The contraction of muscle fibers and muscles are similar. There are two types of muscle contractions, isometric contraction and isotonic contraction. Isometric contractions are contractions where tension increases to the muscle’s capacity, but the muscle neither shortens nor lengthens. Isometric contractions occur if the load is greater than the tension, then the muscle is able to develop. In isotonic contractions, the muscle changes in length and moves the load. There are two types of isotonic contractions, concentric and eccentric isotonic contractions. Concentric contractions is when the muscle shortens and does work, whereas eccentric contractions is when the muscle contracts as it lengthens. (Marieb & Hoehn, 2007)
Method:
The experiment requires a force transducer, electrical stimulator, oscilloscope, electrode, weight cabinet, and a computer, along with students who volunteered to participate in the experiment.
Experimental Arrangement. Force transducer was connected to the computer, and was used to measure force output. The analog signal from the force transducer was converted to digital signal and transferred to the oscilloscope displayed on the computer.
An electrical stimulus was performed via electrical stimulator and electrode. The stimulation voltage (V) and the length of muscle (in mm) was set by adjusting the (+) and (-) buttons. By clicking the stimulate button, electrical shock was delivered to the muscle through the electrode placed on the surface of the muscle. The data was recorded during each experiment and were transferred to the data grid for analysis. (Zao et al., 2008)
Procedure. Prior to any voluntary contractions, each subject was given a series of single, multiple stimuli, isometric contractions and isotonic contractions.
A single stimulus to the muscle (75mm) was administered at approximately 2-3 seconds intervals with slight increased in voltage after each stimuli (starting at 0V) until reached the maximum voltage of 10.0. The data was then recorded to explore the phases of twitch contraction. In multiple stimuli, the muscle (75mm) was stimulated at 8.2 volts for 2-3 seconds. Treppe effect was achieved by adding a single stimulus (8.2V) to the complete relaxation period of the previous stimuli. The wave summation of the muscle (75mm) was achieved with a rapid series of single stimuli of the same intensity (8.2V) before the muscle relaxed completely. To achieve tetanus, a multiple stimulus (8.2V) was made at the stimulus rate of 30 stimuli/sec. To investigate muscle fatigue, a multiple stimulus (8.2V) was made at stimulus rate of 120 stimuli/sec. prior to the relaxation period between twitches.
In isometric contraction, the muscle length shortened to 50mm and was stimulated at 8.2 volts for 2-3 seconds. A small increment (2mm) of muscle length was added to each stimulus until reached 100mm. During isotonic contraction, the effect of load on skeletal muscle was investigated when the fibers of a skeletal muscle are slightly stretched by a weight or tension.
Result:
Figure 1. Twitch Contraction
During a twitch contraction, the latent period was measured to be around 3 msec, the contraction period are between 17 to 30 msec time interval, and the relaxation period are between 30 to 140 msec time interval.
Table 1. Twitch Contraction (Increased in Intensity)
Figure 2. Increase Intensity
Increased in voltage or the intensity of the stimuli produced a greater tension, force reduction.
Figure 3. Treppe Effect
In treppe effect, applying a stimulus after a complete relaxation increases force production.
Figure 4. Stimulation Frequency
In wave summation, superimposed the stimulus while the force generation is being relaxed produced more of an increase in force, also known as unfused tetanus. During tetanus and muscle fatigue, an increased in force production produced a nice smooth graph.
Table 2. Isometric
Figure 5. Length-Tension Relationship
In isometric contraction, an increase in muscles length would most likely result in an increase in force production.
Table 3. Isotonic (Effect of Load)
Figure 6. Effect of Load
In isotonic contraction, constant length but increased in load decreased velocity but produced greater tension. Lifting a light load produced a low force and high velocity contraction, whereas lifting a heavy load will produced a high force and low velocity contraction.
Table 4. Isotonic (Constant Load/ Change in muscle Length)
Figure 7. Effect of Changes in Muscle Length
In isotonic contractions, constant load but increased in muscle length result in an increased and then decreased in velocity and force production.
Conclusion:
Generally, muscle fibers of an individual exhibit an all-or-none response. This means that a muscle can elicit contraction only when the stimulus reaches the threshold value. So when increasing the intensity of stimuli during twitch contraction, the muscle fiber become more excited and produces a greater tension (see Table1).
In treppe effect, the muscle was stimulated multiple times with complete relaxation between stimuli. As a result, an additional increase in tension occur up to a certain amount of time where the force will not increase, also known as maximal contraction (see Figure 2). Such effect occurred because there are some lingering calcium left over in the intracellular sarcoplasmic reticulum, which can used to generate more force. The efficiency activity of the enzyme that involved in the muscle force can also increase force.
In wave summation, the reason why a wave formation occurred when superimposed the stimulus was that there are some residual calcium left in the cell from the previous stimuli, applying another stimulus will generate more calcium into that cell, which produced an increase in force. The enzyme efficiency are also the case, but increasing the frequency of stimulation will increase the amount of force.
Smooth contraction is where tetanus and muscle fatigue occurs. Taking the advantage that there are some residual calcium left within the cell, increasing the frequency of stimulation, bringing the stimuli closer and closer, excited more muscle fibers. When the stimulations are so close together, a nice smooth increase of force emerged, not allowing the muscle to relax at all known to be fused tetanus (see Figure 3). To a point in which the muscle has lost its ability to contract will result in muscle fatigue.
In isometric contraction, when the length of muscle increased the tension also increased but then decreased. This length-tension relationship shows the magnitude of overlap between the actin and myosin filaments. When muscle length increased to 100% of resting sacromere length, all of the available actin molecules overlap with all the available myosin heads, which generates maximum tension (see Figure5). Increasing muscle length over 100% of resting sacromere length will developed no active tension because there is no overlap between actin and myosin filaments.
In isotonic contractions, the force generated by a muscle depends on the resistance that the muscle contracts. When lifting heavy load, the muscle needs to generate more force to overcome the resistance, which prolong the latent period and result in an increase in muscle tension and decrease in initial velocity. So the force-velocity relationship was that lifting light load would generate low force and high velocity contraction, whereas lifting heavy load will generate high force and low velocity contraction (see Figure 6).
Overall, the amount of force generated at certain stimulations are determined by the number of muscle fibers available, the size of the muscle cells, and the number of cross-bridge formations in the muscle fibers.
Reference:
Marieb, E., & Hoehn, K. (2007). Human Anatomy & Physiology (7th ed.), p.281-307.
San Francisco: Pearson Education.
Zao, P., Stabler, T., Smith, L., Peterson, G., Gibson, M., Zanetti, N., et al. (2008).
PhysioEx for A&P Laboratory Simulations in Physiology (7th ed.), p.17-26.
San Francisco: Pearson Education.