Motor Unit Recruitment Strategies During Deep Muscle Pain


When you hurt your arm, leg, or back, the first thing you’d often notice is that ones’ range of motion tends to decrease.   It hurts to move, or your muscles feel achy. Despite this pain previous  medical research has no solid explanation for the fact that despite pain and obvious torn/ damaged muscles, individuals are still able to perform tasks with the same amounts of force as an individual who is not in pain.  Researchers recently have hypothesized that individuals who are experiencing deep tissue pain, in fact recruit more muscles to help out the torn/ damaged muscle do its task.   This might seem painfully obvious but it is not.  Up until this point the current theory/ model of muscle recruitment during pain stated that muscle groups take from other limbs or major muscles to help get a task completed.  Lets dive a bit further into this issue and I”ll attempt to lay it out nice and simple.

Functional Unit of Muscle

Skeletal Muscle Motor Unit

First off we need to understand what a functional skeletal muscle motor unit is, in order to understand how this works.  For the purpose of simplicity we will use the bicep as our model muscle in the following explanations.  As you may know muscles have the ability to flex and relax.  Simply put a muscle is in fact a bundle of fibers which each act independently to add to a larger whole.  The reason you can flex your arm and wave vs. flex your arm to lift a weight is that the more labor, force your body needs , the more motor units are recruited to get the job done.  The neat thing is these muscle fibers actually recruit in an orderly fashion from smaller to larger.

So what happens when you’ve torn a muscle or injured a limb?  Say you need to get a task done but your injured.  How do your muscles deal with this situation?  Researchers set out to answer this exact question.  They did this by getting a patient pool of volunteers who let them inject 5% saline solution into the subcutaneous fat of their forearms and their triceps.  These researchers then looked at the electrical levels of the neurons that innervate each functional motor unit and took recordings from these while their patients were asked to perform tasks with a given force.  These neurons that innervate each motor unit, control the muscles flexion and relaxation abilities.  When a task is performed a certain electrical charge can be measured of each one and with some sophisticated software researchers can tell how many individual units exist and which ones in particular are being recruited.

Diagram of the quadricep muscle

When all was said and done they were astonished.  What they found was that the current model of motor unit recruitment during pain was in fact wrong! As they predicted,  muscle groups that were undergoing deep tissue pain in did in fact show the same electrical activity as normal muscles.  They also were able to generate force just like a non injured muscle could.  However, what they didn’t expect was that the muscles were actually recruiting in a different order than the normal recruitment order from smallest to largest.  This basically means that when your injured your brain knows to switch which muscle unit it chooses to help you get a task done, rather than always recruiting them in the same pattern and order.  This has huge implications for studies on pain and how the human body is able to cope with injury.

The motor unit as seen coming from the spine out to the muscle groups

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Posted on November 23, 2009, in Medicine and tagged , , , , , , , , , . Bookmark the permalink. 6 Comments.

  1. May I ask of the citation for this work? Perhaps I am being dense but I cannot find it. Thank you in advance.

    • Of course you may! I forgot to update the references section with this study’s citation:

      Tuker K., Butler J., Graven Nielsen T., Riek S., Hodges P., Motor unit recruitment strategies are altered during deep tissue pain. (2009) The Journal of Neuroscience. 29(35) 10820-10826.

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