Closed-loop control of movement of skeletal muscle.

Closed-loop (feedback) control of skeletal muscle is critically reviewed. The introductory section examines the advantages and disadvantages of open-loop as compared to closed-loop control in general, defines the problem, and outlines our approach. In the biological systems section, muscle structure and function are defined at the level of the motor nerve, neuromuscular junction, and sarcomere. Time delays, power and efficiency, fatigue, and other effects are also discussed in relation to the development of closed-loop control. This section then proceeds to review biological sensors and finally integrates this information by reviewing the body's own closed-loop control system. The third section critically reviews various approaches to the mathematical modeling of muscle. The control problem (in general) is reviewed with particular emphasis on contemporary control systems engineering. Essential to closed-loop control of paralyzed skeletal muscle is sensor technology. Therefore, the fourth section reviews external mechanical sensors. Specifically, potentiometers and Hall effect sensors, capacitive force transducers, inductive displacement transducers (LVDTs), and various position resolvers are discussed. Finally, the fifth section reviews the application of closed-loop control of skeletal muscle to the human being. The focus of this section is the paralyzed individual: past progress and future directions. An extensive bibliography of cited references is then provided so that the interested reader may pursue his/her particular area of interest in more detail. The authors acknowledge that such an extensive review of so many relevant areas is necessarily not complete and often overly simplistic, but our goal is a "first approach" to a comprehensive understanding of the closed-loop (feedback) control problem for achieving movement in paralyzed skeletal muscle.