[PhysioEX Chapter 2 exercise 3] PEX-02-03

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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.

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 : b. 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: 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
8.5 75 Single 1.83 0.00 1.83
8.5 75 Multiple 3.23 0.00 3.23
8.5 75 Multiple 3.17 0.00 3.17
8.5 75 Multiple 4.58 0.00 4.58
10 75 Multiple 4.68 0.00 4.68
8.5 75 Multiple 5.74 0.00 5.74

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.

Review Sheet Results

1. What is the difference between stimulus intensity and stimulus frequency?
   Your answer:
   Stimulus intensity refers to the strength of stimulus.
   Stimulus frequency refers to the rate of stimulus.
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?
   Your answer:
   When stimulating the isolated skeletal muscle multiple times in a short period with complete relaxation between, each force
   between each stimulus increased slightly in step like form to a max peak. This is 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?
   Your answer:
   In time that the stimulation frequency was increased to the situation in which muscle relaxation did not occur, the amount of
   force increased with each stimulus. When muscle twitches overlap and stimuli occurs one after another in a short amount of
   time, the muscle contraction is higher 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?
   Your answer:
   My prediction was wrong. I did not increase the stimulus voltage, but just increase the rate of stimulus.
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.
   Your answer:
   Frequency-dependent wave summation is dependent on stimulation by the nervous system.
   The number of motor fibers that available influence the motor recruitment.