In situ measurement of the innervation ratio of motor units in human muscles.

Volume conduction measurements were carried out the brachial biceps, tibialis anterior and deltoid, in normal human subjects. Attenuation constant K and time constant tau of the muscle tissue transfer function were measured, and the average electrode uptake area calculated for the three muscles. The average number of muscle fibres in the motor unit, i.e., the innervation ratio, was calculated from the electrode uptake area, data on the motor unit territory, and measurements of fibre density. The innervation ratios for the brachial biceps, tibialis anterior and deltoid were 209, 329, and 239 fibres, respectively. It was found that the anatomical scatter of fibers belonging to the same motor unit was smaller in brachial biceps than in tibialis anterior, whereas the electrophysiological "fibre density" was higher in tibialis anterior. The implications of these findings for the interpretation of normal and abnormal electromyographic findings are discussed.