# PowerPhys:Activity 2 Twitch Contractions and Summation

Topics: Muscle, Myosin, Muscular system Pages: 5 (1509 words) Published: January 22, 2014
LABORATORY REPORT
Activity 2: Twitch Contractions and Summation

PREDICTIONS
Effect of Muscle Fiber Length on Contraction
1. As muscle fiber length increases: contraction force increases, becoming maximum at an optimal length, then decreasing at longer lengths.
Effect of Stimulation Frequency on Contraction
2. As the frequency of stimulation increases, the force of contraction: increases.

MATERIALS AND METHODS
Measurement of Threshold Stimulus
1. Dependent Variable: contraction force.
2. Independent Variable: stimulation voltage
3. Controlled Variables: temperature, frequency of stimulation and muscle fiber length. Effect of Muscle Length on Contraction
1. Dependent Variable: contraction force.
2. Independent Variable: muscle fiber length.
3. Controlled Variables: temperature, stimulation voltage, frequency of stimulation. Effect of Stimulation Frequency on Contraction
1. Dependent Variable: contraction force.
2. Independent Variable: frequency of stimulation.
3. Controlled Variables: temperature, stimulation voltage.
4. What structure was stimulated to cause a muscle contraction? The motor unit was stimulated.
5. Explain why the temperature of the water bath was 35ºC (95 ºF). The water bath was 35ºC because is near the mouse’s normal body temperature. 6. At a stimulation frequency of 15 Hz how many stimuli were there per second? There are 15 pulses (stimuli) per second.

RESULTS
See Table 4: Measurement of Threshold Stimulus
See Graph: Effect of Stimulation Value on Twitch Contraction Force 1. What is the threshold stimulus? 400mV.
2. What is the maximum force generated at the threshold stimulus? The maximum force generated was 0.084 gf.
3. How does increasing voltage above threshold stimulus affect force development? The stimulus is measured in volts, and its role is to cause depolarization of the muscle fiber or the nerve. A minimum voltage is needed in order to induce an action potential. This minimal voltage is called a threshold stimulus. Once the depolarization reaches the threshold stimulus, the muscle fiber or the nerve will generate an action potential, and will cause a contraction. As the stimulus voltage is increased past the muscle’s threshold voltage, an increase in the amount of force in the entire muscle can be observed. By increasing the voltage above the threshold stimulus, also the force generated from the muscle increases. An increase in voltage means that it is delivered to the whole muscle, and more motor units are being activated. As a result the total force produced by the muscles increases.

See Table 5: Muscle Length and Contraction Force
See Graph: Effect of Muscle Fiber Length on Contraction Force 4. What was the force of contraction at a muscle length of 7.0 mm? 0.075 gf. 5. What was the optimal muscle length (muscle length that generated the maximum force)? It was between 8.0mm-9.0mm.

6. What was the maximum force generated at optimal length? 0.084 gf. 7. How does increasing muscle fiber length affect force of muscle contraction? A muscle fiber will develop its greatest tension when there is an optimal zone of overlap between thick and thin filaments. The length of a muscle impacts its force-producing capability because of the corresponding overlap of thick and thin filaments at varying lengths. When a muscle fiber is stimulated to contract and that muscle is at an optimal length, indicated by the greatest possible overlap of thick and thin filaments, maximal strength is produced. If a muscle is too short or too long, less than optimal force is produced due to the lack of filament overlap and binding site availability.

See Table 6: Stimulation Frequency and Contraction Force
See Graph: Effect of Stimulation Frequency on Contraction Force 8. What was the force of contraction at a stimulation frequency of 22.5? In table 6 I do not have a value for 22.5. I have 15 stimuli per second and 30 stimuli per second. So the value for 22.5 stimuli per second would be...