A region of a muscle where there is a concentration in the distribution of motor end-plates (neuromuscular junctions). The characteristics of surface EMG signals are affected by the position of the recording electrode relative to the innervation zone. A term that is often confused with the innervation zone is the “muscle motor point”. The electrical motor point is defined as the location on the skin above the muscle where a muscle contraction is most efficiently generated through electrical stimulation. The anatomical motor point corresponds to the location where the motor branches of the innervating axons enter the muscle belly or where they are most superficial.

A measure of the opposition to the flow of current into each input terminal of an amplifier, as a function of frequency. Related terms: Resistance, differential amplifier gain.An ideal EMG amplifier has infinite input impedance, i.e., no current flows in or out of the input terminals of the amplifier.

The inverse value of the interspike interval between two successive action potentials discharged by a single motor unit. Typically measured in discharges per second, pulses per second (PPS) or Hertz (Hz), see Mean Firing Rate.

It is the technical term for the composite EMG signal that results from the summation of action potentials from multiple motor units firing asynchronously.

The time between two successive action potentials discharged by a motor unit. Typically measured in units of seconds or milliseconds.

A contraction in which a muscle exerts a force with no overall change in muscle–tendon length and no detected change in joint or limb position. At the beginning of a muscle contraction, muscles can shorten substantially and tendons can lengthen with no change in joint or limb position (quasi-isometric).

A contraction in which the muscle tension remains constant.The term “isotonic” implies constant tension, however, in practice, constant muscle tension is rarely maintained. The term “isotonic” is often used to refer to a constant torque contraction under conditions that can isometric, concentric (muscle shortens) or eccentric (muscle lengthens) and for which the velocity of the movement can vary. This type of contraction is often performed using isokinetic dynamometers, which measure the torque a subject produces at a single joint.