The Complexity of Biomechanics

inide-of-watches-beautiful-and-complex-mechanisms-7I really enjoy studying biomechanics. I find it totally fascinating to learn, for example, that a certain muscle is very well suited to stabilize a joint, but not to move it through a large range of motion; or that it is active in one movement but not another; or that it becomes atrophied in people with chronic pain.

I think this kind of detective work is very cool and I always look forward to applying it to help one of my clients. But that is where things get frustrating, because with biomechanics, as with so many other subjects, the more you learn, the more you realize you don’t know as much as you would like to know. And sometimes you don’t know enough for your treatment protocols to make any sense.

Following are a few random observations that remind me that manual therapists such as myself need to stay humble about the extent of our biomechanical knowledge.

Joints Interactions are Complex 

I recently picked up a very intensely biomechanical book called Human Locomotion by Thomas Michaud. Very cool book! And humbling. This book is about four hundred pages of detailed biomechanics analysis, most of which you need about two advanced degrees to understand and apply.

One takeaway that I received from browsing through this book is that different people have different bony shapes, the differences are hard to detect, and these differences have important biomechanical consequences. For example, the book is literally filled with passages like the following:

The subtalar joint motion may be limited by bony restrictions that block pronation and/or supination. The most common bony restriction that limits supination is the tri-articulated subtalar joint. This anomaly occurs in approximately 36% of the population and produces a restriction that prevents continued subtalar joint supination when the anterolateral facets of the calcaneus contacts the anterolateral facet of the talus.

Another example of a bony restriction that limits the range of subtalar joint supination is the rudimentary talocalcaneal bridge. … This bony anomaly, which is very difficult to identify with conventional x-ray techniques, acts as an osseous block that maintains the heel in an everted position.

The author provides numerous examples of how understanding these differences may be important in devising an intervention to improve gait. For example, a person with one type of foot might benefit from forefoot striking, while another would be better off heel striking.

Another interesting takeaway from this book is that expert opinion about even simple biomechanics can be completely overturned by new technologies that provide better measurements. For example, recent 3-D imaging techniques to determine the effects of orthotics on gait revealed findings that were totally surprising to orthotics experts, including that it is far harder to prevent pronation with an orthotic than was previously thought.

Muscles are (surprisingly) complex 

We all know the function of a multi-joint structure like the foot is complex, but even the simple actions of a muscle can be very hard to determine. I have recently come across at least three examples of how common assumptions about muscle function have been challenged by research.

The psoas

We are still learning about the basic function of the psoas, and debate continues, even amongst the foremost experts in the world. But there is growing evidence that it functions more as a spinal stabilizer and hip stabilizer than a hip flexor, and that it works to create posterior pelvic tilt as opposed to anterior tilt during standing. This information clearly calls into question the biomechanical rationale of “releasing” the psoas to help with back pain supposedly caused by anterior pelvic tilt or hyperlordosis.

The upper trapezius

Research by Bogduk argues that the upper trap serves more to stabilize and retract the scapula than to elevate it and upwardly rotate it. This calls into question the biomechanical rationale of many treatments of upper cross syndrome.

The supraspinatus

We are all taught that the supraspinatus initiates abduction of the shoulder, but new research indicates that it does not activate any earlier than the deltoid. Perhaps this has implications for the biomechanical rationale underlying treatments for shoulder impingement.

The point here is not so much to argue in favor of any of the new interpretations offered in the above research, but to simply note that some very basic biomechanical issues have not yet been settled, and that many treatment protocols may be proceeding on a faulty base of unsupported assumptions.

Complexity makes prediction difficult in the presence of error

One of the simplest models that drives a great many treatments in manual therapy is based on the lower cross syndrome. For example, a client comes in with lower back pain, and assessment reveals that the client’s pelvis is anteriorly tilted and the lumbar spine is hyperlordotic. The therapist decides that the psoas is “short” and needs to be lengthened or released.

Now this is about as simple as any biomechanically based treatment gets in manual therapy, but let’s look at how many things would need to be true for this story to make sense. There are many links in the chain of reasoning, and regular readers will immediately notice that almost all of them are flawed.

First, there is very little reason to believe that low back pain is caused by excess lumbar lordosis or anterior pelvic tilt, because most studies find very little correlation between these variables.

Second, given considerable individual anatomical variation in vertebral shape, sacral base angle and bony landmarks on the pelvis, we have reason to question someone’s ability to assess that a particular client is standing in anterior tilt or hyperlordosis, as opposed to a posture that is neutral for their particular structure.

Third, as we just learned about the psoas, it might play no role in increasing anterior pelvic tilt in standing, and in fact might even perform the opposite function.

Fourth, there is little reason to believe that manual therapy could lengthen a short psoas.

So the chain of reasoning does not hold up in any of its links, much less all of them.

And this is one of the simplest biomechanical models that we see, one that is taught to novices. More sophisticated biomechanical models, that people may study for years, often have even more links in the chain of reasoning, creating even more places to go wrong. For example, overactivity of muscle A in the pelvis inhibits the activity of muscle B, which moves joint C into flexion, which causes joint D to compensate by moving into extension, which eventually brings us to . . . the lateral pterygoid! Each successive link in the chain makes it far more likely for error to be introduced. And errors are particularly problematic in a complex system.

One of the characteristics of a complex system is that you cannot predict its behavior unless you have perfect knowledge of all the variables and how they interact. Even very small errors in measurement of the initial conditions of the system will lead to huge errors in prediction after a little time passes. Therefore, to the extent that the body’s interactions are complex, which they often are, it is unlikely that we can predict how changes in one area will affect a distant area. We might be confident that one will affect the other, but exactly how might be very hard to determine. 


I am not saying that we are hopelessly ignorant about biomechanics or that they are always too complex to understand and apply at a practical level. Far from it. There are obviously many situations where we can be confident in our biomechanical analysis. (I just haven’t listed any here!)

What I am saying is that we need to be aware of how fragile biomechanical models are to mismeasurement and incorrect information. If your intervention is based on a pretty complicated biomechanical analysis, and you don’t have rock solid knowledge of all the relevant variables in the system and how they interact, you probably aren’t affecting mechanics in the way you think you are.

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27 Responses to The Complexity of Biomechanics

  1. That’s got the grey matter working! Am always very cautious with my “knowledge” of biomechanics and often feel evidence is inconclusive. This helps confirm my findings. Would love to know more about the psoas possibly increasing posterior pelvic tilt.

    • Thanks Claire. If you want to know more about the psoas, click the link in the post. There is a full text of a very good review article (which links to other papers.)

  2. Great article Todd! Having delved deeply into biomechanical research over the past year, I can’t tell you how confusing certain topics are such as segment interaction, and how various technologies all have inherent weaknesses including EMG, inverse dynamics, muscle modelling, etc. Don’t get me wrong, we should continue on our quest to increase our understanding of biomechanics as it is very important. But we should frame the knowledge properly and be aware of the limitations, especially pertaining to chronic pain.

    • Thanks Bret! As I said at my comment at your blog, the article was partly inspired by guys like you and Greg Lehman, who know more than almost anyone else about biomechanics, and are the first to admit the limits of your knowledge. Keep up the good work!

  3. Nice article!

    But, You sure never goona make any money or be the next ‘guru’, if you act like you haven’t understood everything 100% or you don’t have answers to everything. :)

  4. Don’t we, as practitioners, try to improve function rather than length through movement and stretch? Does stretching not have an effect in the increase of blood supply to joints and muscles thereby leading to the body self healing as it was designed to do?
    I look at the biomechanics and quail, then think that I will never know enough, and that every patient is individual and I learn every time that I look!

  5. Can practitioners who insist that this model has helped and “cured” their clients/patients be deluded in the Texas Sharpshooter Fallacy?

    This term refers to a story where someone once painted a bullseye around a few bullet holes on a barn, and claim that his marksmanship is accurate.

    • Hi Nick,

      Never heard that one! Sounds like it implies dishonesty so I would not use it. But for sure there are many honest and well meaning prax out there of all kinds who have gotten good results and did not know the actual mechanism of effect.

      • That´s the point. Even if the psoas (or whatever other muscle) isn´t really doing what we up to now think it does, why the hell does it help to work on it in a way that theoretically would have no effect? “Releasing” the psoas often has an inmediate, clear effect on the client, unlike other treatment options where the client’s feeling often is “well, I kinda sorta feel a bit better”. I`ve read the articles concluding that the psoas can`t shorten by to much sitting etc. but nonetheless that´s the effect I witness a lot of times. So, I do not doubt science, but taking such findings to their logical end would imply doing nothing, because the available treatment options don`t achieve what they are supposed to. An intriguing conuncrum…

  6. Thanks Tod for your observations,
    I also picked up Thomas Michaud book about a year ago and i am going through it….slowly…
    I find the information humbling yet liberating in that there is no one way of treatment, you got to put on your detective hat and creative hat and start peeling the layers.

    below is a link to an interview with Thomas Michaud

    Also, going deeper into Gait and Biomechanics:
    they reference Thomas Michaud book a lot, can help break it down.
    Search “psoas” on their blog, you will find a lot of interesting information.


    • Hi Inbar,

      I did listen to that interview, which was fantastic. That’s why I got the book. Thanks for the ink to the gaitguys I have seen their blog but not this particular series of posts.

  7. Terrific post, Todd. I’ll plug this on shortly. You found a couple wonderful examples in Michaud’s book.

    As usual, though, I’m like your more cynical evil twin: same basic thinking, but I’m afraid I think biomechanics usually are “too complex to understand and apply at a practical level.” Not impossible but damned close. Effectively impossible for the average clinician, most of the time? As you argued yourself quite nicely, even the most rudimentary biomechanical equations are fraught with unsafe assumptions and capricious variables. And if that’s true — and I really think it is — then, well, yikes!

    • Hi Paul,

      Thanks for the compliment and the plug. Evil twin I like it!

      Some biomech interactions are complex while others are simple, so I think there are many biomech predictions we can make. If I see someone touch their toes without flexing very much at all from the hip, I can predict that they are flexing more in their back to compensate. Or if someone has very limited dorsiflexion, I can predict they will have a problem with an OHS. A golf swing with limited spinal rotation and weight transfer will be weak. Inability to extend the thorax will make an overhead reach tough. All pretty simple, and potentially useful clinical observations and biomech predictions. The tough thing is when you start to blame any of these poor movement patterns on the aberrant behavior of a particular muscle. Now things are getting complex!

      • For example, just found this today, courtesy of Phil Snell on FB:

        “The results of this study suggest that limited hip flexion in LBP can contribute to excessive lumbar flexion and posterior pelvic tilting during hip flexion in the sitting position. Further studies are required to confirm whether improving the hip flexion range of motion can reduce excessive lumbar flexion in patients with LBP accompanying limited hip flexion.”

      • I do agree that some biomechanical equations are simple enough to work with, but they tend to be relatively uninteresting. There is an anology here to one of our favourite concepts: the deepity. For others reading, here’s a quick reminder: a deepity is a statement that has two possible interpretations, one that is true but trivial or another that is “deep” but wrong. This also fits most statements about biomechanics insofar as they tend to either by true but obvious or unimportant … or a profound and interesting but hopelessly unreliable and impractical insight into why someone hurts. If all discussions about biomechanics were stripped of the latter type — anything with a fragile chain of reasoning, anything that would be nifty if only there were fifty ways it could be wrong — there might not anything left worth blogging about. :-)

        • Hi Paul,

          I appreciate the deepity reference but can’t agree that it applies here. If I say that limited dorsiflexion is the reason for poor ROM in a squat, this doesn’t alternate between two meanings – there is only one meaning, which is clear. Further, the level of triviality required for a deepity implies that the truth is obvious to everyone and that knowing the truth doesn’t help solve a problem. The observation that a client is flexing too much in the low back in a forward bend because of poor hip mobility is not trivial in this sense. It is something that many therapists might miss, and that other therapists might use to get good results in improving mechanics during bending. To use another example, it is useful and non obvious to know that inadequate dorsiflexion can cause a bad squat, that poor thoracic extension will impair overhead pressing, or that sacral sitting causes a thoracic kyphosis. None of these concepts take a degree in anatomy to understand, but they are useful concepts that are not trivially obvious.

  8. I also think that we sometimes get too amazed by people’s biomechanical theories to the point where we abandon common sense. For example, when I do shrugs I feel my upper trap firing hard and it feels like the upper traps are elevating my scapulae. What else is going to move 500 lbs – the levator scapulae with its tiny moment arm and low PCSA? Come on Bogduk 😉

    • Hi Bret,

      Great point! I recall having some similar thoughts reading that paper for the first time. (It wasn’t during a 500 lb DL though :))

  9. You take a look at Douglas Heel’s work with muscle activation, that I now treat people with, psoas is definately the major hip flexor in the body, we can not move forward without it, when it is tested in this treatment it is not functioning ad it should be and soon ad we go in and activate it, the psoas starts to function correctly and many posture issues go away but we work on all the major muscles in the body not just the one!

  10. Great post – and another book on the shopping list. Just one question: Could you give a short touch on the topic how the psoas is tilting the pelvis posterior? I can’t figure that out, but it’s quite an interesting idea.

  11. Great post. I have had a great deal of success with manual release of the psoas. I am rendered unable to walk, or even to flex and extend the spine, from time to time. Mostly linked to extreme situations of excessive sitting (e.g. completion of Bachelor and masters thesis). The only “cure” I have been able to find is som light glute stretches and a huge amount of “Coach stretch” (Hip flexors). Also I have been combining this self threatment with manual release of the psoas. It takes no more that 2-3 days to unlock this apparant “psoas cramped” syndrome. I must also add that my history of back injuries and grusome torment stems from a very real, and very instantanously disc herniation of L3,4 and 5, caused by trembling weights in the bottom position of the squat (think no collars on the bar, and a left/right lateral flexion with hips fully flexed). How come I find this so alleviating? Im talking a transformation after self threatment from “30 kg deadlift NO GO”, to “200 kg deadlift, oh yes” the next day?! When I have these periods of absolut no function in the back, I look severly twistet through my entire upper body as well. I spend most my days now on a FMS program fixated on Active straight leg raise and shoulder (Thoracic) function. I really don`t know if there is an actual question in there, and I certainly don`t expect an answer. Im am just surprised and realise that my own “diagnosis” is not up to par. Hopefully you can shed some light on this, because I always talk about anterior pelvic tilt in relationship with sitting, thight flexors, sleeping gluts (AND the back injury) etc and always blame Psoas for the worst struggles and that`s what I have been attacking.

    • Hi Lars,

      Thanks for the comment. I don’t know why the psoas release helps you because the CNS is very complex. Click on my Pain category in the menu bar, peruse through some of the articles, and I think you will get some ideas about potential mechanisms.

  12. The individual variability is what I think dooms studies on biomechanical causes of pain. Given the differences in bony structures, people’s nervous systems, and the thousand other variables we don’t yet know about, do you think a study done today (or even in 15 years) could possibly filter subjects into subgroups appropriately and accurately enough to test a biomechanical hypothesis? Clinically, I think there are a good number of people who look like they’ll respond to a biomechanical approach and do respond to a biomechanical approach with enough investigation….However, there are are definitely some for whom the biomechanical approach does little to nothing.

    And then one wonders whether it’s not a biomechanical problem (e.g. I have a client who eventually had her issues diagnosed as a case of too much copper in her blood that led to nerve pain) or whether the investigator just doesn’t yet know enough about the relevant biomechanics…?

    Thanks for the book rec!

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