Answer:
Type IIA- Fast twitch, fatigue resistant unit
Explanation:
During intense activities such as exercises where there is an increase in the  contractile muscle strength, there is an activation of additional muscles to accomplish the task.
This is referred as motor unit recruitment.
It can be type I where it involves slow twitch fatigue resistant units with small twitch tension and slowest contraction
Or Type II A- where fast twitch, fatigue resistant fibers are activated to enable a larger force and faster contraction.
