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Is symmetry an important therapeutic goal?

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Graham, Oct 16, 2008.

  1. Graham

    Graham RIP

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    Taken from our discussion:

    David Holland stated:

    So! Is symetry an important consideration in our therapeutic outcome?

    Do we have any evidence either way regarding symetry and gait?

  2. Craig Payne

    Craig Payne Moderator

    I do not think we know. We assume that symmetry might be a good thing, but if we have a patient with one bad foot and one good foot and we make an orthotic to make the good foot into a bad foot, then we have achieved symmetry - is that a good outcome?

    This comes back to what clinical gait parameters do we need to change to get a clincal symptomatic outcome??

    We have some good assumptions (that may well be correct), but we really do not know.

    We have looked at asymmetry in relationship to the timing of heel off loading anf have certainly hypothesised that this asymmetry could cause an assymetry in proximal muscle activity to compensate - this may or may not be a facter in proximal problems (see this thread on Asymmetrical density heel raises)
  3. Atlas

    Atlas Well-Known Member

    Trying to attain symmetry to the nth degree might be futile and unrewarded.

    But to simply dismiss it is wrong IMO.
  4. davidh

    davidh Podiatry Arena Veteran

    Hi Atlas,

    Opinion noted.

    Any evidence?

  5. Tissue stress model might be useful here. If there is an asymmetry which is pushing tissue stress past safe thresholds then correcting that asymmetry should be beneficial. By the same token if symmetry exists but tissue stress thresholds on one side are reduced it may be theraputically useful to create an asymmetry.

    I would say that symmetry has inherent value only in that it equalises forces between the two sides distributing stress evenly. Not an ignoble goal but not the beall and endall in all cases.

    Also symmetry in gait is only useful if it creates symmetry in the internal forces and movements in the pathological structures (wherever they may be.) If the body was symmetrical then symmetry of gait function would acheive this. If however the body is asymmetrical structurally (most / all are) we cannot rely on this.

    For example, if there is a structural LLD and the longer leg pronates (shortening it) there will be an asymmetry in gait which creates a symmetry in pelvic level. Creating a symmetry in foot posture and gait here (measured for eg with force plate) will create a potentially harmful asymmetry in the pelvis, back, etc etc.

    I suppose it depends on whether one approaches it from a root "style" (return the system to "normality" irrespective of the specific pathology) or the tissue stress style (assess the pathological stress and reduce it).

  6. Of course, symmetry is important since we are all bipedal organisms and the kinematics and kinetics of one foot and lower extremity will directly affect the kinematics and kinetics of the contralateral foot and lower extremity. However, focusing solely on making the feet and lower extremities function symmetrically, at the expense of increasing symptoms elsewhere for the patient, does not make good clinical sense.

    The best practitioners of clinical biomechanics are artists that use science as the basis for their clinicial decision-making process.
  7. And assuming the left and right feet are ambulating upon identical surfaces, which they seldom are.
  8. Yes, the molecules under the left and right feet are different, Dr. Spooner.;) In many circumstances (i.e. the flat surfaces that David Holland likes to always emphasize), the surfaces are basically identical to each other.
  9. You need to get outside more Prof. Kirby, there you may find a whole host of surface geometry on which to place your feet.
  10. David Smith

    David Smith Well-Known Member


    This is my view:

    Symmetry is a nice place to visit but you wouldn't want to live there.
    Like any position of reference, symmetry is a global position of reference and is a place to start from but not neccessarily a destination we need to arrive at to complete our journey. Like a star in the sky it is a guide to where we are relative to somewhere else but not somewhere we ever are likely to travel to.

    That's the metaphors done,

    Symmetry is or can be an artifice that satisfies our intuition in that it gives the illusion of balanced forces, which may or may not exist. Certainly symmetrical gait and posture with symmetrical internal forces can only exist in a symmetrical evironment.

    Humans are only grossly symmetrical but then grossly asymmetrical in detail.
    It would be unusual, wouldn't you agree, to find someone with identical feet, identical bilateral musculature, identical joint shapes, ambidextrousness, exactly equal limb length, equal muscle strength either side. No one even has a perfectly symmetrical face.

    If not then kinematic symmetry what about kinetic symmetry? Are symmetrical forces a desired or optimal goal? If we have slightly different limb lengths and slightly different joint shapes and slightly different soft tissue structure, with slightly different mechanical properties, why would you expect symmetrical and identical forces.

    How often do we observe a patient with unilateral symptoms yet very simmilar foot function or is even asymmetrical but the foot that appears far outside our reference of normal is not the one that is symptomatic? What then?

    We can only address the symptomology in a logical fashion, diagnosing the pathology, deducing the aetiology and by intervention, reducing the forces for instance. Can we then concern ourselves with symmetry, I think not, symmetry was only our reference not our goal.

    Having said that I often like to look for symmetry, it satisfies my intuition but that is just my own logical deceipt, which fortunately I can recognise before it becomes a rigid yet fanciful convention.

    Cheers Dave
  11. In order to better understand the question of whether trying to achieve symmetry in gait should be a therapeutic goal of ours as clinicians of the human foot and lower extremity, we must first divide our discussion into two parts: kinetics and kinematics.

    Symmetrical kinetics would indicate that the forces and moments acting on and within each and foot and lower extremity would be identical to each other during the specific weightbearing activities where each limb is performing the same biomechanical function as the other limb, such as walking and running. We may use in-shoe pressure insoles, pressure mats, force plates, strain gauges, or when combined with three-dimensional kinematic analysis, we may use inverse dynamics to gain useful information regarding the level of kinetic gait symmetry in the individual.

    The question becomes, is having symmetrical kinetics something that is desirable as a bipedal organism during walking and running? I do believe that possessing symmetrical kinetics is desirable in the feet and lower extremities of our patients since, in mechanics, engineering and in nature, symmetry of forces between two halves of a machine or organism will tend to equalize the stresses acting on the structural components of each of the two halves. Equalization of stresses between two parts of a machine or organism will reduce the tendency for one of the halves to have reduced stresses at the expense of the other half having increased stresses. These increased asymmetrical stresses acting on a structural component of the human foot and lower extremity (i.e. bone, cartilage, ligament, muscle, fascia and tendon) will, over time, lead to increased likelihood that the structural component may develop internal damage to it, which leads to what we call "injury". Therefore, from a kinetics view, it is clear that if we can make the forces, moments and stresses between the two lower extremities symmetrical, or equalize them, then we can also minimize the risk of "higher stress" lower extremity musculoskeletal injury developing over time.

    Symmetrical kinematics would indicate that the movement patterns of each foot and lower extremity would be identical to each other during the specific weightbearing activities where each limb is performing the same biomechanical function as the other limb. We may use visual gait examination, video analysis, or two-dimensional or three dimensional gait analysis techniques to determine the kinematics of each lower extremity. Being that visual gait analysis is the easiest, quickest and least expensive clinical gait evaluation technique, often this is what most of us rely on to detemine whether symmetrical function is occuring or not. Certainly, from a theoretical standpoint, having symmetrical kinematic function would be most desirable but if the structure of the two limbs is not symmetrical, this may not be a realistic or desirable goal.

    For example, if one foot has a normal subtalar joint (STJ) axis location and functions normally during walking but the other has a significant medially deviated STJ axis and functions maximally pronated at the STJ throughout the stance phase of gait, then should we try to make each foot function the same? Since tissue stress theory is based on the fact that we need to 1) reduce the stress on the injured structural component, 2) optimize gait function, and 3) not cause any other pathology to occur, then the key phrase here is "optimize gait function".

    In other words, we don't want to over-invert the medially deviated STJ axis foot so much in trying to make it function kinematically identical to the foot with the normal STJ axis because, in doing so, we may be subjecting the medially deviated STJ axis foot to abnormal and increased forces and stresses (i.e. abnormal kinetics) that are may cause injury elsewhere within that foot. For example, if we over-invert the foot orthosis for the medially deviated STJ axis foot, we may cause excessive painful plantar pressures on the medial heel and medial arch from the orthosis, excessive painful pressure on the lateral metatarsals due to over-supination, or may cause increased stress in the peroneal tendons by too much increase in STJ supination moment from the orthosis, in our desire to have the two feet function with symmetrical kinematics.

    Therefore, when treating mechanical pathologies in patients with asymmetrical kinematic function, we should have the therapeutic goal of optimizing kinematic gait function, not necessarily focus on making the patient's kinematic function symmetrical at the expense of making the patient's gait kinetics less symmetrical that results in new injury. In this way, we can make our patients less painful, more productive, prevent new injuries and make them function as well as they possibly can during their weightbearing activities given their characteristic structural composition and their characteristic neuromuscular capabilities.
  12. All this talk of symmetry as got me thinking about a thought experiment I was playing with last week: If we had a foot that had equal pressure beneath every single point of it, where would the centre of pressure be? If this situation was constant with time, would it be capable of movement?
  13. Atlas

    Atlas Well-Known Member

    Get your mother/wife to wear one stiletto on one foot; and a flat on the other foot whilst at home for the next few months. Alternatively, put a heel raise in one of their shoes full-time for the next few months.

    If symmetry doesn't totally matter they wont notice any changes and neither will you.

    Physiotherapist (Masters) & Podiatrist
  14. davidh

    davidh Podiatry Arena Veteran

    Hi Ron,

    Pretty extreme example, but I concede the point.

    How does this relate to orthoses and F-scan:confused:

    In his original post Graham was asking if evidence existed to show the importance of symmetry - I assumed he was talking about scientific evidence, but perhaps not...

  15. One thing i think which has become clear through this thread is that to answer the OP one needs to define what is meant by symmetry.

    The point i made and Kevin expanded on is that symmetry in one place / measured one way does not necessarily equate to symmetry throughout the system. We need to be clear on what symmetry we are talking about when we ask the question "is symmetry and important theraputic goal."

    I contend that one cannot say that symmetry in, for example gait kinematics or vertical force measurement tracks, is an important theraputic goal as a standalone statement. It often is, but only if the circumstances around it indicate!

    As so often with biomechanics it boils back down to tissue stress. Symmetry is good IF it has a benificial effect on internal tissue stresses.

    I saw a patient last week who had a relevant presentation. structural LLD with a more pronated foot in WB on the longer limb. In that case seeking symmetry of foot posture could have had a deleterious effect on the proximal structures by increasing pelvic tilt. Seeking symmetry of the whole kinetic chain by seeking symmetry of the foot posture AND the pelvic level might have been more benificial.

    All depends on where we look for symmetry.

  16. David Smith

    David Smith Well-Known Member


    Oh! Why can't I wear the stillettos? she always get to wear them it's just not fair:D

    Making something asymmetrical when it was innately symmetrical in a symmetrical environment is the same as requiring something innately asymmetrical to be symmetrical in a symmetrical environment. What is the same is that you changed something that didn't need changing.

    Take a box, say a kithchen cabinet, and twist it, take a tree and make it straight - both will be broken in the attempt. Are humans more like a tree than a box?
    Does a body need to be perfectly symmetrical to avoid damaging stress? no. However a change to that symmetry, like a hip replacement for instance might result in some pathology. It wasn't the shape that caused the destructive stress it was the change in shape and the resulting local forces. This then might be viewed as a degree of asymmetry and a gross assymetry might indicate pathology in humans. It may well do this but I would say that, where the asymmetry is not due to recognizable change, this would also be a gross assumption. How can we recognise, just from observation of shape, where is the demarkation between pathological and non pathological asymmetry?

    Symmetry is more relevant when the diagnosis indicates that the forces that lead to pathology also resulted in asymmetry or vice versa ie the change in symmetry or posture caused forces that led to pathology. This is the case where the symmetrical body in a symmetrical environment was changed into an asymmetrical body in a symmetrical environment. This is the case we as clinicans become familiar with and so form the opinion that asymmetry = pathology and all things asymmetrical would be better off if it were symmetrical, which is in my opinion a logical fallacy.

    The logical fallacy of accident, also called destroying the exception or (This bit is specially fo Robert Isaacs and Graham) a dicto simpliciter ad dictum secundum quid, is a deductive fallacy occurring in statistical syllogisms (an argument based on a generalization) when an exception to the generalization is ignored. It is one of the thirteen fallacies originally identified by Aristotle. The fallacy occurs when one attempts to apply a general rule to an irrelevant situation. (http://en.wikipedia.org/wiki/Accident_(fallacy))

    Cheers Dave

  17. Dave and Colleagues:

    Good thoughts, Dave. Rather, a better question in this regard is does a person that have perfect symmetry in their feet and lower extremities reduce that person's risk of musculoskeletal injury when compared to that person that has grossly abnormal asymmetry in their feet and lower extremities? I believe that most of us would argue that perfect symmetry is the ideal, that gross asymmetry is non-ideal and that gross asymmetry will lead to increased risk of abnormal tissue stresses and increased risk of musculoskeletal injury.

    Therefore, as I said in my last posting, striving toward perfect kinematic symmetry is a reasonable therapeutic goal if we only knew, in striving to achieve that perfect kinematic symmetry, that abnormal asymmetrical kinetics did not result within the individual's body that could then lead to increased magnitudes of tissue stress and musculoskeletal injury. I really don't think we currently can determine whether our treatments result in kinetic symmetry of our patients at this stage of our abilities, other than observing for pathology or asking our patient if pain or discomfort is occurring.

    Great discussion. In fact, I feel that this is one of the most clinically important theoretical discussions that have taken place here on Podiatry Arena in some time.
  18. David Smith

    David Smith Well-Known Member


    If memory serves - To estimate CoP (non calculus)

    Define a matrix of discreet force areas, choose one longitudinal line in that matrix, find the discreet force for each unit area (uf), find the distance of each unit area from a common reference (dr) (usually where all the forces of interest cause a moment in the same direction) sum all the forces X distances [Sum(uf*df)], sum all the unit forces to find total force, find the distance that the total force times that distance is equal to the moments by [sum(uf*df)]. This = CoP for that line. Repeat across all longitudinal lines. THis will define a line of CoP. Repeat for all transverse lines, this gives a transverse line of CoP. Where the two lines cross = the characterisation of the total summation of CoP.

    Therefore if all are equal then it is still possible to estimate a CoP where that is depends on the shape of the foot. If the foot was symmetrical about x and y axes, then the CoP would be in the centre of the total area. If asymmetrical, as it is then the CoP can't be guessed at. Although you could divide it up into symmetrical shapes, find the center of each shape then find the summed centre of the individual centres of CoP. This would give a good approximation more easily.

    For the second part do you mean the whole surface area of the foot or the surface area in contact with the ground. Clearly the former could not move and the latter could in theory. (probably)

  19. I arrived at the same conclusion

    I was talking about plantar pressure. Considering the later, you suggest that foot movement is possible without centre of pressure displacement or velocity? Is this through vertical lift off?
  20. markjohconley

    markjohconley Well-Known Member

    Couldn't there be motion about its centre of mass? but not motion of same, mark c
  21. David Smith

    David Smith Well-Known Member


    Re diagrams - Is this what you meant Simon?

    I meant that as the foot progresses thru stance phase it may be possible under unusual circumstances for the pressure to remain uniform across the plantar surface on the ground and CoP at the centre of the plantar pressure matrix at any given point in time. Of course the pogression of CoP must move relative to a fixed reference, but this would be the same regardless of symmetry.

  22. markjohconley

    markjohconley Well-Known Member

    My post was based on me thinking "movement" meant relative motion between parts of the foot, rather than displacement of the foot. I'll be quiet now.
  23. efuller

    efuller MVP

    Well if we moved around on our legs like they were pogo sticks we could get close to even amount of pressure on the bottom of the foot for the entire stance phase. However, to have ankle push (power added to the leg in stance phase.) the heel has to lift off of the ground while the forefoot remains on the ground. It would be impossibe to have the same magnitude of pressure at all points throughout stance phase becasue of upward and downward acceleration of the body. The force changes with acceleration and the area of the foot does not change so the pressure would change.

    Now, if you had 100 legs and you could pick one up and put it down without upward or downward acceleration of the center of mass you might be able to keep the pressure equal between all areas of the foot, but the magnitude would change as the foot and leg loaded.

    If the sampling rate of pressure was low enough you could measure no change.

    Anyway, fun thought experiment

  24. efuller

    efuller MVP

    On symmetry: I recall a study, sorry don't have reference at hand, where there was a comparison of treatment of back pain with manipulation and physical therapy. There was more pain relief with physical therapy and the gait was closer to symmetrical with manipulation. Would rather be symmetrical, or would you rather have less pain?

  25. If the foot was an immovable, rigid platform that was attached at the ankle as a frictionless joint and the individual had no ability to exert either ankle joint plantarflexion moments or ankle joint dorsiflexion moments, then, yes, the center of pressure would show little to no movement on the plantar foot during "walking". The anterior to posterior movement of the center of mass over the planted foot during the stance phase of gait, in this case, could not be resisted by the foot and so the center of pressure would change very little since the foot would do nothing to either decelerate or accelerate locomotion than would a "peg leg". A person with this type of foot could perform a type of bipedal locomotion, but the muscle power for this activity would all need to come from proximal to the foot and ankle and any resistance to foot motion relative to the leg could not occur. I don't think that most of us would still call this type of bipedal locomotion "walking", even though it would still allow the individual to move from point A to point B.
    Last edited: Oct 22, 2008
  26. Dave, Mark, Eric and Kevin,

    Thanks for taking the time to think about this, t'was just a bit of fun. I'd been thinking about a "magic" insole that could constantly change stiffness to equalise the pressures beneath the foot. Kevin, do you remember that sketch of the pistons beneath the met heads, you once sent me? That was the inspiration to this little head game :dizzy:. Anyway back to the real world, my nail drill just died and it's going to cost me the best part of a grand for a new one- ouch.:craig:

  27. Simon,

    This drawing was based on an idea I had during my Biomechanics Fellowship (1984-1985) when I did the study on the Anterior Axial Radiographic Projection (The Kirby view: A radiographic view for flatfoot evaluation. The Journal of Foot and Ankle Surgery , Volume 43 , Issue 6 , Pages 436 - 439.J . Clark , J . Gerbert , W . Jenkin).

    At the time, I was thinking how I could design an x-ray projection that could be done to allow the weightbearing metatarsal heads be evaluated radiographically with the same plantar force on each metatarsal head to see what "level" they would be at relative to each other in this "constant plantar force position". One thought was to use plungers under each metatarsal head that would apply equal force to each and then design the orthosis/surgery around this constant metatarsal head force position. In theory it makes a lot of sense.....however, there is only so much time in the day for me and someone with more time and a less busy life will need to take up this project for us in the future.
  28. Graham

    Graham RIP

    Nature 438, 1148-1150 (22 December 2005) | doi:10.1038/nature04344; Received 23 September 2005; Accepted 20 October 2005
     Zoran Popovihttp://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=search&term=Popovic+Z
     more authors of this article

    Dance reveals symmetry especially in young men

    William M. Brown1, Lee Cronk1, Keith Grochow2, Amy Jacobson1, C. Karen Liu2, Zoran Popovi2 & Robert Trivers1
    1. Center for Human Evolutionary Studies, Department of Anthropology, Rutgers University, 131 George Street, New Brunswick, New Jersey 08901-1414, USA
    2. Department of Computer Science and Engineering, University of Washington, Box 352350 Seattle, Washington 98195-2350, USA
    Correspondence to: William M. Brown1 Correspondence and requests for materials should be addressed to W.M.B. (Email: wmbrown@rci.rutgers.edu).
    Top of page

    Dance is believed to be important in the courtship of a variety of species, including humans, but nothing is known about what dance reveals about the underlying phenotypic—or genotypic—quality of the dancer1, 2, 3, 4, 5, 6. One measure of quality in evolutionary studies is the degree of bodily symmetry (fluctuating asymmetry, FA), because it measures developmental stability7, 8. Does dance quality reveal FA to the observer and is the effect stronger for male dancers than female? To answer these questions, we chose a population that has been measured twice for FA since 1996 (ref. 9) in a society (Jamaican) in which dancing is important in the lives of both sexes. Motion-capture cameras created controlled stimuli (in the form of videos) that isolated dance movements from all other aspects of visual appearance (including FA), and the same population evaluated these videos for dancing ability. Here we report that there are strong positive associations between symmetry and dancing ability, and these associations were stronger in men than in women. In addition, women rate dances by symmetrical men relatively more positively than do men, and more-symmetrical men value symmetry in women dancers more than do less-symmetrical men. In summary, dance in Jamaica seems to show evidence of sexual selection and to reveal important information about the dancer.
  29. Graham

    Graham RIP

    Predominantly extra-retinotopic cortical response to pattern

    Christopher W. Tyler,a,* Heidi A. Baseler,a Leonid L. Kontsevich,a
    Lora T. Likova,a Alex R. Wade,a and Brian A. Wandellb
    aSmith-Kettlewell Eye Research Institute, San Francisco, CA, USA
    bDepartment of Psychology, Stanford University, Stanford, CA, USA
    Received 15 December 2003; revised 26 April 2004; accepted 17 September 2004
    Available online 26 November 2004

    Symmetry along one or more axes is a key property of objects and
    biological organisms. We report on a bilateral visual region of occipital
    cortex that responds strongly to the presence of multiple symmetries in
    the viewed image. The stimuli consisted of random dots organized in
    fourfold and onefold mirror-symmetric patterns, against random
    control stimuli. The contrast between symmetric and random patterns
    produced negligible or inconsistent activation of the primary visual
    projection area V1 or of other medial occipital projection areas.
    However, there was strong symmetry-specific activation in extraretinotopic
    lateral occipital cortex. The high level of activation in this
    region of cortex may represent part of a general class of computations
    that require integration of information across a large span of the visual
    D 2004 Elsevier Inc. All rights reserved.
  30. Graham

    Graham RIP

    J Biomech. 2006 Jul 13; : 16843472 (P,S,G,E,B) Symmetry-based resistance as a novel means of lower limb rehabilitation.

    [My paper] Ann M Simon, R Brent Gillespie, Daniel P Ferris
    Department of Biomedical Engineering, The University of Michigan, Ann Arbor, MI 48109-2214, USA; Department of Mechanical Engineering, The University of Michigan, Ann Arbor, MI 48109-2214, USA.
    Robotic devices hold much promise for use as rehabilitation aids but their success depends on identifying effective strategies for controlling human-robot interaction forces. We developed a robotic device to test a novel method of controlling interaction forces with the intent of improving force symmetry in the limbs. Users perform lower limb extensions against a computer-controlled resistive load. The control software increases resistance above baseline in proportion to lower limb force asymmetry (balance between left and right limb forces). As a preliminary trial to test the device and controller, we conducted two experiments on neurologically intact subjects. In experiment 1, one group of subjects received symmetry-based resistance while performing lower limb extensions (n=10). A control group performed the same movements with constant resistance (n=10). The symmetry-based resistance group improved lower limb symmetry during training (ANOVA, p<0.05), whereas the control subjects did not. In experiment 2, subjects (n=10) successfully used symmetry-based resistance to alter their lower limb force production towards a target asymmetry (ANOVA, p<0.05). These studies suggest that symmetry-based resistance may hold rehabilitation benefits after orthopedic or neurological injury. Specifically, performing strength training therapy with this controller may allow hemiparetic individuals to focus better on increasing strength and neuromuscular recruitment in their paretic limb while experiencing symmetric limb forces.
  31. davidh

    davidh Podiatry Arena Veteran

    Hi Graham,

    You cut and pasted three refs:

    The first concludes "dance in Jamaica seems to show evidence of sexual selection and to reveal important information about the dancer. "

    The second:
    "The high level of activation in this region of cortex may represent part of a general class of computations
    that require integration of information across a large span of the visual

    The third:
    "Specifically, performing strength training therapy with this controller may allow hemiparetic individuals to focus better on increasing strength and neuromuscular recruitment in their paretic limb while experiencing symmetric limb forces. "

    I assume these were introduced as scentific proof to strengthen your argument that symmetry (as measured by F-scan) is important.

    They don't do it for me I'm afraid. Anyone else?
  32. Graham

    Graham RIP


    As with most practices based on assumption, you are incorrect. I posted these for interest only.

  33. davidh

    davidh Podiatry Arena Veteran

    Oh - ok:confused:.
  34. David Smith

    David Smith Well-Known Member


    Getting back to symmetry tho, I recently had a Physio and a Chiropracter ask about orthoses to balance a leg length discrepancy on their respective patients. They thought it may resolve their back pain and It has been my experience in the past that this if often the case ie fit heel lift, resolve back pain.

    However how does this work? we presume that if we see symmetry it is probably a good thing but this can only be true in a symmetrical environment. The floors of our office and those of other building like supermarkets and shopping centres give this symmetrical environment. However as soon as we are out in the world the ground is not flat so how can we expect postural symmetry in a non symmetrical environment and if we are not symmetrical in this non symmetrical environment does this lead to patholigocal stress. I would guess that in the main it does not. Does an LLD only cause pathology when we tend to spend more time on very flat even surfaces? How flat then would those surfaces need to be to envoke the Aymmetrical pathology proposition?

    Maybe this would imply that we are on average, symmetrical, ie the mean sum of all our postural changes approximates symmetry and so there is no pathology. What then if we spend two hours with the lumbar spine full flexed to the right and then two hours fully flexed to the left and continued this sequence over many weeks. The average position would be straight up and errect, even tho we never attain this position except in transition, could we then expect no pathology, I doubt it.

    Hmmm! is there then some critical 'deviation V's time' curve where the exceeding of such boundary will tend to result in pathology? A curve depicting a limit of asymmetry as it were. Could this be the genisis of our intuition? we see a large deviation from symmetry, we know its been there for a long time, we assume that this leads or tends to lead to pathology. This assumption is based on the convention that large deviation equals large stress or at least a stress time curve that cannot be exceeded. Is it safe to make that assumption? maybe it is more important to ask is it unsafe to have symmetry and assume this goal represents normal and non pathological.

    This puts me in mind of David Holland's argument that we are not evolved for flat surfaces and so we cannot use a paradigm that involves some arbitrary orthogonal convention of alignment to justify and validate a normal position. Therefore if we can assume the body is not primarily made to be compliant with a symmetrical environment is it correct or useful to expect this postural position, which appears abitrary, to be advantageous or benificial?

    Cheers Dave
  35. Graham

    Graham RIP


    Could you tell me what surfaces you regard as "out in the world"?

    If the body is asymetrical and meant to interact with an asymetrical enviroment without pathology, but this asymetry, on a flat surface, develops pathology, would it not reason that to create symetry of the body to function on the artificail flat surfaces, which it does most of the time, would prevent pathology?
  36. Dave:

    You are the engineer among all of us. Certainly I would imagine that, if you, as an robotics engineer, were to design a robot that needed to be in contact with the ground with only two "legs", with each of these "legs" making periodic oscillating rotational movements, and with each "leg" connected to the body of the robot with mechanical joints of some sort, that you would want to design those "legs" symmetrically, wouldn't you? Would the design then of such a walking robot be considered "arbitrary", or would there be a good mechanical reason to design such a walking robot with symmetrical legs?

    I would think that factors such as improved mechanical efficiency, decreased wear on structural components of the robot, and less feedback from central processing unit of the robot to guide a level and straight walking pattern would all be advantages of having symmetrical limbs in a walking robot. I don't think that any good robotics engineer would purposefully design a walking robot with asymmetrical limbs, with asymmetrical moments of inertia, asymmetrical masses, asymmetrical power outputs to each limb, and with asymmetrical lengths.:confused:;)
  37. An interesting point. Raises the old spectre of whether the body is "meant" to function in a certain way. Meant, of course, implying inteligent design. Or is it all an accident of evolution in which case the body is not "meant" to do anything but rather is just evolving to the present demands upon it (until we came along with our orthotics of course ;))

    Your point has several presumptions, not least of which is that an asymetrical body will operate in an aysymetrical environment without pathology! Not sure we know thats true. If that a priori (sorry, pompus Brit using latin again;)) premice is flawed then the subsequent one becomes irrelevant.

    But designing the robot is beyond our ability. What we are doing, to stretch your analogy, is taking a robot which is structurally asymmetrical and has been for many years due to manufacture and design inconsistancies, which has developed CPU patterns on that basis and has possibly undergone structural changes in line with that... and then trying to level the limbs up!

    Now that does, from time to time, work. However i can think of many other times when it might not. Depends on so many factors. How entrenched are the adaptations? What, exactly, is causing the pathology? Etc?

    Interesting idea. I suppose one needs to know the cause of the pathology. Is it potential for individual excessive excursion from the "average" (in which case starting one side of a central position will make one more at risk because there is less far to go)? Is it, as you suggest, a composite of stress and time? Is it a question of thresholds established over time then breeched in critical event?

    Here's something else to mull. I throw it out just as a what if.

    We have out patient with their structural LLD. Lets call him Bob. Bob's spine has been in that position for years, his muscles on one side are tighter, looser on the other, his knees are used to working asymmetrically etc etc. We stick him a lovely pair of orthotics all balanced and dandy. The muscles in his back start balancing. The compression on his spinal nerves on one side is eased. His lower limb function becomes symmetrical. Wolfs law kicks into play and everything adapts to its new position. All is right with the world.

    Bob wears his insoles religiously for a few years. His body adapts to the symmetrical position. Because his pain is improved he has a better outlook on life, gets a better job, earns $$$$$ and marries a supermodel named Cherie who has a toy poodle. For their honeymoon they go to Brazil. Unfortunatly their luggage, containing his orthotics goes to Denmark.

    It could happen.

    Is he now more or less at risk of developing problems when the ole structural asymmetry rears its head again? Will he return from his holiday in a better or worse state than if he had never been "corrected".

    Great debate! Thanks Graham.:drinks


    ps. If the analogy of Bob seems a bit far fetched turn the example into someone who only wears their insoles in some shoes and goes for a long walk without them. Boring, but more plausible.
  38. Graham

    Graham RIP


    I guess if he stays in Brazil long enough! If he stays off the beach and only pounds the side walk!

    That brings me back to David, and my previous question:

    Could you tell me what surfaces you regard as "out in the world"?

  39. This is not what I asked of David to explain to us. I just wanted to know if a smart engineer would purposely design robotic legs symmetrically or asymmetrically and what would be the mechanical or scientific reason for doing so.
  40. David Smith

    David Smith Well-Known Member

    Kevin and Graham

    Graham wrote
    Pretty much anywhere that is not inside a building.

    I don't know for sure. This is the question I am pondering Graham.
    (1)How flat and even is your world. Sure beneath one foot, on man made sufaces, it is as flat as one would consider significant. However. between each foot, how unsymmetrical would the topography have to be before you considered it assymetrical or uneven?

    (2)While the ususal human body is grossly symmetrical, it is not symmetrical in detail. This detail is greater than microscopic and is in the realm of macroscopic in that it can be measured with everyday apparatus and even visible to the naked eye.

    Is there a tipping point where the gross symmetry becomes gross asymmetry and causes pathology. Tipping points are part of chaos theory and is where a system can operate quite well and in an ordered and predictable manner even tho within there are variables that have random actions but those random actions do not upset the whole system until they reach a certain point of pertubation, which then send the system into a state of unbalance and unpredictability. This point itself is difficult to predict and is known as the tipping point.

    How far removed from gross symmetry must we go before assymetry becomes a tipping point where the musculo-skeletal system cannot control internal forces and so they become pathological. By definition this tipping point is unpredictable in both its effect and the time of its genisis.

    Conversely how much can we restrict and set boundaries for the degree of assymetry allowed in a body without perhaps causing a tipping point in the opposite direction.

    If we consider symmetry as a rigid confined system with conventional and predictable boundaries and each possible variation of asymmetry as a degree of freedom. Then how many degrees of freedom are allowed before the system reaches a tipping point and how many degrees of freedom are required for normal non pathological operation.

    This is what I am pondering on and I believe is the essence of your original question.

    It may be that the optimum degrees of freedom are variable with variation in environment. Therefore the more the environment is restricted in its asymmetrical degrees of freedom the more the body must coordinate with this. In which case how far are we, as a biological machine, capable of complying with this criteria of orthoganal symmetry?

    Kevin wrote

    Only one among many including you and Eric. In engineering reasoning is important and clear reasoning is your forte.

    Kevin, my robot would, by convention, be concieved in a symmetrical environment and so using euclidean geometry and Newtonian mechanics I would tend to design a robot within those boundaries. This may be why even the best robots today struggle to cope with topography outside those limited boundaries.

    Now if I were to design a robot that had some inherent asymmetry and was able to allow more degrees of freedom of asymmetry to coordinate with it's environment then perhaps my robot would easily cope with walking up the Bens of the Trossachs or the sands of the Sahara or just walk down to the shops without falling 30 times.

    The only constant parameters that I can see in terms of symmetry are gravity and time. I'm really intrigued by this subject but before I make my brain hurt any more by thinking about these things I'll await your thoughtful replies.

    All the best Dave

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