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Neurological Efficiency and Recruitment.

Posted Dec 29 2008 5:54pm

In the previous post, I alluded to the terms neurological efficiency and neurological recruitment.  These terms sound similar but are quite different, not in the least in that neurological efficiency cannot be improved, whereas recruitment can be improved.  Consider this post an attempt to clear up confusion.

Both terms refer to what goes on in a muscle and its corresponding motor units.  Skeletal muscles are voluntary; in other words, you can will them to contract and relax (try doing that to the smooth muscle in your intestines).  Something has to carry the signal to contract from your brain to the muscle, and that something is the motor unit.  The impulse to contract goes from your brain, into the motor neuron, through the axon, and finally into the muscle fiber(s), the end result being contraction of those muscle fibers.

It’s probably clearer to show a diagram, so here ya go:

It may come as a surprise to you that you can’t really contract every single muscle fiber you have in your body all at the same time (not under “normal” conditions, anyway).  Neurological efficiency is the ability to contract a percentage of a given pool of muscle fibers. Neurological recruitment, however, is the ability to activate a greater number of individual motor units.

Say you have 100 muscle fibers in your biceps brachii muscle, activated by one particular motor neuron.  Someone with poorer neurological efficiency might only be able to contract 40% of the available fibers in any one contraction.  Someone with better neurological efficiency might be able to contract 80% of their available fibers.

Here’s where it gets a little murky - imagine now that the aforementioned 100 fibers was part of just one motor unit, and you had 5 motor units feeding your biceps (500 total fibers).  An untrained person might only be able to activate 3 of those 5 motor units at any given time (giving him a potential of 300 fibers working together), whereas a trained person might be able to activate all 5 (giving her a potential of 500 fibers working together).

Of course, the math gets complicated when you combine neurological efficiency and recruitment.  In the above examples, let’s assume the untrained guy is neurologically efficient (80%), and the trained female is neurologically inefficient (40%):

Untrained male = 300 x .8 = 240 fibers contracting with any one impulse

Trained female = 500 x .4 = 200 fibers contracting with any one impulse

Keep in mind these are just made up numbers to help illustrate the point; no athlete is out having his or her neurological efficiency tested.

An analogy:  Imagine a kiddie pool filled with water, and you’ve the task of emptying it out with a bucket.  You’re holding a 5 Gallon Home Depot bucket.  The bucket correlates to neurological efficiency - no matter what, you’ll can’t purge any more than 5 gallons with any one bail.  Neurological recruitment correlates to number of buckets - you can increase the amount you bail by simply increasing the number of buckets you use at any one time (using 4 buckets simultaneously being quite the sight).

Since most coaches and trainers like to focus on things you can actually change, the main focus in exercise science is on improving recruitment.  And that can be done through consistent, hard training.  Neurological efficiency is genetic and unchangeable, and (understandibly) ignored.

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