FT (or type 2) fibers are often separated into divisions. 2a fibers have intermediate endurance but more force production than ST (type 1) and 2b fibers have the poorest endurance and are he hardest to recruit. Think of the grimace and concentration it takes to lift a heavy weight. Those strongest 2b fibers take a lot to get them into the game. Where as you can talk and think and do long division in your head while walking because those type 1 fibers take little to engage. This is known as action potential. The higher the force fiber the higher the action potential (the electrical signal in the nerve needed to jump the synapse gap).

I have trained a number of FT dominant athletes who went on to be extremely successful endurance athletes. It took a while for them to develop that aerobic base and they had to spend a lot of time in Z1 and 2. But eventually they got there and were top competitors.

The good news is that 2a fibers can convert trough training to develop more and more endurance characteristics. So when you recruit those lower force FT fibers they are getting stimulus that causes them to look more like ST fibers

I would not concern yourself with if and when you might start recruiting FT fibers in a work out if the intensity is low you are building an aerobic base.

Scott