Exercise 2: Skeletal Muscle Physiology: Activity 3: The Effect of Stimulus Frequency on Skeletal Muscle Contraction Lab Report

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1 Name: Courtney.Sims Exercise 2: Skeletal Muscle Physiology: Activity 3: The Effect of Stimulus Frequency on Skeletal Muscle Contraction Lab Report Pre-lab Quiz Results You scored 100% by answering 4 out of 4 questions correctly. 1. During a single twitch of a skeletal muscle You correctly answered: b. maximal force is never achieved. 2. When a skeletal muscle is repetitively stimulated, twitches can overlap each other and result in a stronger muscle contraction than a stand-alone twitch. This phenomenon is known as You correctly answered: c. wave summation. 3. Wave summation is achieved by You correctly answered: a. increasing the stimulus frequency (the rate of stimulus delivery to the muscle). 4. Wave summation increases the force produced in the muscle. Another way to increase the force produced by a muscle is to You correctly answered: d. increase the number of activated motor units. 11/11/13 page 1

2 Experiment Results Predict Question: Predict Question 1: As the stimulus frequency increases, what will happen to the muscle force generated with each successive stimulus? Will there be a limit to this response? Your answer : d. As the stimulus frequency increases, the muscle force generated by each successive stimulus will first increase and then decrease as the stimulus frequency becomes very high. Predict Question 2: In order to produce sustained muscle contractions with an active force value of 5.2 grams, do you think you will need to increase the stimulus voltage? Your answer : a. yes Stop & Think Questions: Was there any change in the force generated by the muscle during the second stimulated twitch? You correctly answered: c. Yes, the second twitch generated more muscle force. Is the total muscle force generated by the higher frequency stimulation greater than the force generated in previous stimulations? You correctly answered: a. Yes, it is greater than the previous stimulations. Does the force generated by the muscle change with each additional stimulus? You correctly answered: b. As the stimulus frequency increased, the muscle tension generated by each successive stimulus also increased, and a limiting maximum value was observed. Experiment Data: Voltage Length Stimulus Active Force Passive Force Total Force Single Single Single Multiple Multiple Multiple /11/13 page 2

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5 Post-lab Quiz Results You scored 100% by answering 4 out of 4 questions correctly. 1. Which of the following is not one of the ways that the body can increase the force produced by a skeletal muscle? You correctly answered: d. application of higher voltages to the whole muscle 2. When a muscle receives a stimulus frequency that causes non-overlapping twitches to follow each other closely in time such that the peak tension of each twitch rises in a stepwise fashion up to a plateau value, the result is known as You correctly answered: c. treppe. 3. In this experiment the isolated skeletal muscle was repetitively stimulated such that individual twitches overlapped with each other and resulted in a stronger muscle contraction than a standalone twitch. This phenomenon is known as You correctly answered: c. wave summation. 4. Wave summation is achieved by You correctly answered: a. increasing the rate of stimulus delivery (frequency) to the muscle. 11/11/13 page 5

6 Review Sheet Results 1. What is the difference between stimulus intensity and stimulus frequency? Stimulus Intensity - Is the strength of Apmlitude of the stimulus. Stimulus Frequency - Is the number of times per unit time that the stimulus is presented. 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? When stimulating the isolated skeletal muscle multiple times in a short period with complete relaxation between each stimuli the amount of force between each stimulus increased progressively in step like form to a maximum peak in which a plateau of the force occurred. This is known as Treppe. 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? When the stimulation frequency was increased to the point in which muscle relaxation did not occur the amount of force progressively increased with each stimulus. When muscle twitches overlap, no muscles relax, and stimuli occurs one after another in a short amount of time, the muscle contraction is higher than if a single stand alone contraction had occurred. This is known as 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? No, I did not have to increase the stimulus voltage above 8.5 volts. In order to achieve an active force of 5.2g the stimuli needed to be delivered in rapid succession in a short amount of time and not allowing for full relaxation of the muscle. In order to get 5.2g of active force, wave summation needed to be performed. 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. 11/11/13 page 6