Muscle Function

I wrote this post about muscle function about 6 months ago. Have a read and feel free to comment. I’m going to follow it up with some updated thoughts, but I’d love to hear what you think 🙂

I want to preface the following comments with the fact that it is only my opinion. Part of the reason for writing this is that I have found doing this blog helps me clarify my own thoughts and muscle function is soooooo complicated, I thought it might help me get a grip!!

Muscles, their associated fascia and their action have always given me the impression I know nothing! The more I learn about them the more I feel I don’t know anything? I’m not talking about the mechanism for contraction or anything on a microscopic level…I’m talking about what is happening on a gross, activation/action level.

When you learn anatomy you are always told about the origin, insertion, etc. However, when it comes to muscle action you are always taught the concentric action and usually in the plane with the most movement. For example, the hamstrings extend the hip and flex the knee. This totally underestimates the potential of the hamstrings, there is definitely potential for the hamstrings in the frontal and transverse planes. Whether this is simply that the teaching of anatomy remains extremely traditional or because it is so difficult to scientifically prove what a muscle does in function that we’ve stuck to the traditional model of teaching. Whichever it is, I’m convinced we can teach a model of anatomy that translates to movement far better giving trainers/therapists a far better chance of understanding of function. I believe this is a major problem with the traditional teaching of anatomy; it is so abstract to how the body moves.

Having spent time learning to understand functional movement with Gary Gray, Dr David Tiberio and most recently/extensively with John Hardy I think muscle function is the hardest thing to understand, particularly if you come from a more traditional model of teaching. In our Diploma in Functional Therapy we go through every muscle…and we think about how it is loaded in all three planes. Recently, I talked about Fryettes laws of spinal motion, and then later that day John and I were chatting on the phone about muscle function, which made me think about the laws that might govern muscle function. I’ve never really read about any that cover what I am thinking about, so I started messing around with the following:

The Hardy-Poulton Laws of Muscle Function:

First Law of Muscle Function:
In function, muscles are activated through feeling force in one, two or all three planes.
Muscle can be activated by longitudinal forces, lateral forces, twisting forces or a combination.

Second Law of Muscle Function:
When activated significantly in one plane a muscle is more easily activated in the other planes.
For example, a small force in the frontal plane is more likely to cause a strong activation of the muscle if it has already been taken to end range in the sagittal plane.

Third Law of Muscle Function:
A muscle can be lengthening in one or two planes and shortening in another allowing it to maintain tension while still allowing joint movement.
Therefore, isometric muscle contraction* is rare in function, it is more likely that there is a trade off between the planes of motion.

* By this I mean no change in length of the muscle or its tendon. It is my belief that in function muscle fibres can often remain in a relatively isometric contraction in order to utilize the reactive nature of its associated tendon.

Obviously, this is only a model of what I think might be happening, as currently we cannot scientifically prove how muscles truly function during complex movements such as gait.
I don’t know all the answers on the best way muscle function/anatomy should be taught, but I think we can get to a model of teaching that allows the trainer/therapist to transfer their knowledge of anatomy to functional movement much more easily. 
The way we teach it is to not talk about a muscle in terms of its concentric action, this is ultimately misleading, even though it is not wrong.

The way we start is to think about how the local joint movement can turn on (lengthen) the muscle according to the above laws. We will do this in all three planes, for example, to turn on the gluteus maximus in all 3 planes you would take the hip through flexion, adduction and internal rotation. I think this allows you to see anatomy in movement straight away, showing how the anatomy can be applied…much more useful! 
Once you have done that you can start getting more complicated as your knowledge grows, integrating the role of fascia, the influence of distant joints and the effects of multiple joint movement.

For example, the soleus will directly affect the foot and ankle; however, by the soleus slowing the anterior rotation of the tibia during gait and the mass and momentum of the trunk taking the rest of the body forward over the fixed foot, the soleus will indirectly extend the knee. This type of muscle action occurs in many areas of the body with muscles influencing joints and body segments way beyond the joints that they actually cross. It also means that a muscle that crosses two joints is not as straightforward as one might think.

For example, the hamstrings are able to extend the knee in function and the peroneus longus can decelerate sub-talar eversion in spite of the fact that its concentric action will be listed as eversion in any anatomy book you look at. Have a think about these last two examples…it’s interesting, but it makes a lot of sense.

Research is happening all the time and as our knowledge and technologies progress we may get closer to understanding the complexities of muscle function, however, until then I think we will have to make do with a model of understanding that leads us to strategies of treating and training that work for our patients and clients.

I would love to hear your thoughts and comments on this post, you can comment on the blog or you can email me at

Related Posts:

Applying the Laws of Muscle Function



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17 Responses to Muscle Function

  1. Ben Thompson December 6, 2011 at 2:16 pm #

    I love this post. For a long time I looked for a thought process that would help me begin to understand muscle function and this just made the most sense to me. Big thank you to both Jh and yourself for teaching me

    • physioblogger December 6, 2011 at 2:20 pm #

      Pleasure mate…hope we get to catch up soon 🙂


  2. John December 6, 2011 at 5:34 pm #

    Hi There,
    Are these ‘extra’ or indirect extensions a consequence of movement or CNS control? Do you think the brain is taking all of these indirect contractions into acount?


    • physioblogger December 6, 2011 at 8:07 pm #

      Hi John,

      Massive question!! I think if you could definitively answer that question you would be very famous!

      My opinion is that there is a lot of direct tissue to tissue interaction, though I’m sure the CNS is aware of this…it’s really tough to know!! I’m sure there will be some interesting development in this area as research and technology improve.

      Great question….I’m stuck 🙂 Hope we find more out in the near future!!


      • John December 6, 2011 at 8:35 pm #

        Hi Neil,
        Thanks. I have to admit I rushed that so I could get to Sainsburys before it got busy.

        Be interesting to look at muscle stimulation in individuals who have lost concious control of that particular muscle, and, conversely, individuals who suffer from phantom limbs.


  3. Simon December 7, 2011 at 5:52 pm #


    Fantastic post. This really does highlight how huge the task is of analysing how we move and how it goes wrong. As I read your post it popped into my mind how the function would change with joint position ( a hamstring on a closed chain action in medial hip rotation would have a far different set of forces acting on the various planes than in neutral or lateral rotation) and how the interplay of length /tension would work with that to produce or control movement. Great job! This has really got me thinking about stuff that I wasn’t applying in practice and how easy it is to fall back into the thought processes of how you were taught.

    • physioblogger December 7, 2011 at 7:44 pm #

      Hi Simon,

      Thanks for reading. I’d love to hear how you get on trying it out in practise.

      All the best,


  4. Nathan Wharerimu December 18, 2011 at 10:43 am #

    Hi Neil
    Like your post. Was wondering similar thing to John. Role of CNS . The laws you described dont seem to include feedfoward mechanisms in preparation like tensing before impact. Also , the role of peroneal longus in eversion control. Can you explain further please. My dulled , traditional anatomy conditioned brain is finding it difficult to fathom.

    • physioblogger December 18, 2011 at 6:08 pm #

      Great question…To be honest I wasn’t thinking about that kind of thing when i wrote them, maybe there’s a 4th law in just waiting to be written 🙂 My main aim of that post was to point out that the way anatomy is taught traditionally makes it very difficult to develop a functional understanding of movement.
      How the nervous system deals with all the information and prepares for all movement I have no idea!! There is likely to be a big roll for the feed forward mechanisms as well as for tissue to tissue communication, which I’m sure we haven’t even scratched the surface of as yet.
      As for the peroneus longus…well in gait, as a front foot, the sun-talar is everting while the mid-tarsal joint would be inverting. Since the peroneus longus would decelerate mid-tarsal inversion it will indirectly decelerate sub talar eversion….simple as that!!!! Not sure how important it is to understand such things…but it’s fun 😉


  5. Kedar January 3, 2012 at 5:36 am #

    Hey Neil, amazing note. It actually racked my brain in reading it. Great thoughts all of you.

    I have few points to put down. Here in India we are taught Anatomy by faculty with medical background. They have hardly any idea about the joint motions and its application in field of rehabilitation. What I learned about the motions is through observation and extensive reading. There is a huge lacunae in teaching functional anatomy

    I think, we cannot rule out the effect of gravity. In my view the CNS sends commands for muscle activation, but what constitutes is other factors such as gravity, intended movement and the most important EXPERIENCE. I happen to read some where about muscle memory. Can’t recall much.

    In addition none of the muscle placement is body is straight including the rectus (meaning straight). All muscle are placed spirally. This helps them get activated easily in all the three planes, but is strongest in one plane.

    I would add few comments but got to teach my students some functional anatomy now.


    • physioblogger January 3, 2012 at 10:24 am #

      Hi Kedar,

      THank you for your kind comments, it’s interesting to hear the point of view from teachings in India. It sounds like your students are lucky to be taught by you 🙂 Thanks for reading and contributing to the blog,


  6. orlaith February 1, 2012 at 3:58 pm #

    Wow! I think my brain just melted while attempting to get my head around this. It is as though you put in words the directions to an itch I couldnt scratch!
    I just found this blog and there is so much to read & make sense of.
    I am very interested in learning more.
    I agree with what you say at the beginning, so much is evidence based yet if someone doesn’t throw out a theroy, we have no path to follow for research. I find it daunting to put thoughts out there as it is met with opposition when not proven! Thanks for a brilliant blog

    • physioblogger February 2, 2012 at 1:59 pm #

      Thank you so much for your kind words, glad you are enjoying the blog. Feel free to throw in ideas…they are always welcome 🙂


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