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Muscles and bones act together in the locomotor machine of the human body. The bones provide the structural basis and support. They are divided into segments separated by articular joints to provide the multiple degrees of freedom required to generate motion for mechanical daily activity. Muscles are attached to the bones and, by acting across the joints provide the torques required to either stabilize the multi-segmental structure (under static conditions) or to act as dynamic actuators for generating motion.
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The following question may then be asked: Are there means to enhance muscle activity so as to increase its protective action under fatigue or other conditions of muscle deficiency?
Artificial activation of muscles by functional electrical stimulation (FES) may alter the loads on bones and joints. For instance, by activation of the tibialis anterior, tensile stresses can be reduced in the tibia. Moreover, activation of antagonistic muscles can modify the type of stresses on bone, depending on the modes of activation, e.g. symmetric, asymmetric or anti-symmetric. Thus, muscles can be activated either as limb actuators, or as shock absorbers. In the first case, activation may achieve better loading balance between the 2 legs by enhancing weakened functions. In the second case, muscle activation may serve to reduce impact loads. An interesting additional issue is whether electrical stimulation at 20-50Hz compensates for the reduced muscle activity in that frequency?
The present book addresses these questions by combining the knowledge and experience gained in the author's laboratory in the following two areas: dynamic loading of the musculo-skeletal system in human locomotion and FES of muscles. The first chapter presents a treatise on the biomechanical implications of impacting loads on the human body. The concept of mechanical impedance is used to quantify the factors involved in body protection from dynamic loading. The second chapter introduces the phenomenon of electrical stimulation of muscles, particularly by discussing two associated key issues: muscle recruitment and muscle fatigue. The third chapter presents the work accomplished on modeling and force predictability in electrically stimulated muscle. Mechanical, myoelectric and metabolic manifestations of the contractile engine are used to set a comprehensive structural/phenomenological model. The fourth and last chapter ties together the issues discussed in the previous chapters. First, it analyses the mechanical manifestations of muscle fatigue, particularly in relation to shock load transmission. Muscle fatigue is further treated as an example of muscle deficiency. Additional contributors to muscle deficiency are also discussed. Finally, the prospective of the application of FES to enhance muscle activity and bone remodeling and reduce bone and joint loading are discussed.