Zatsiorsky explained that maximal MU recruitment is possible in small muscles with loads as low as 50% of Fmm and as low as 80% Fmm in large proximal muscles.
This is why sub-max loading works and I went into depth discussing this in my lecture at South Alabama a couple months ago (will be for sale before too long)
Beyond max recruitment threshold exists rate coding/firing rate and in extreme intensities- synchronization. When weight training is the means, and not the ends, it is even more justified to train sub-max and leave the highest intensity training to the primary objective.