**2.2. Muscle electrophysiology**

An innervated muscle fibre is activated when the currents created by the activity of its NMJ create a transmembrane action potential that then propagates in both directions along the muscle fibre away from the NMJ initiating and coordinating contraction of the fibre. In other words, action potentials propagate along the axon of a motor neuron to activate the muscle fibres of a MU. The currents creating the action potentials of the activated muscle fibres linearly contribute to a spatially and temporally dynamic electric field created in the volume conductor in and around a muscle. The strength and spatial and temporal complexity of the created electric field is determined by the number of MUs active and their size and spatial extent. Electrodes placed in this electric field can be used to detect a time changing voltage signal (i.e. an EMG signal).
