Does orderly recruitment of motoneurons depend on the existence of different types of motor units?

It has recently been proposed that the recruitment of motoneurons is correlated with their type rather than their size. To test this hypothesis the axonal conduction velocities (CVs) of 92 pairs of soleus motoneurons (all of the same histochemical type) were compared with their recruitment order. By means of spike-triggered averaging it has been shown that in 89 out of 92 pairs of soleus units the motoneuron with the lower CV was recruited first. The results indicate that orderly recruitment occurs in the absence of different types of motoneurons and is correlated with cell size within a cell type.