Muscle mechanics

A muscle twitch is a brief, weak contraction produced in a muscle fiber in response to a single action potential. While the action potential lasts 1 to 2 msec, the resulting muscle twitch lasts approximately 100 msec. However, a muscle twitch in a single muscle fiber is too brief and too weak to be useful or to perform any meaningful work. In fact, hundreds or thousands of muscle fibers are organized into whole muscles. In this way, the fibers may work 

Number of muscle fibers contracting. As the number of contracting muscle fibers increases, the strength of skeletal muscle contraction increases. Two major factors determine the number of muscle fibers activated at any given 

A motor unit is defined as an alpha motor neuron and all of the skeletal muscle fibers it innervates. The number of muscle fibers innervated by an alpha motor neuron varies considerably, depending upon the function of the muscle. For example, the muscles of the eyes and hands have very small motor units. In other words, each alpha motor neuron associated with these muscles synapses with only a few muscle fibers. As a result, each of these muscles is innervated by a comparatively large number of alpha motor neurons. Densely innervated muscles are capable of carrying out more precise, complex motor activities. On the other hand, antigravity muscles have very large motor units. For example, the gastrocnemius muscle of the calf has about 2000 muscle fibers in each motor unit. Muscles with large motor units tend to be more powerful and more coarsely controlled. 

Multiple motor unit summation involves recruitment of motor units. As the number of motor units stimulated at any given moment increases, the strength of contraction increases. Asynchronous motor unit summation refers to the condition in which motor unit activation within a muscle is alternated. In other words, at one moment, some of the motor units within the muscle are activated, while other motor units are relaxed. This is followed by the relaxation of previously activated motor units and activation of previously relaxed motor units. Consequently, only a fraction of the motor units within the muscle generate tension at any given moment. Therefore, this type of summation may generate submaximal contractions only. 

Amount of tension developed by each contracting muscle fiber. As the 

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Although some species of cestodes (e.g. Hymenolepis diminuta, p. 236) undergo diurnal migrations within the intestine - and therefore their suckers may not always be in a state of sustained contraction - others (e.g. Echinococcus granulosus) apparently remain in the same place and appear to be able to maintain contraction. How this is achieved physiologically is not known, but it has been speculated that this may operate through a catch muscle mechanism (as in lamellibranchs) or by the involvement of special stretch receptors (796). 

Brenner B (1986) The cross-bridge cycle in muscle. Mechanical, biochemical, and structural studies on single skinned rabbit psoas fibers to characterize cross-bridge kinetics in muscle for correlation with the actomyosin-ATPase in solution. Basic Res Cardiol 81 (Suppll) 1-15... 

Zahalak, G.I., ModeUng muscle mechanics (and energetics). In Winters, J.M. and Woo, S.L.-Y. (Eds.), Multiple Muscle Systems Biomechanics and Movement Organization, Springer-Verlag, New York, pp. 1-23,1990. 

Fig. 9.7. Descriptions of muscle mechanics passive stretch curve, energy loss and elastic efhciency...

Fig. 9.8. Description of muscle mechanics experimental contraction modes of a skeletal muscle...

Fig. 9.10. Description of muscle mechanics a force-speed diagram and b standardized force-speed diagram...

The solution of the 3-D elastic problem of a nonlinear nonhomogeneous anisotropic material presents a major engineering challenge. The interaction between the geometry of the moving dynamic muscle and muscle mechanics can be evaluated by different approaches. The obvious difficulty lies in the uncertainty of the spatial values of the material properties in a nonhomogeneous body. 

FEIGL I have a little trouble with your terms elastance and resistance . It seems to me that elastance is similar to what cardiac muscle mechanics people might be tempted to call the length-strength relationship, since you ve derived it from isovolumic contractions. Similarly, resistance would be called the force-velocity relationship. What is your reason for choosing the terms elastence and resistance ... 

Min BG, Kresh JM, Fich S, Kostis JB, Welkowitz W (1978) Relation between computed zero-load aortic flow and cardiac muscle mechanics. J. Biomech 80 227-235 Sandler J, Dodge HT (1963) Left ventricular tension and stress in man. Circ Res 13 91 Shastri SJ (1969) A thevenin equivalent model of the left ventricle derived from hemodynamic measurements obtained by use of a ventricular assist pump. Ph D Thesis, New Brunswick, NJ Rutgers Univ... 

FEIGL I d like to reply to Dr. Mirsky. As one of the few experimentalists in the room, I think it goes both ways. It s difficult for the experimentalist to keep up with the mathematics and the sophisticated techniques that you use. Your papers are hard for us to read so we don t design our experiments to answer exactly the questions that you would like answered. On the other hand, you often present us with models which don t relate to a whole generation s worth of experimental work that s been done in cardiac muscle mechanics. There is a lot of muscle mechanics that s been done and Dr. Mirsky s particularly aware of this. The potential beauty of the Haifa Model is that the linear papillary muscle type cardiac muscle mechanics may be incorporated into a global model. It can be tested at these two levels global perfomance and fiber cardiac mechanics. That gives us to two points to test and compare and that has a nice potential. It remains to be seen how much fruit it bears. 

In our model we aim to incorporate the interrelationship between cardiac muscle mechanics and coronary blood perfusion. In this model geometry of the heart plays an essential role. The method of ultrasound velocity tomography can be useful, as is demonstrated to measure the three-dimensional geometry of the heart in the laboratory with sufficient accuracy. The accuracy (the spatial resolution) of the tomogram depends on the total number of measurements. As the set of equations (lb) can be solved only if the number of variables does not exceed the... 

Wilkie DR (1974) The efficiency of muscular contraction. J Mechanochem Cell Motil 2 257-267 Wong AYK (1973) Some proposals in cardiac muscle mechanics and energetics. Bull Math Biol 35 375-399... 

Panerai, R.B., Amodelof cardiac muscle mechanics and energetics,/. Biomech., 13,929-940,1980. 

Muscle tension, 16-6 Muscle tissue stimulation, 28-17 Muscle, mechanics of, 48-3—48-4 energy transformation in, 66 2 fiber architecture, 48-3 force—length relationship of,... 

Tirosh, R Liron, N. Oplatka, A. A hydrodynamic mechanism for muscular contraction. In Cross-Bridge Mechanism in Muscle Contraction, Proceedings of the International Symposium on the Current Problems of Sliding Filament Model and Muscle Mechanics, Tokyo, Japan, 1978. University of Tokyo Press Tokyo, 1979 pp. 593-609. 

Hill, A.V. First and Last Experiments in Muscle Mechanics. Cambridge University Press London, 1970. 

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