Psychophysiology: Human Behavior and Physiological Response

By John L. Andreassi | Go to book overview

8
Muscle Activity and Behavior

The muscles of the body are critical to almost every form of human behavior. In fact, without muscle activity there would be no observable behavior. Albert Szent-Gyorgyi, who worked on muscle biochemistry, was fascinated by muscle activity. In his own words: "Muscular contraction is one of the most wonderful phenomena of the biological kingdom. That a soft jelly should suddenly become hard, change its shape and lift a thousand times its own weight, and that it should be able to do so several hundred times a second, is little short of miraculous" ( McElroy, 1988, p. 82).

Studies of electromyograms (EMGs) and behavior have yielded interesting findings. Examples are the patterning of facial muscle action during the expression of different emotions, the differential patterning of EMG while an individual views pictures of positive and negative stimuli, and the EMG gradients that have been observed in motivated behavior. The first evidence that contraction of human muscles produces electrical activity was provided by in 1849 by Du Bois-Reymond ( Caccioppo, Tassinary, & Fridlund, 1990). In an interesting account of historical antecedents to modern techniques for measuring muscle activity, Cacioppo and colleagues described the cumbersome procedure used by Du Bois-Reymond to record the first human electromyogram (EMG) with a galvanometer. He placed cloths on each forearm and immersed both in containers of saline solution. Each of these electrodes was connected to a galvanometer, and Du Bois-Reymond noted deflections on a meter whenever the muscles of the hand were flexed. Later experimenters built on and refined the techniques of Du Bois-Reymond to make the recording techniques much more practical.

The most common way by which modern psychophysiologists measure muscular activity in behavioral studies is through EMG recordings from the skin surface. These recordings are obtained through the placement of recording electrodes over a muscle group of interest and amplifying the tiny electrical signals that muscles produce when they are active or at rest. The EMGs may also be obtained by inserting tiny needle electrodes directly into the target muscle. However, this sometimes painful procedure is used mainly in clinical diagnosis and in some forms of muscle rehabilitation to measure activity of small portions of a larger muscle, known as motor units.

This chapter discusses briefly the anatomy and physiology of muscle and the measurement of muscular activity in the form of surface EMGs. Psychological studies of the relationship between the EMG and various behaviors, including speed of reaction, tracking, conditioning, cognition, speech, emotional expression, sleep, and conditions of motivation are topics of this chapter. More specifically, some of the questions dealt with in this chapter include: What is the relationship between ongoing level of muscle activity and readiness to respond, or quality of motor performance? Do small levels of muscle activity in the speech apparatus during reading affect reading speed? To what extent can muscle activity be conditioned? What pat

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