A mechanical model of the biceps brachii muscle
This contribution deals with the development of mechanical model of the biceps brachii muscle. In order to account for the finite speed of propagation of the activation pulses, the model has been conceived as an assembly of contiguous discrete elements, e
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1 Laboratorio di Meccanica Funzionale, University ofUdine, Udine, Italy Istituto di Medicina Funzionale Riabilitativa, 'Gervasutta' Hospital, Udine, Italy
Abstract. This contribution deals with the development of mechanical model of the biceps brachii muscIe. In order to account for the finite speed of propagation of the activation pulses, the modeI has been conceived as an assembly of contiguous discrete elements, each one excited independently and defined accordingly to the HiII' s muscular model and the Huxley's sarcomer assumptions. The proposed model has been verified with reference to experimental data gathered during in vivo laboratory experiments. 1
Introduction
The human muscles can be considered as mechanical actuators able to generate at their ends only contractive forces. Their functionality depends on their physiologic properties as well as on the level of activation, the fatigue state, the current clongation and velocity of contraction. Due to the inherent complexity of the muscles operation, engineers often do represent them with simplified models which abstract roughly the underlaying phenomena, missing this way some aspects which could be critical in some applications. In this work a model is proposed which translate the Hill's and Huxley's assumptionsabout the muscle rheology and physiology. The aim is to set up a framework able to represent the intera::tions between human muscles and mechanical devices as well as a tool useful to help the physician in his or her diagnostic tasks. This research was focused on a simple but representative skeletal muscle, i.e. the biceps brachii. 2
A brief introduction to the muscle rheology and physiology
Every single muscle is structured on a scalar architecture made of connective, nervous and vascular tissues as well as of muscular fibers. The fibers are cells of lengthened shape containing sequences of sarcomers, which are the smallest contractive units inside the muscle. On its own, each sarcomer consists, internally, of narrow bands of filaments of myosin and, at its ends, of narrow bands of thin filaments of actin. As a response to an external electrical stimulus, chemical links are established between the myosin and actin mulecules with a subsequent shift of the thick filaments with respect to the thin ones, generating the muscular contraction mechanism. On the other side, while no nervous StiIIlllus is applied to the muscle, the muscular force, if any, is due essentiaIly to the elasticity of the cormective tissues ofthe whole muscle. P. B. Pascolo (ed.), Biomechanics and Sports © Springer-Verlag Wien 2004
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M. Gatti, P. Pascolo et al.
The smallest functional unit of the muscle is caUed motor unit and is composed by one motorneuron and the muscular fibers connected to it. The motorneuron Iinks the motorunit to the central nervous system which in turn acts its control through electric al pulses called action potentials. These ones last about 1+3 ms and propagate inside the muscular fibers at a finite speed, called conduction velocity, whose value
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