Carbonfiberboy is correct. To make another overgeneralized but still accurate for its purpose statement, it's actually not very easy to recruit fast twitch fibers. Similar to what cfb said, anything over 2 min starts to transition to slow twitch fibers, and eventually becomes almost entirely slow twitch by 5 min. For those first 2min of sprinting, your fast twitch fibers overpower the slow twitch.

Even your LT power sprints are mostly slow twitch. You'd have to get in the 200+% of your FTP to utilize fast twitch fibers.

James