Cambridge University Press 2017. [T]he principal function of the nervous system is the coordinated innervation of the musculature. Its fundamental anatomical plan and working principles are understandable only on these terms. (Sperry, 1952, p. 297) We have a brain for one reason and one reason only - and that is to produce adaptable and complex movements. There is no other reason for the brain. (Wolpert, 2011) INTRODUCTION The sophistication of the somatic system enables the vast repertoires of adaptive reflexes and skilled actions characteristic of behavior. The electrophysiological signals associated with active muscles have been of interest for centuries due to the complexity of their organization and dynamics, their clinical applications, and their value as indices of and possible contributors to behavioral processes. In this chapter, we provide an introduction to psychophysiological research on the somatic system. We begin by reviewing the history of this research and by articulating some of the major issues, limitations, and advantages of the various ways researchers have investigated the output of the somatic system, including the use of electromyography (EMG), motion capture, and various video coding systems. We then review briefly the physiological basis of the somatic system. Because psychophysiology has tended to favor the use of surface EMG, we focus here primarily on guidelines for EMG recording in humans. We continue with a discussion of the social context for the measurement of muscle activity and of psychophysiological principles and common paradigms that have emerged from research on the somatic system. For recordings made of muscle activity to be of theoretical significance, one must consider conjointly the various contexts in which these signals are acquired. HISTORICAL CONTEXT In this section we identify two distinct themes in the development of the measurement of somatic activity in psychophysiology. The first is the history of the physiology of the muscles, which derives from the writings of the early Greek philosophers, and from the scientific renaissance in the seventeenth century. The second is the history of psychophysiological research, which began in earnest with the work of such figures as Duchenne (1990 [1862]), Spencer (1870), Darwin (1873 [1872]), and James (1890), all of whom emphasized relatively subtle patterns of somatic actions as a way of characterizing and understanding human behavior generally.
