Neurons that integrate information from sources within the nervous system and transmit information to the muscle fibers that they innervate. The alpha motoneuron cell body is located in the spinal cord, or brainstem, and the axon leaves these structures to innervate a group of muscle fibers. In the context of EMG, the term motoneuron typically refers to alpha motoneurons, which innervate extrafusal muscle fibers, rather than gamma motoneurons, which innervate smaller intrafusal fibers within the muscle spindle. Alpha motoneurons are known as the final common pathway in the processing of motor commands, as all activation signals from the brainstem/spinal cord to muscle are transmitted through them.

A motor-evoked potential (MEP) is the electrical response recorded from a muscle following stimulation of the motor cortex, typically using transcranial magnetic stimulation (TMS) (Rossini et al., 1994). The MEP reflects the compound muscle action potential elicited by descending motor signals and is commonly recorded using surface EMG. MEPs are used to assess the excitability of the corticospinal pathway, and they are distinct from the mechanical twitch response (i.e., force) that may also follow cortical or peripheral stimulation. Note: Although TMS is the most common technique, MEPs can also be elicited using electrical stimulation of the cortex in some research or clinical settings.

A motoneuron and all of the muscle fibers it innervates.All muscle fibers belonging to a motor unit are activated every time the motoneuron discharges (in normal, healthy physiology). The muscle fibers of a motor unit (without the motoneuron) define the muscle unit.

The summation of individual muscle fibre action potentials from all the fibers of a motor unit in response to a discharge of its motoneuron.The motor unit action potential detected by an electrode system will depend on the electrode configuration and the position/orientation of the electrode relative to the muscle fibers.

Activation of a motor unit to participate in a muscle contraction. Related term: Recruitment threshold of a motor unit.During voluntary muscle contractions where there is a gradual increase in force, motor units are usually recruited in an orderly fashion referred to as “Henneman’s size principle”. This states that the threshold of motoneuron activation is determined by the size of the neuron, with recruitment occurring in approximate order of size –from the smallest motor units (typically innervated by the smallest motoneurons) to the largest (typically innervated by the largest motoneurons). It should be noted that the size principle may not apply to all types of muscle contractions (e.g., during ballistic or electrically stimulated muscle contractions).

The size of a motor unit is often characterized by the number of muscle fibers innervated by a motoneuron, though different definitions have been used. The number of fibers a motoneuron innervates is generally correlated with the diameter of the innervating motoneuron axon. Motor unit size is often quantified using the twitch force generated by the motor unit or the amplitude of the motor unit action potential. The twitch force of a motor unit depends on the number of muscle fibers innervated by the motor axon and the cross-sectional area of the muscle fibers. The amplitude of the recorded motor unit action potential depends on the number of muscle fibers innervated by the motor unit, the diameter of the fibers, the distance/orientation of the fibers relative to the recording electrode, and the acquisition parameters of the EMG detection system.

A tendency for two or more motor units to discharge together or within a few milliseconds of one another, with a rate of occurrence above that expected due to chance.

The volume within a muscle in which the muscle fibers of a single motor unit are distributed.

A simple method to smooth EMG data, that acts as a low-pass filter, reducing random fluctuations in the rectified (absolute value) or squared EMG signal. Each sample of the rectified EMG is replaced by the average of N + 1 samples (N/2 samples before it and N/2 samples after it). Related terms: Filter, EMG signal envelope, EMG amplitude: Root mean square value, EMG amplitude: Average rectified value (ARV) or mean absolute value (MAV).It is usually applied to rectified (absolute value) or squared signals. The width of the moving average window selected (and the degree of overlap between windows) should be stated when reporting results. The appropriate length for a moving average window will be shorter when analysing muscle activity during dynamic contractions than during isometric contractions.

The degree to which a muscle is excited, encompassing both the number of activated muscle fibers and the rate of their discharges. Muscles are activated by neural drive from the motoneuron pool.