADP + P ADP + P ADP + P +2 Ca Ca 2 binds to troponin Tropomyosin pulled aside Binding sites on thin filament exposed + Ca 2 ADP + P ATP + Ca+2 Continued contraction If the stimulus for contraction is maintained, Ca+2 continues to bind to troponin. 6 ATP splits, which provides power to “cock” the myosin heads and store energy for the next power stroke ATP Muscle contraction + Release of Ca 2 from sarcoplasmic reticulum exposes binding sites on thin filament: Contraction cycle ATP +2 Ca +2 Ca ADP + P 2 Exposed binding sites on actin allow the muscle contraction cycle to occur 5 New ATP binds to myosin, breaking the connection to actin ADP + P ATP ADP + P ADP + P 3 Myosin heads bind to actin, forming cross-bridges, connecting myosin to actin P ADP P ADP 4 ADP and P release from myosin and crossbridge pulls thin filament (power stroke) 119 3. When stimuli occur at increasing frequency, muscle fibers cannot completely relax between twitches. The individual muscle twitches combine in a sustained contraction. The force of the individual twitches combines in a process called summation. Figure 4b shows how the process of summation causes force variance in muscle contractions. 4. Figure 4c shows how the force of muscle fiber contractions is affected when frequency of stimulation increases. Notice that the force generated levels off after achieving a maximum value. Procedure A-Force Variance in Muscle Contraction 1. Force variance in muscle contraction has a variety of causes. First, examine Figure 3 to visualize how the force of a single muscle twitch varies over time. Analyze how the pulling force of a single muscle fiber varies from the time of stimulation to the end of the period of relaxation. 2. Figure 4 ATP shows a series of single muscle twitches. Compare this to the single muscle twitch graphed in Figure 3. Locate the time of stimulation, period of contraction, and period of relaxation of each of the twitches shown in Figure 4a. Add labels to one of the muscle twitches shown in 4a. Myograms of (a) a series of twitches (b) summation, and (c) a forceful sustained contraction Force of contraction FIGURE 3 FIGURE 4 A myogram of a muscle twitch Force of contraction Latent period (b) Period of contraction Time of stimulation Period of relaxation Force of contraction Force of contraction (a) Time (c) 120 Time FIGURE 5 Force variance due to muscle fiber length at time of stimulation Optimal length Overly stretched Force Overly shortened Muscle fiber length 5. Then, analyze Figure 5 to determine how muscle fiber length at the time of simulation is associated with force variance in muscle contractions. Communicate your findings by discussing the information shown in the graph with a partner. 7. Trade roles with your partner. Again, record your results in the data table. 8. Graph the results. Analyze your data to determine how muscle fatigue affected the amount of force your muscles could exert. Procedure B-Force Variance in Muscle Contraction Due to Fatigue FIGURE 6 1. Exercise or other use of a muscle for a prolonged period may cause a muscle to contract with decreased force. This condition is called muscle fatigue. Prolonged exercise causes muscles to produce lactic acid as muscle metabolism shifts from aerobic to anaerobic ATP production. Research is currently being carried out to further clarify the role of lactic acid in muscle fatigue. Muscle fatigue is a factor that can causes variance in the force of muscle contractions. 2. Work with a partner. Obtain a stopwatch and a clothes pin. 3. Grip the clothes pin as shown in Figure 6. Squeeze to open the clothes pin. Think about the role of muscle contraction plays in opening the clothes pin. 4. You will carry out ten 10-second sessions. In each session you will count how many times you can open and close the clothes pin as your partner times you. 5. Between each 10-second session of squeezing the clothes pin, rest for 5 seconds. 6. Record your results in the data table. 121 9. To further investigate the cause and effect of muscle fatigue, you will carry out a similar investigation using a tennis ball. 10. Work with a partner. Obtain a stopwatch and a tennis ball. 11. Grip the tennis ball as shown in Figure 7. Squeeze the tennis ball and identify the muscles that are involved in this action. Discuss this with your partner. 12. You will carry out ten 10-second sessions. In each session you will count how many times you can squeeze the tennis ball as your partner times you. 13. Between each 10-second session of squeezing the tennis ball, rest for 5 seconds. 14. Record your results in the data table. 15. Trade roles with your partner and carry out the steps of the investigation again. Record your results in the data table. 16. Graph the results. Analyze your results to determine how muscle fatigue affected the amount of force your muscles could exert while squeezing the tennis ball. FIGURE 7 122 Name Laboratory Assessment 13 Date Section The A corresponds to the indicated outcome(s) found at the beginning of the laboratory exercise. Muscle Fatigue and Force Variance Part A Assessments 1. Identify one muscle structure and describe the role the structure plays in muscle contraction. 1 2. Describe how the force exerted by a single muscle twitch varies from the time of stimulation to the end of the period of relaxation. 2 Force of contraction 3. Explain the process of summation and how it affects the force exerted by muscle fibers. Reference the graph in your explanation. 2 3 Force of contraction (a) (b) Force of contraction 4. Explain how the force generated by contraction of muscle fibers is affected by the length to which the muscle fibers are stretched (c) when they are stimulated. 2 3 Time 123 Part B Assessments 5. Record the results of your first investigation in the table. 4 10-second Trial Number of Clothes Pin Squeezes, Participant 1 Number of Clothes Pin Squeezes, Participant 2 1 2 3 4 5 6 7 8 9 10 6. Record the results of your second investigation in the table. 4 10-second Trial Number of Tennis Ball Squeezes, Participant 1 1 2 3 4 5 6 7 8 9 10 124 Number of Tennis Ball Squeezes, Participant 2 7. Analyze your data. What trends do you notice? 4 8. Explain why the number of squeezes can be used to make inferences about force variance. 4 9. Write a sentence that communicates the causes and effects of the force variance you observed in the investigation. 5 Critical Thinking Assessment Compare the difficulty of climbing steps one step at a time and climbing steps two steps at a time. Relate the relative difficulty of these two tasks to the force variance caused by the length to which the muscles fibers are stretched when stimulated. 3 Think about the impact of muscle fatigue on the force generated by muscle contractions. Identify and describe three everyday situations on which muscle fatigue has an impact. 5 125 NOTES 126
