Introduction
Muscle contractions are a result of the buildup of tension within the muscle, and for muscles to contract, they must have a continuous supply of energy in the form of a molecule called adenosine triphosphate or ATP (Silverthorn, D.U., 2010). Through muscle contractions, we are able to run, walk, lift, push, sit, and even chew our food (Stabler, et.al, 2009). In addition to an energy requirement, skeletal muscles must be stimulated to contract (Stabler, et.al, 2009). Skeletal muscles are stimulated from an action potential that originates from within motor neurons (Stabler, et.al, 2009). Motor neurons are those that send electrical signals to skeletal muscle cells (Stabler, et.al, 2009). An action potential is the electrical signal that occurs when positively charged ions flood into the motor neuron as a result of a chemical, electrical, or other type of stimulus (Stabler, et.al, 2009). This signal, an area of intracellular positivity, self propagates down the length of the neuron towards the muscle cell (Silverthorn, D.U., 2010). Once this signal reaches the muscle cell, it is converted into a muscle contraction through a process called excitation-contraction coupling (Stabler, et.al, 2009). The interior of muscle cells also becomes very positive resulting in a muscle contraction.

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Muscle contractions have 3 different phases which include the latent period, contraction phase, and the relaxation phase (Silverthorn, D.U., 2010). The latent period occurs between the start of an action potential and the beginning of a muscle contraction (Stabler, et.al, 2009). This is the phase that will be studied later. The contraction period begins at the end of the latent period and ends when muscle tension ends (Stabler, et.al, 2009). The relaxation period occurs begins at the end of the contraction period until the muscle becomes free of tension (Stabler, et.al, 2009). To initiate a muscle contraction, the stimulus must reach its threshold. This is the minimal stimulus required to generate the action potential within a muscle cell causing the internal cellular environment to become positive (Stabler, et.al, 2009). In addition, the change in stimulus intensity can play a role in how strongly the muscle generates force when it contracts which is referred to as the active force (Stabler, et.al, 2009). As a stimulus is repeatedly applied to a muscle, fatigue will eventually occur. Fatigue can refer to a deficit in muscle functioning or a gradual decline in the force sustained by a muscle (Enoka and Duchateau, 2008). Other research has shown that fatigue could be the result of metabolic changes that occur within the contractile mechanisms within the muscle fibers such as changes in ion concentrations (Allen and Westerbland, 2001).
If the latent period length is dependent upon the strength of the stimulus, increasing the electrical stimulus intensity should also increase the latent period, and since a threshold stimulus needs to be reached for a contraction to occur, then there will be a minimal amount of electrical stimulation required to generate a muscle contraction. In addition, if the active force strength is dependent upon the strength of the stimulus intensity, an increase in stimulus intensity should increase the active force. If muscle fatigue is occurring due to repeated stimuli over a period of time, then applying a stimulus at a constant rate should result in a decrease of sustainable force within the muscle. These experiments will be carried out using an electrical stimulus by passing a known amount of voltage through an isolated skeletal muscle attached to a metal holder that will transmit the data to a recorder and an oscilloscope screen for analyses (Stabler, et.al, 2009)
Materials and Methods
In order to understand muscle contraction physiology, I evaluated 4 different experiments. The first 3 experiments were designed to use a single stimulus to evaluate the latent period of a muscle contraction, to evaluate the threshold stimulus of a muscle contraction, and to evaluate the effects of increased stimulus intensity on a muscle contraction. The fourth experiment was designed to demonstrate the effects of muscle fatigue. The following materials were used for these experiments: an isolated skeletal muscle (75mm in length), a metal holder to measure force generated by the skeletal muscle, an oscilloscope, an electrical stimulator (single and multiple stimulus), and a data collection box. The first experiment was designed to determine the latent period of a muscle contraction. First, the muscle was attached to the metal holder. The electrode from the electrical stimulator was rested on the surface of the muscle. The electrical stimulator was set to 6.0 volts. A muscle contraction was induced by applying a single electrical stimulus using the electrical stimulator. The data generated a tracing on the oscilloscope screen which was used to determine the latent period by selecting the point where the flat line began to rise. The data were recorded using the data collection box. I repeated this experiment using the following voltages: 1.0 volts, 3.0 volts, and 10.0 volts. These voltages were used to see if changes occurred within the latent periods. For the second experiment, the data generated was used to determine the threshold voltage. The threshold voltage occurred when the active force measured in grams was greater than 0. The equipment setup was the same as the last experiment, and the electrical stimulator was set to 0.0 volts. At 0.0 volts, the muscle was stimulated and the results observed and recorded using the oscilloscope and data recorder respectively. This experiment was repeated multiple times by increasing the voltage by 0.1 volts until the minimal threshold voltage was determined. For the third experiment, the effects on muscle contractions due to an increase in the electrical stimulus intensity were explored. Again the same equipment setup was used. The initial voltage was set to 0.5 volts followed by stimulation of the skeletal muscle. The data were observed and then recorded. This experiment was repeated multiple times by increasing each subsequent voltage by 0.5 volts. This continued until the data showed there was no change in the increase in active force. For the final experiment, fatigue was induced in the skeletal muscle. The equipment setup for this experiment was similar to the first three experiments. However, a different electrical stimulator was used which incorporated a multiple stimulus option as well as a single stimulus option. The multiple stimulus option added the ability to start and stop the stimulus activity. This experiment was designed so that several stimuli per second were being applied to the skeletal muscle if so desired. The electrical stimulator voltage was set to 7.0 volts, and the number of stimuli per second was set to 100. The muscle was then stimulated for approximately 400 seconds by selecting the multiple stimulus option, and the graphical data were recorded from the oscilloscope.
Results
For experiment one, the latent period was recorded in milliseconds and was compared to its corresponding stimulus voltage. The time measurement (latent period) reflected the start of the flat line until it began to rise. Below is a summary of the recorded data.
Latent Period Determination
Stimulus Voltage (V)
Latent Period (msec)
1
3.89
3
2.78
10
2.22
For experiment two, the threshold stimulus determination data was collected by measuring the electrical stimulus voltage and its corresponding active force generated. Once the active force became greater than 0, the experiment was stopped. Below is a table with the collected data.
Threshold Determination
Stimulus Voltage (V)
Active Force Generated (gms)
0
0
0.1
0
0.2
0
0.3
0
0.4
0
0.5
0
0.6
0
0.7
0
0.8
0.02
For experiment three, the data were collected in order to determine the effects of increased stimulus voltage on muscle contractions. The data reflected 0.5 volt interval increases in the electrical stimulus until 10 volts were reached. Below is the summary of the data.
Muscle Contractions – Increased Stimulus Effects
Muscle Contractions – Increased Stimulus Effects
Stimulus Voltage (V)
Active Force Generated (gms)
Stimulus Voltage (V)
Active Force Generated (gms)
0.5
0
5.5
1.59
1
0.15
6
1.65
1.5
0.43
6.5
1.7
2
0.66
7
1.74
2.5
0.87
7.5
1.78
3
1.04
8
1.81
3.5
1.19
8.5
1.82
4
1.32
9
1.82
4.5
1.42
9.5
1.82
5
1.51
10
1.82
For experiment four, data was graphed in order to demonstrate the effects of fatigue. The rate of the multiple stimulus was 100 stimuli/second at a constant setting of 7.0 volts. The data were recorded over a 400 second interval. Below is a graphical representation of the collected data.
Muscle Fatigue – Effects of Prolonged Stimuli Over Time (Stabler, et.al, 2009)
C:Shea’s StuffHuman PhysiologyFatigue.jpg
Citations
Allen, D.G. and H. Westerbland. (2001). Topical Review: Role of phosphate and calcium stores in
muscle fatigue. Journal of Physiology 536.3: 657-665.
Enoka, R. and J. Duchateau. (2008). Muscle Fatigue: what, why and how it influences muscle function.
Journal of Physiology 586.1: 11-23.
Silverthorne, D.U. 2010. Human Physiology: An Integrated Approach. 5th Edition. Pearson
Benjamin Cummings, pp. 408-422.
Stabler, T., Smith, L., Peterson, G., and Lokuta, G. 2009. PhysioEx 8.0 for Human Physiology —
Laboratory Simulations in Physiology. pp. 17-22.