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.

The extracellular waveform detected from a single muscle fibre as an action potential propagates in both directions away from the neuromuscular junction.

The speed at which an action potential propagates or travels along a muscle fibre. A ‘global’ estimate of muscle fibre conduction velocity may be obtained from the time taken for a surface EMG signal to travel between two spatially displaced recording electrodes. Conduction velocity is expressed in m/s or mm/ms with values typically centred around a mean of 3–4 m/s.

An M-wave, also known as a compound muscle action potential (CMAP), is an electrical response recorded from a muscle following direct electrical stimulation of a motor nerve or, in some cases, the muscle itself (stimulating the motor axons) (Palmieri et al., 2004). It reflects the synchronous activation of multiple motor units and is recorded using surface EMG. The M-wave is commonly used to assess neuromuscular transmission, peripheral nerve excitability, and to monitor muscle response during reflex or voluntary testing. Note: While a mechanical twitch may accompany the stimulation, the M-wave refers specifically to the electrical potential, not the force output.