Quantitative Motor Unit Number Estimates

Abstract Quantitative motor unit number estimates (MUNE) are neurophysiological techniques designed to quantitate the number of viable motor axons that innervate a muscle or muscle group. A variety of techniques have been developed to accomplish this, each with its own advantages and disadvantages. Traditionally, the MUNE is calculated from the supramaximal compound muscle action potential (CMAP) by dividing the CMAP by the mean size of the motor unit potentials. The resulting unitless number represents the number of motor units within a muscle or muscle group. These techniques vary by how this mean motor unit potential size is calculated. These methods have been applied most widely to disorders of the motor neuron, such as amyotrophic lateral sclerosis (ALS), spinal muscular atrophy, and polio. MUNE has been used as a research measure in animal and human studies. Additionally, it has been used as an outcome measure in clinical trials for ALS and spinal muscular atrophy. More recently, two techniques have been developed that rely on statistical calculations from the CMAP and are fundamentally unique from the traditional methods. These have been called the MUNIX and MScanFit methods. These newer methods are faster to apply and can be performed with less patient discomfort. This chapter reviews the traditional and newer technique of motor unit number estimation.