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.


Both recruitment and firing rate contribute to the overall "strength" of muscle action, but there are early and late stage depending on the load under which the muscles are operating. This allows more precision for lighter objects (where muscle recruitment is the dominant mechanism). For heavier objects, most of the muscles get recruited and all the body can do at that point is increase the firing rate more. Note that throughout the whole process, both mechanisms are occuring: it's just a matter of which mechanism dominates at which stage/degree of muscle exertion.

And this makes sense: for smaller objects that you will be manipulating, you want more precision, so having muscle recruitment control gives you that spatial precision that allows you to manipulate objects (like tools) in novel ways. Whereas when you're just trying to lift a giant rock, to block a small stream, there isn't much precision required, you're just trying to get it to go a general direction (up and over), so you recruit all muslces and drive the spiking frequency as high as it will go. Whether you fail or not at these extreme loads depends on your personal pain tolerance and your muscle's tensile strength (people with a higher pain tolerance are more likely to snap a tendon or muscle, or even bone).

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  • $\begingroup$ So let me get this straight its the load that decides which process dominates right? Is it possible to achieve precise spatial control like writing and using tools just with firing rate? What i mean is can just firing rate alone decide the precise amount of force required for skilled work or like u said use tools in novel ways? $\endgroup$ – user24896 Apr 25 '14 at 13:13
  • $\begingroup$ More precisely, it's how much effort you put into a particular load. Precision manipulation requires a lot of activating and inactivating as you go back and forth writing, sharpening a spear, or tying a knot. So different muscles will engage at different times, so that's heavily recruitment dependent by its very nature. Lifting a heavy rock, you'll start out recruiting muscles, but once they've mostly been recruited, all you can do is increase firing rate. $\endgroup$ – Keegan Keplinger Apr 25 '14 at 19:05
  • $\begingroup$ I do not know whether your area of expertise includes bionics, but I was planning on doing a paper on a bionic arm. In case of a bionic arm, the type of amputation considered is above elbow and so this means that a lot of muscles are also removed. Therefore the bionic arm cannot operate with muscle recruitment but instead depend only on the firing rate. (Continued in the next post...character constraint) $\endgroup$ – user24896 Apr 27 '14 at 5:01
  • $\begingroup$ Although achieving a precise control using a bionic arm is extremely ideal, i just want to design an algorithm to make it as precise and human friendly as possible. That is the reason i asked whether it is possible to control the force and speed of motion precisely using the firing rate alone. $\endgroup$ – user24896 Apr 27 '14 at 5:01

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