Motor unit number estimation (MUNE): where are we now?

Counting the number of motor units (MUs) in a muscle appears a deceptively simple task. Count the number of muscle fibers and count the number of motor axons supplying the muscle, then divide the former by the latter. But there's a problem. This erroneously assumes that each MU is approximately the same size. However, the innervation ratio varies between muscles (Feinstein et al., 1955). Also, type II MUs are larger than type I MUs and the proportions of these two basic MU types varies between muscles (Brooke and Engel, 1969; McComas et al., 1971).

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