This chapter discusses a brief description of muscle structure and physiology and the use of electromyography in biomechanics. The main bulk of the muscle is belly, which is composed of muscle fibers, the functional part of the muscle. During a contraction in vivo, the muscle belly can often be seen or palpated below the skin as a pronounced, hard bulge. The tendons anchor the muscle to the skeleton at the origin and the insertion. The functional unit of muscle is the muscle fiber, of which there are several types. The nonenclature of the various types of muscle fiber can be confusing, as there are a variety of naming regimens, with many authors referring simply to fast and slow fibers. The action potential sweeping over the surface of the muscle causes a response in the contractile system by the process of excitation-contraction coupling.