The detection and recording of electrical activity generated by muscle fibers. The basic elements of motor control in the body are the motor units which comprise motor neurons in the brainstem or spinal cord, their axons, and from ten to several hundred muscle fibers supplied by each motor neuron. Motor units vary in the size and properties of their motoneurons, the sizes and conduction velocities of their axons, the morphology of their nerve muscle junctions, and the structure and physiological properties of the muscle fibers supplied by each motor neuron. Voluntary activity recruits motor units in an orderly manner, beginning with smaller, twitch units with more slowly conducting axons recruited in weak contractions and progressing to much-harder-to-recruit, larger and faster-twitch motor units with more rapidly conducting axons.