Variation of motor unit size in the human extensor digitorum brevis and thenar muscles.

One physiological method for estimating the motor unit number in a muscle depends on dividing into the maximum compound muscle action potential, the potential average of the first few motor unit potentials excited by a motor nerve stimulus above motor threshold. To be valid, such an average unit potential size must be representative of the whole motor unit population. This assumption may not be justified. The present study has shown that there are single motor units in healthy and abnormal, thenar, and EDB motor unit populations, many times larger than any motor unit excited close to the motor threshold. This finding suggests that previously reported motor unit estimates may not only be an overestimate of the true motor unit population number, but have excluded much larger motor units with higher thresholds. Low motor unit estimates in neuropathies may result from a change in the order of activation of motor units with the appearance of larger motor units, normally of higher threshold among the first few just above the motor threshold.