This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: All India exam, Subject name: Anatomy
Fast fatigable motor unit recruited 
Select and explain. 
A. First, B. Last, C. During active pa of contraction, D. During relaxation
B: Last
Answer: B: Last 
Explanation: Motor UnitEach single motor neuron and the muscle fibers it innervates constitute a motor unit.Each spinal motor neuron innervates only one kind of muscle fiber so that all the muscle fibers in a motor unit are of the same type.On the basis of the type of muscle fibers they innervate, and thus on the basis of the duration of their twitch contraction, motor units are divided into S (slow), FR (fast, resistant to fatigue), and FF (fast fatigable) units.(Refer: Table 17)(Refer: Ganong&;s Review of Medical physiology 24th edition, pg no: 108-109)