The motor unit and quantitative electromyography

Abstract Electromyography (EMG) assesses the anatomic motor unit (A‐MU), but knowledge of its anatomy, physiology, and changes with pathology is limited. The electrophysiological motor unit (E‐MU) and its motor unit potential (E‐MUP) represents a fraction of the A‐MU. Routine EMG assesses a limited number of E‐MUP waveform characteristics (metrics) and their magnitudes qualitatively scaled in a nonlinear manner. Another approach is quantitative EMG (QEMG), whereby 20+ E‐MUPs are extracted and both basic and derived metrics obtained and values expressed quantitatively. In diseased muscle, many E‐MUP metrics may be normal, which complicates diagnostic interpretation. In QEMG, E‐MUP metrics can be clustered and statistical analyses performed to assign probabilities that E‐MUPs (and the muscle) are normal, neuropathic, or myopathic. In this article we review what is known about the A‐MU, the restricted E‐MU, E‐MUP metrics, and what QEMG offers currently and in the future.