The generation of physical movement in animals involves the activation and control of muscle forces. Understanding the mechanisms behind force generation and control is essential for professionals who work to promote health. The human body can be represented as a system of articulated segments in static or dynamic balance. Within this system, movement can arise from internal forces acting outside the joint axis, causing angular displacement of these segments, or by forces external to the body. Knowledge of the contribution of muscle forces to joint position and movement is of great importance for the study of muscle activity during exercise and also for understanding the coordination of muscle activities during functional movement. However, muscles forces cannot be easily measured in vivo; rather, they must be assessed, calculated or modeled (Amadio & Duarte, 1996; Amadio & Barbanti 2000).
Closely associated with the generation of force by a muscle, is the generation of an electrical signal that can be observed by placing electrodes on the skin surface to detect underlying electrical activity displaying the associated waveform on a computer monitor. This process is called electromyography (EMG) and the waveform is the electromyogram. The assumption that there is an association between EMG and underlying muscle forces is the basis for many applications of EMG, allowing inferences regarding various aspects of muscle physiology. However, it is not possible to measure muscle force directly using EMG. Since 1952 there have been studies that show some cases where there is a linear relationship between force and EMG (Lippold, 1952), however this relationship is not always simple and linear. In recent years methods for detecting and processing EMG signals have been refined considerably, with the availability of better equipment, tools, mathematical, statistical and computational techniques. Although the determination of muscle strength using EMG measurements has also evolved, it has not yet fully exploited the technological potential available. In this chapter, we describe a practical approach to the quantitative evaluation of muscle force through analysis of the EMG signal.