This electrical potential is produced in consequence of electrical signals sent from the brain, along efferent nerves that control motor units (Baylor & Hollingworth, 2011; Smith et al. 2013). (strengthandconditioningresearch.com)
These α - motor neurons send action potentials down efferent fibers to the neuromuscular junction to produce a muscle twitch, which can be detected by EMG electrodes (Palmieri et al. 2004). (strengthandconditioningresearch.com)
Polleux F., Giger R.J., Ginty, D.D. Kolodkin A.L. and Ghosh A. (1998) Patterning of cortical efferent projections by semaphorin - neuropilin interactions. (polleuxlab.com)