ESTIMATING THE NUMBER OF MOTOR UNITS IN A MUSCLE

A loss of motor units is the primary defect in many neurogenic disorde rs. The number of motor units remaining in a muscle or group of muscle s can be estimated by a number of methods on either needle electromyog raphy or as part of nerve-conduction studies. Comparison of the method s of motor unit number estimates (MUNE) and their underlying assumptio ns shows that each method can provide reliable and useful clinical inf ormation. Each method relies on obtaining an estimate of the average s ize of single motor units, and compares that size with the size of a m aximal compound muscle action potential in which all motor units are a ctivated, The potential errors of all the methods are similar and rela te primarily to adequacy of sampling of the size of single motor units . The method of statistical estimates of MUNE does not attempt to isol ate and measure individual motor unit potential sizes, Rather it estim ates the size of single motor units from the variation in size of a mu scle action potential when individual motor units in a group of motor units fire randomly.