From what I have read, its been given that muscle functioning and coordination involves two processes: frequency summation and muscle recruitment. What I want to know is that, do these two processes work simultaneously or are these two stages separate? As in, both frequency summation and muscle recruitment contributes to modulating the amount of force which makes me wonder why two processes are required to satisfy the same purpose?The conclusion I could come to was that, frequency summation applies to only a single muscle fiber and when the fiber saturates and no more force can be recruited more muscles are recruited to increase the amount of force applied. An example scenario of this occurring would be a person is lifting a very heavy object. I want to know whether my inference is right or wrong? If wrong I would like someone to explain to me the exact mechanism. 
What exactly is the neurobiological mechanism behind the functioning of the human muscle?
neurobiology