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Recently, both high quality physiological data and human-operator-describing function data of low variability and large dynamic range have become available. These data lead to control engineering descriptions for neuromuscular actuation systems that are compatible with the available data and that provide insight into the overall human control structure (e.g., the types of feedback systems used for various inputs). In this paper, some of these physiological and human-operator data are briefly reviewed, and a simple neuromuscular actuation system model is presented. The physiological data of interest include recent anatomical and physiological data for the muscle spindle and input-output studies of the muscle. These data indicate that simple linear models can describe the basic behavior of these two elements in tracking tasks. This paper contains two key developments: 1) the variation in system parameters as a function of average muscle tension or operating point; and 2) the role of the muscle spindle both as an equalization element and in its effects on muscle tone or average tension. The simplest neuromuscular model suggested by and compatible with the data is one in which muscle spindles provide four functions in one entity: 1) the feedback of limb position; 2) lead/lag series equalization; 3) the source of at least one command signal to the system; and 4) a signal for adjustment of the spindle gain, equalization, and steady-state spindle output which produces the average muscle tension.