MMG simply stated, is the sound that resonates from the muscle during a stimulated contraction. The act of pulling on a taught string and letting go has been related to model the sound generated by the muscle. This sound can be caused by the cycling of the actin myosin cross bridges, or perhaps by the gross lateral shift of the muscle, or maybe even the brief shock generated by the thickening of several active muscle fibers. These are three of the hypothesis that try to account for the sound emitted from the muscle. The exact cause of mechanomyography is yet to be determined, but there are a few attributes that can be spoken for. It is understood that the muscle sound is related to muscle activity and its properties are related to the properties of the contraction. (8) The main frequency of MMG is 25 Hertz, British physicist, chemist, and physician William Wollaston determined this. This can possibly be attributed to the ATP turnover rate. ATP can hydrolyze and refurbished at a rate of about 40msec, which translates into 25Hertz. (8,10)
The MMG signal can be recorded using a variety of transducers. One is the piezoelectric crystal contact sensor (microphone).Some researchers have used a hydrophone in studies when the muscle was submerged in water in vivo, others have used condensers.(11) It will be discussed later some of the differences between the types.
Factors that influence the MMG signal can include stiffness of the muscle, fluid in an around the muscle, temperature of the tissue, fiber type, gender, technique and equipment used, type of contraction performed, amount of fatigue, and cross talk between muscles.
Stiffness can alter the MMG by lowering the amplitude. As the muscle produces a greater force the stiffness of the muscle increases linearly with the force increase. When subject reaches 80 100% of MVC this stiffness can cause the amplitude of the signal to decrease due to the fusion of motor unit twitches, which will limit the oscillation.(2) This stiffness can also be attributed when talking about the different types of contraction.
Types of contraction have been shown to provoke different amounts of peak torque. Eccentric forms of contraction elicit greater force production than that of concentric. This could cause a greater stiffness in the muscle during eccentric contractions, which could lower the MMG. Also there is the mechanism behind the contraction that could be the factor. As stated earlier, the cycling of the cross bridges is supposed to produce the MMG signal. With eccentric contraction this cycling of the cross bridges would certainly be altered form that of concentric contraction. With the breaking of the cross bridges, instead of the binding and cycling.
Another factor that affects the MMG signal could be the amount fluid in and around the muscle. It has been hypothesized that the sloshing of the fluid may be the cause for increases in the signal with velocity of contraction. This is definitely an area that needs further study. What are the effects of hydration status on MMG?
Temperature has been shown to have a linear relationship with MMG. The amplitude is especially influenced by temperature.(8,11) Orizio and Stokes show a decrease in amplitude from 100% to 15% with a decrease in temperature from 25C to 7C respectively.
Fiber type can be a determining factor in MMG signal. Marchetti states the cause of the MMF signal is the change in tension of the sarcomere, which would incorporate the intrinsic functions of the fiber, namely the speed of contraction.(7) Also, it has been shown that fast twitch fibers are characterized by a higher mean and median power frequency than are slow twitch fibers.(5) Thus we could interpret fast and slow twitch fibers.
Gender could also play a role in the propagation of the signal. The differences in gender can be attributed to a number of factors. Evetovich stated a gender difference in velocity related patterns of MMG with concentric muscle contraction. The reasons for these differences could be greater muscle mass and thicker adipose tissue.(4) First gender difference can be accounted for by the difference in the amount of adipose tissue.(4) Perhaps the increased amount of adipose in females can act as a low pass filter and may filter some of the MMG signal out. Also, this extra adipose can cause the contact sensor to be further form the active site thus lessening the reception. And the amount of force production could augment the signal, as females cannot produce as muck peak torque as males (generally), which lowers the stiffness of the muscle allowing greater amplitude propagation of the signal through the seemingly more relaxed muscle fiber. Menstrual cycle can also be a factor. Does the female body become dehydrated during menstruation and if so will this effect MMG due to less sloshing of the fluids?
Types of contraction have been shown to provoke different amounts of peak torque. Eccentric forms of contraction elicit greater force production than that of concentric. This could cause a greater stiffness in the muscle during eccentric contractions which has been shown to lower the MMG.(5) Also, there is the mechanism behind the contraction that could be the factor. As stated earlier, the cycling of the cross bridges is supposed to produce the MMG signal at 25 hertz. Supposed with eccentric contraction this cycling of the cross bridges would certainly be different that that of concentric contraction. Could the action of the tearing apart of the cross bridges have an effect on the MMG signal?
The technique used when applying the contact sensor may result in changes in the recorded signal. If the contact sensor is applied with different amounts of pressure it can yield an altered signal. This phenomenon is similar when the muscle is under a substantial amount of stiffness. The sensor must be placed with consistency to reveal reliable data. Also, the location of the contact sensor along the surface of the muscle will affect the signal. Orizio states that the signal is greatest toward the belly of the muscle and decreases as you move toward the tendons. You must also be careful when filtering the recorded signal, this can have an obvious effects on the processing of the signal. It has been shown that most of the useful data will be between 2-100 Hz.(8) So anything that may be out side of this area may be discarded, as it may have been the result of the gross lateral shift at the beginning of the contraction. Simply state your reasons why you filtered the signal and the frequency that you filtered it. And finally, simply the type of sensor that you use is important. There are three types that are used, accelerometer, transducer, and composite probe. The accelerometer gives its measure in m/s2, the transducer gives its measure in mV, and the composite probe detects both EMG and MMG. The accelerometer can be used for smaller muscles and the transducer can be used for larger muscles. I propose an adaptation of the equipment that could benefit further research could be a pressure sensor incorporated into the contact sensor, so that when it is placed on the subject the same amount of pressure could be applied every time and we could eliminate the possibility that the changes in the pressure when microphone is attached was the determining factor behind any change in MMG. This could prove for greater reliability between tests.
MMG has been shown to decrease with fatigue. The relationship of MMG with fatigue simulated that of torque. It was such an identical result that Orizio concluded that MMG might be a good determinate for fatigue.
Cross talk is another variable that may affect MMG. When the contact sensor records the signal of more than the desired muscle this is called cross talk. When corresponding muscles around the intended muscle contract, they also emit a sound frequency that can be picked up by the microphone, thus is integrated into the signal and can give a false response.
Practical uses of MMG can be wide spread, such as fiber typing. Using MMG one might be able to fiber type without having to perform an invasive procedure such as biopsy. MMG could prove to become an inexpensive and convenient way to fiber type. MMG can also be used to diagnose muscle disease. Barry has shown in his research that the electrical efficiency was decreased with pediatric muscle disease. Perhaps the mechanomyographic response can give us a better look at the location and cause of the inefficiency. We can also look at muscle atrophy. Marchetti witnessed a clear reduction of the high frequency signal with muscle atrophy.(7,8) Using this information about atrophy we can then look into the elderly population. Possibly do a comparative study of young subjects and elderly and the MMG responses between the two. We could hypothesize that the elderly population would have a decreased MMG due to the loss of the muscle mass and the amount of recruitment that would be available. This may also give us an insight into the degenerative process of muscle atrophy, is it neural or mechanical or both?
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Advanced Exercise Physiology II