ABSTRACT
Every living system is under the influence of a variety of physical fields. Information about the environmental changes (external and internal) detected by various receptors is transferred to different levels in the nervous system, and the decision to react or not to react is taken. Any human activity in the external environment involves the motor system. Utilization of the motor system allows humans to change their position in space (locomotion) and to interact with the environment in a controlled mode (work). The human motor system is complex, with about 280 biomechanical degrees of freedom (number of coordinates necessary to describe the motion of a body in space) under thousands of neural channels. Such a complex system contains many subsystems, inputs, and outputs, and is multilevel. The motor system has the ability to learn and to change its structure. Also, the motor system can act in a wide range of external conditions while the internal condition changes are kept within small ranges, necessary to maintain basic living functions (homeostasis). No consistent theory of analyzing such systems exists. The properties of the human motor system are described by its biomechanical parameters. The central nervous system (CNS) monitors biomechanical variables such as force, position, and velocity. The manner in which the CNS keeps these variables constant, stable, and reproducible in a variety of motor tasks is the subject of intensive study. Despite the unclear control rules of the generation of human movement, the quantification of the human motor system is under intensive development. The human motor system consists of two basic subsystems: structural and functional. The structural system contains morphological, passive and active musculoskeletal elements and can be described with a great, but limited, number of parameters. The functional system describes the spatial and temporal ordering of the structural elements and is practically infinite. Quantitative description of the system can be achieved to different degrees at the range of its structural and functional subsystems. The basic structural properties of the motor system include segment dimensions,
ranges of motion, segment dynamics (moment of inertia, stiffness, damping), maximum voluntary, and reflex activation. At the range of the functional subsystem, the most frequently quantified human functions include walking, and initiation of walking, standing upright, reaching, climbing stairs, and grasping.
