Exploring Turvey's paper on Action and perception at the level of synergies

Yes..

Even if my calculations and estimates were off we can look at the real world. People rupture disks bending over. People can squat 1000 lbs without rupturing their disks. Do more people injure their backs lifting a heavy object as opposed to just lifting their own body weight?

70 N human. Estimate part of body above L1 (or pick your own vertebra and alter the estimates accordingly) is 15 N. Bend forward at L1 and at the hips so that the center of mass is 20 cm anterior to L1. The bending moment, from a static load, that has to be resisted by the disk at L1 is 3 Nm The distance between the back muscles and the disk, estimate 5cm. Load on disk = weight plus resistance to bending moment. Bending moment load = 60 N + 15 N for a static load. (It would be greater for a dynamic load. However for a bending load the compressive forces would be concentrated at furthest edge of the disk and not be evenly distributed about the disk. So, if the force was concentrated over 1 sq cm of the disk then the pressure at the edge of the disk would be 75 newtons per sq cm.

Now 1000 lbs = 450 N If the back is straight and the load is distributed evenly over the entire disk. weight plus body = 465. Estimate 16sq cm then pressure on disk is 29 N per sq cm. Much less muscle activity is required when the spine is straight. There would still be some muscular compression of the disk.

Understanding leverage is key to understanding mechanics. With leverage you can increase the force applied. Anyone who has ever lifted weights should have an intuitive sense of this. It's much easier to lift a 10 foot pole at the middle than at the end.

Regards,

Eric

 

Eric,
Thank you for your detailed answer. Now we can have an intelligent discussion on the forces involved on the discs.
As I have said “when the engineering model is correct, then it would be easy for anyone to make the predictions with engineering principles.” Your model of the squat is not correct. If the spine was straight, the person would fall backwards, because the center of mass would be posterior to the base of support.
The back in the squat is not perpendicular to the ground, therefore there are moments in the spine, and the erectors have to generate tension to keep the back relatively straight.
Let’s look at some 1000 lb squats and tell me that the extensors do not fire. Look at the relation of the weight to L5 and tell me that no moments are developed.

http://www.youtube.com/watch?v=APLHDfKpOqg

http://www.youtube.com/watch?v=un5P30LIQhU&feature=related

http://www.youtube.com/watch?v=L0lF4lm3efA&feature=related

http://www.youtube.com/watch?v=ZGHxoizDkh8&feature=related

Here is a high rep work out with light weights, and you will see that each squat is done the same as the heavy squats. :deadhorse:

http://www.youtube.com/watch?v=Z78bfF5ygKE&feature=related

I am sure you can lift more than a woman with your new squatting technique that does not require the use of the back muscles. By the way, even the Brits have the weight anterior to L5, and would have to use their extensor muscles.

http://www.youtube.com/watch?v=2fmJy-4UQeE&feature=related

In the part of Turvey’s paper that you read (1.1) Turvey showed how complicated all movements are by showing all the muscles that are required for something very simple like making a sound. The squat requires more of the body to work, and hence even more muscles are required.
Sitting produces more intradiscal pressure than standing. How is this possible? If you look at the following article:
http://www.anatomytrains.com/uploads/rich_media/AnatomyTrainsOverview.pdf
You will see that the superficial front line works as a unit only when standing. This fascial line works with the abdominals. The fascial lines in the body are parts of structural system of continuous tension

If you use proper technique, you will make progress in your lifting. :empathy:

Regards,

Stanley

 

Eric,

When I correct a tension element at a distance from it, then I will call it tensegrity. This eliminates confusion and makes it simpler to understand.
How exactly do you make this correction?

Regards,

Stanley

 

Kevin, eric, Stanley, Mart, others,

Eric and Kevin have made me think a little the last few days about why there is such resistance to a concept - that we readily say needs clarification- that is simply stated, obvious for anyone to see. One reason may be that we cannot put a mathematical model to it. Some see this as a detriment, others say that it is evidence that we are finally getting somewhere close to the truth because being able to define it with current capabilities means that the hypothesis/equation is too simple to be anywhere near the truth. The problem with any Tensegrity equation is that it branches and anastimoses, producing a non-linear solution. We can’t even write that equation right now. I am no mathematician, but I have been looking for anything that would suffice ….nothing.

That said, why do I still even look at it if I am going to call myself a scientist? First, not having the answer and looking for it IS science. Obviously, the method is important, but what did we use before computers, for instance? Slide rules; fingers? Did that make those men any less scientific?
When I take one of my cadaver feet, separate every bone, begin putting them back together and note that every joint is very loose, then I should expect the whole to be very loose, yes? Yet, when I get it back together and substitute cheap twine for a few ligaments and tendons, the foot becomes remarkably stable. I cannot explain this with current models.
To make it even simpler, if I remove ALL soft tissue, leaving only bone and the articular capsules, the foot won’t even stand up on its own. Yet, if I put a screw in the CC joint AFTER I manipulate all joints into a normally congruent position, the foot becomes so stable that I can’t collapse it easily at all.
I am not a caveman looking for God to explain thunder, Tensegrity is a model and it does fit. That’s all any of us are saying. Do we need to tweak the definition a bit? Maybe. It is done on this site all the time. How many time, Kevin, have I read your proposal to alter the defining terms for a topic? Perhaps we can get you to help us with this one. There is no doubt that both you and Eric can clarify and articulate very well. You are excellent at debate. However, we really ought not to be debating this,help us simply solve the puzzle – or take our best shot. Would it be too much to ask that you critique this in a positive light in a manner that teases out the issues rather than tearing it down?
With respect,
Kevin M

 

Definately.

When I started getting interetsed in this I had no idea what a mine field it is, the more I have probed the more ignorant I realise I have been.

David emailed me an intersting chapter from a paper and I'm not sure what it's origins are so don't want to go posting it without asking him if that would be OK.

I found Ingber's paper really worth reading just from a physiological perspective. I thought I might try a shot at condensing it but that seems pointless really, it is all meat no veg. What I enjoy, but feel a little paralised by is reading a paper like that and thinking "Wow what a great explaination", then reading a critical review of the paper (this was cited earlier) and thinking "wow what a great rebuttal of that idea. It is clear that this issue is contentious is a much wider field than our own. David if you are reading this I was thinking of the chapter about tensegrity architecture with definitions, and the nice visual with the washing line being gradually morphed into a true "classical" tensegrity. That beautifully simply but elegant series of the transition from the clothes line helped me get a profound understanding a pretty tricky abstract idea to get hold of. I'll drop you a line off thread you seem to be amidst email address change.

cheers

Martin


The St. James Foot Clinic
1749 Portage Ave.
Winnipeg
Manitoba
R3J 0E6
phone [204] 837 FOOT (3668)
fax [204] 774 9918
www.winnipegfootclinic.com

 

Eric, Stanley,

Along out theme that "stabdard" mechanics cannot explain everything we see and your addition of weight lifting, I give you a synopsis of Gracovetsky, Kapanji and others. I did not write this, and I would have added some other details about forces, but this is good never the less. It is too long to copy, but an easy read. enjoy.

http://medicalsciences.med.unsw.edu.au/SOMSWeb.nsf/resources/ANAT314101/$file/FA2-4+IVD.pdf


Kevin M

 

I have created an FTP area on my website for large files, it is password protected to keep out unwanted visitors. I have put a selection of chapters on the biomechanics of MSK injury which I have just finnished reading. They are a nicely put together 100 pages which form a basic "tissue biomechanics 101 course" and a useful bridge in to this thread. please feel free to download this (approx 80MB file so you will need broadband) - I'll leave it there for a couple of weeks.

link to my website URL

www.winnipegfootclinic.com

click on resourses button on left of page
then click on Podarena icon

FTP Username: podarena@winnipegfootclinic.com
Password: footstuff

cheers

Martin


The St. James Foot Clinic
1749 Portage Ave.
Winnipeg
Manitoba
R3J 0E6
phone [204] 837 FOOT (3668)
fax [204] 774 9918

www.winnipegfootclinic.com

 

Stanley,
I did not say vertical, I said straight. You can be relatively straight without being vertical. And the videos support my point about pressure within the disk. I also agree with your point about extensors muscles acting. In the videos the extensor muscles would have to be acting. However, if there were any more forward flexion of the spine they would have to have more tension than there with the technique shown in the videos.

Additionally the videos also show a slight lordosis of the lower back. In the low back there is an angulation of the tops and bottoms of the vertebrae that corresponds with the lordotic curvature. If the lordotic position were not attained there would be increased pressure in the disks and increased chance of rupture. I still believe my model is valid.

So, Stanley, do you believe technique is important or do you just have to be strong and fast? Will you be able to lift more with better technique?

Regards,

Eric

 

Stanley,
Our world views are so different we really have a hard time understanding each other.

Regards,

Eric

 

I can explain it with simple engineering. See my paper on the Windlass Mechanism. It's simple bridge building, you have compression members and tension members. When you cut away all the tension members, it falls apart. When you replace them with twine It can stay up. Cut the twine it falls again.

Kevin M,

One of my biggest critiques of your ideas on tensegrity is that it cannot be defined. We cannot critique something in a positive light if we don't know what it is.

Regards,

Eric

 

Eric,

Working out a better definition is what I was hoping you would help with. I have gathered so much detail that I am having a hard time clarifying. You could ask questions and, as I suggested, tease this apart until we get something agreeable.

regards,
kevin m

 

Eric

Most of the mechanical analysis of foot and lower limb function which I have read necessarily regard the components of the system as rigid bodies.

In considering the biomechanics of injury we need to explore the mechanics of deformable solids since injury and the body’s response is to considerable tissue deformation.

I agree that we need some definition because Ingbers notion of tensegrity (which appears to be closest to those of us who are seeing this as worthy) seems at odds with the classical definitions in the issue of zero compression at the nodes.

Do you agree with the basic notion that Turvey has presented us with the likelihood that levels of organisation are present in our body systems which are not only necessary for its optimal function but not fully integrated into explaining it’s function?

Do you agree that once we consider the microscopic inhomogeneous and deformable nature of our body components, the simple mechanics, which give us the tangible calculations which you rightly value, may fall apart at the sites of injury, and ideally we should be thinking about this?

These questions motivate me to pursue this for a while.

Cheers

Martin


The St. James Foot Clinic
1749 Portage Ave.
Winnipeg
Manitoba
R3J 0E6
phone [204] 837 FOOT (3668)
fax [204] 774 9918

www.winnipegfootclinic.com

 

What's wrong with considering bones as rigid bodies. Yes they bend a little, and yes they occaisionally break, but when they don't break treating bones as rigid body is a close enough to an accurate assumption. Are you thinking of a specific instance where you cannot treat bones as rigid bodies?

Yes, the body tissues are organized into different structures; bone, skin, muscle, etc. and they do have to work together for the body to function. I don't understand what is meant by "not fully integrated into explaining its function?"

Things break. They break when placed under more stress than they can handle. The engineering equations, with a few assumptions can help us predict when they will break. They can also help guide us in treatment to reduce stress on an injured structure. (Varus heel wedge for PT Dysfunction) So, no I disagree with the idea that the formulas fall apart when examining sites of injury.

In the case of a fractured tibia: you use crutches to...
a. Reduce stress in the injured tibia to allow it to heal.
b. Replace the internal compression member with an external compression member to maintain biotensegrity.

Yes you do have to incorporate some physiology of wound healing into the engineering explanation. But we do know that when a fracture site is allowed to move you can get a pseudo arthrosis.

One of the points that I have been making in this tensegrity discussion is how do we use this information? Tell me how we use tensegrity to make treatment decisions. The engineering approach explains why we use a varus heel wedge over a valgus heel wedge in the treatment of PT dysfunction.

Cheers,

Eric

 

Eric, the point about straight and vertical is that if the back is not vertical (flexed)as a whole or in part, the extensors have to generate tension which results in an increase in joint compression forces.

How does it do that?

Your original calculations did not take this into account.

The assumption is the angulation of the lumbar vertebra is equal to the lordosis. This may or may not be the case. If at any time the back loses the lumbar lordosis, then your theory is
:sinking:.

Ideally, it is important to be strong, fast and have good technique. The answer to your question first starts with your definition of good technique. So tell me your definition of good technique, and we can go from there.

Eric, the original question was, “With your engineering background can you mathematically show me how the stress on the lumbar spine of squatting 1000 pounds with good technique is less than that of the head on the neck?”
Your point was the lifter had a straight back, and therefore the extensors did not have to place tension on the back to stop flexion, and therefore there were no muscular joint compression forces, just the weight. Then you compared this to a lumbar spine (not a cervical spine) in a flexed position.
We agree about lumbar lordosis, and I have to commend you for seeing with it. I did not mention this, as I didn’t think you would pick it up, and we would have a discussion on how this can or cannot occur. It is called hooking the back. I am wondering why you didn’t mention the “an angulation of the tops and bottoms of the vertebrae that corresponds with the lordotic curvature” when you first modeled the lumbar spine. Is this the case of the over simplified model? Still, the back is not straight, and the extensors generate tension. Common sense says that squatting 1000 lbs should be almost impossible, and bending your head should be no problem. Depending on some unknown factor (the integrity of the tensegrity system) 1000 lb squats may not be damaging, and bending your head may damage the cervical spine.
Regards,

Stanley

 

Yes, my original equations did not take the firing of the extensors into acount. However, with a back flexed the extensor muscle tension will be greater than with back more straight. So, the calculation numbers may have been wrong but the concept is still valid in explaining why sometimes you get injured lifting your own body weight and other times you can lift a 1000lbs. on your back without injury.

That's ok Stanley, we can use your definition of good technique. You lift more often than I do. (The last time I lifted was in undergrad when I was on the crew team. I'd rather be on the water, pulling on an oar, making pretty boats go fast than be in a musty room with a lot of mirrors.) So, can you lift more weight, with less chance of injury with good technique as opposed to bad technique?

My point went further than just compressive force on the disk. I included an increase in pressure within the disk when there was flexion of the spine. I don't think my explanation is sunk yet.

So, from real life observation that people damage their spine with no load and may not injure themselves when lifting huge loads on their back. How do you explain this with tensegrity?

"Depending on some unknown factor (the integrity of the tensegrity system)"

You could insert any word for tensegrity in the above sentence. The integrity of the "magic" system. The integrity of the "neutral position" system. The giving of a phenomenon a name does not necessarily give an understanding of the phenomenon.

Here's a bit better one. The integrity of the "disk" system. This one is sort of "duh", because we have a painful back and an MRI with stuff squirting out of the disk. Of course there is a loss of integrity in the disk, but why did it happen. How do we use tensegrity to explain the rupture of the disk?

I would wager that, with your definition of lifting with proper technique, the pressure in the disk would be minimized. It may have been found through trial and erorr, but it can be explained using engineering principles.

Regards,

Eric

 

Hi Eric

Agreed with everything you said, interesting that you used the dreaded T word in your description of effects of using crutches (no offence just winding you up a little ).

I think after reading around this in more depth that my concern over the “mechanical approach” is centered on using free body diagrams, calculating moments and making assumptions about compression, tensile and shear forces at specific instances in time to make assumptions about the dynamic equilibrium which exists between all these elements during gait.

For me this has been intuitive concern and the only thing that I had come across with seemed to address this was in the arena of what I could understand of Turveys abstract reasoning based on observations and the ideas postulated by Ingber and others about this equilibrium, to me this seemed to attempt to fill a void. Though I agree that doesn’t make it true or useful. Perhaps this is just a sign of my limited knowledge of what has been written .

The past couple of days I have looked at some of the studies using FE to explore functional complexity in the foot and what I am beginning to see is that there are very sophisticated mechanical models being developed which are starting to unlock the level complexity which for me the tensegrity ideas hint at.

I have cut and paste a couple of abstracts below for papers which I feel do this.

I feel what is needed at this point is to tidy up what we might think "tensegrity" represents and when I get a bit of time will post a separately some definitions which Dave Smith sent me which may be helpful.


cheers

Martin

The St. James Foot Clinic
1749 Portage Ave.
Winnipeg
Manitoba
R3J 0E6
phone [204] 837 FOOT (3668)
fax [204] 774 9918
www.winnipegfootclinic.com






Real-time subject-specific monitoring of internal deformations and stresses in the soft tissues of the foot: A new approach in gait analysis G. Yarnitzkya, Z. Yizharb, A. Gefena,� Department of Biomedical Engineering, Faculty of Engineering, Tel Aviv University, Israel Department of Physical Therapy, Faculty of Medicine, Tel Aviv University, Israel Accepted 18 August 2005

Abstract
No technology is presently available to provide real-time information on internal deformations and stresses in plantar soft tissues of individuals during evaluation of the gait pattern. Because internal deformations and stresses in the plantar pad are critical factors in foot injuries such as diabetic foot ulceration, this severely limits evaluation of patients. To allow such real-time subject-specific analysis, we developed a hierarchal modeling system which integrates a two-dimensional gross structural model of the foot (high- order model) with local finite element (FE) models of the plantar tissue padding the calcaneus and medial metatarsal heads (low- order models). The high-order whole-foot model provides real-time analytical evaluations of the time-dependent plantar fascia tensile forces during the stance phase. These force evaluations are transferred, together with foot–shoe local reaction forces, also measured in real time (under the calcaneus, medial metatarsals and hallux), to the low-order FE models of the plantar pad, where they serve as boundary conditions for analyses of local deformations and stresses in the plantar pad. After careful verification of our custom-made FE solver and of our foot model system with respect to previous literature and against experimental results from a synthetic foot phantom, we conducted human studies in which plantar tissue loading was evaluated in real time during treadmill gait in healthy individuals (N ¼4). We concluded that internal deformations and stresses in the plantar pad during gait cannot be predicted from merely measuring the foot–shoe force reactions. Internal loading of the plantar pad is constituted by a complex interaction between the anatomical structure and mechanical behavior of the foot skeleton and soft tissues, the body characteristics, the gait pattern and footwear. Real-time FE monitoring of internal deformations and stresses in the plantar pad is therefore required to identify elevated deformation/stress exposures toward utilizing it in gait laboratories to protect feet that are susceptible to injury. r 2005 Elsevier Ltd. All rights reserved.

1: J Biomech. 2008 May 23. [Epub ahead of print] Links Finite element analysis of plantar fascia under stretch-The relative contribution of windlass mechanism and Achilles tendon force. Cheng HY, Lin CL, Wang HW, Chou SW. Graduate Institute of Mechanical Engineering, Chang Gung University, 259 Wen-Hua 1st Road, Kwei-Shan, Tao-Yuan 333, Taiwan.

Information on the internal stresses/strains of the plantar fascia under stretch is useful in enhancing knowledge on the stretch mechanisms. Although direct measurement can monitor plantar fascia changes, it is invasive and gathers only localized information. The purpose of this paper was to construct a three-dimensional finite element model of the foot to calculate the stretch effects on plantar fascia and monitor its stress/strain distributions and concentrations. A three-dimensional foot model was developed and contained 26 bones with joint cartilages, 67 ligaments and a fan-like solid plantar fascia modeling. All tissues were idealized as linear elastic, homogeneous and isotropic whilst the plantar fascia was assigned as hyperelastic to represent its nonlinearity. The plantar fascia was monitored for its biomechanical responses under various stretch combinations: three toe dorsiflexion angles (windlass effect: 15 degrees , 30 degrees and 45 degrees ) and five Achilles tendon forces (100, 200, 300, 400 and 500N). Our results indicated that the plantar fascia strain increased as the dorsiflexion angles increased, and this phenomenon was enhanced by increasing Achilles tendon force. A stress concentration was found near the medial calcaneal tubercle, and the fascia stress was higher underneath the first foot ray and gradually decreased as it moved toward the fifth ray. The current model recreated the position of the foot when stretch is placed on the plantar fascia. The results provided a general insight into the mechanical and biomechanical aspects of the influences of windlass mechanism and Achilles tendon force on plantar fascia stress and strain distribution. These findings might have practical implications onto plantar fascia stretch approaches, and provide guidelines to its surgical release

 

Martin:

There is no need for integrating "tensegrity" into an understanding of foot biomechanics especially considering we now have many foot-related papers using finite element modelling. Asics already uses finite element models of the foot with nearly all the ligaments, bones and tendons modelled to help design their shoes.

In addition, you may want to read a great thread on what holds up the longitudinal arch of the foot where we discuss finite element modelling in relation to this question. You will notice that we did not need to mention tensegrity or biotensegrity in our lengthy discussion on this subject in order to answer this question.

You may also want to check out my article on Emerging Concepts in Podiatric Biomechanics for further explanation of Finite Element Modelling.

 

Hi Kevin

I am beginning to think that perhaps this is true, however since I started this sucker I may as well see it through as much as seems reasonable.

I'll check out your suggested reading in between figuring out how to examine the twins and keeping the wolf from the door.

cheers

Martin

 

Gentlemen,

I am not sure this article can be used in any way to further the conversation. I only include it to show that Tensegrity is being evaluated with finite element analysis. http://www.sciencedirect.com/scienc...=5846282&md5=7f8734c62dd3c2dc6b2b5a3ea753188c

Cheers,
Kevin M

 

I fully expected you to give me the modified equations. This time please do it with the cervical spine, as we see this occur in patients that have had a few lumbar surgeries.
Also please explain why most people can flex their neck with no problems, as the simplified engineering model and the tissue stress theory does not distinguish between the people who will be able fully flex their neck with no problems, and those that cannot.

Eric, I hate to tell you that real lifters do not have lots of mirrors in their work out room, and they do not use gloves or spandex. I didn’t know you did crew. To do crew, you had to do some real lifting to be good.
To answer your question, eliminating obvious flaws in technique will help to prevent some injuries. For instance allowing the knees to rotate inwards will strain a piriformis muscle. The relatively weak piriformis muscle will not be as effective in moving the larger weights. On the other hand, as weight increases, there are changes in technique that are required to lift the heavier weights that should, but don’t necessarily cause injuries. Some deviations in technique are necessary for certain individuals. Long femurs will result in a posterior displacement in the center of mass at 90° that necessitates more forward flexion so as not to fall backwards. The back angle may not be ideal, but the lifter will lift no weight without making this compensation. So there is no good definition of proper technique, just some rules (sound familiar?).

During a maximal squat there is a point where there is no flexion to a slight degree of flexion. Contrary to what you think, this allows for more weight to be lifted.
http://drsquat.com/articles/squatcourse.html
Flexion of a spine with 1000lbs should blow out a disc with your predictions. However, it doesn’t seem to occur as often as you would expect

The tensile elements help to prevent the joints from compressing. Failure of the tensile elements allow the joints to bear the full forces (axial weight + moments negated by muscle tension resulting in joint compressive forces). If you compared a Snelson sculpture to a picture of the ligaments of the spine, there is a major similarity.
http://www.intensiondesigns.com/itd-biotensegrity/biotensegrity/resources_levinpapers/paper5.html
The spinal segments are supported in relation to the other spinal segments by the tension elements. Failure of a tension element predisposes the spine to damage.

Don’t wager, unless you know. Part of proper technique is the use of the extensors of the spine. According to engineering principles the back extensors will counteract flexion moments, and the result will be joint compression forces. The increased tension should put the back at risk. However, the opposite occurs. This is a picture of Dave Rigert’s back.
http://beyondstrong.typepad.com/photos/muscle/dorian_yates_193_shirochajs.jpg
Using your engineering principles please explain how strong extensors help to eliminate back problems.

Regards,

Stanley

 

I'm not that familiar with back injuries. What anatomical structure is injured in the above scenario? I can't explain the physics of the situation unless I know the anatomical structure in question.

Stanley, I've always thought you were a real lilfter. Rowers use spandex. It's nice to have tight fitting clothes so that you don't catch a shirt on the sliding seat or your thumb in your shirt. The best rowers were tall and could move the oxygen. There was a guy on my freshman team who was 6' 7" and skinny and the only activity he did before joining the college crew team was a paper route. When the team was tested his VO2 max was 67 and the team average was in the mid 50's. The national team coach was recruiting him as sophmore. He didn't lift all that much.

It sounds like you have good intuitive sense of the mechanics of center of pressure and center of mass. Mechanical analysis seams to work in weight lifting

I don't recall making that prediction. What I said was that there would be increased intervertebral disk pressure when there was flexion of the spine as opposed to when there is even pressure within the disk. My prediction is that weight lifters will be more likely to be injured with forward flexion of the spine when they try and perform squats. Do you want to prove me wrong?

I'm still waiting for a analysis of anatomical structures that are supposed to be in a tensegrity formation. In a tensegrity structure the compression members are held appart by the tension members. Just drawing lines on a vertebral column is not enough. Where is the proof that the there is enough tension in the ligaments to hold the disks apart.

There's a guy who has mirrors in his gym.

Assumption: forward flexion of the spine will cause increased disk pressures. (By placing more force on one side of the disk; causing compression on one side fo the disk and stretching and tension on the other side of the disk. As I recall fluid dynamics, there will be increased pressure on the wall of the disk at the point where it is stretched and has a larger radius of curvature. )

Yes the muscle activity will cause incrased compression of the disks. However, if the disks are compressed symetrically, the disk pressure will be lower than if there is forward flexion with load. The strong extensors keep the disks evenly loaded.

Cheers,

Eric

 

Eric, you were able to explain the lumbar disc, the cervical disc should be the same except the magnitude of the forces.

Sometimes, not always.

I’m sorry, I extrapolated from what you wrote. You have mentioned that the reason for the disc failure on a mechanical basis is two factors: the pressure and the localization of the forces. You gave calculations based on the fact that the spine was straight.
Regarding the pressure, we discussed that the amount of joint compressive forces is based on the moments and the tension required resisting the forces. I showed you that the spine is flexed in relation to the ground in a squat which requires tension to stop the spine from flexing.
Regarding the localization of the forces, you discussed a flexed spine having one tenth the area bearing the weight. I showed you that the spine flexes in some individuals at the sticking point of the squat (actually, it is the pelvis coming forward that causes the joint flexion).
So the question remains unanswered. Why is it that some people can damage their discs in their neck doing next to nothing, and some power lifters can squat 1000lbs with no injuries? It seems that the stress on the tissues with your calculations does not explain it.

Eric, I don't know if it has been done yet. This doesn’t mean that biotensegrity doesn’t exist. I once had a discussion with an Endocrinologist about reverse T-3 and some other recently discovered hormones. He told me that they first figured out that the hormones had to exist, and then they found it. As far as the soft tissue holding the spine in a tensegrity formation, we are discussing whether it would have to exist that way. You are trying to show me that things can be explained with basic engineering principles.

Eric, you really have me on this one? Are you are saying that the extensors will cause even compression on the discs, and hence lower the pressure? So are you saying that the contraction of the extensors causes less pressure in the disc?

Thanks for the good discussion.

Regards,

Stanley

 

I thought that Eric and Stanley would find this of use:

http://linkinghub.elsevier.com/retrieve/pii/S0021929003000150

 

Well one reason that I had trouble explaining it is that people don't put the 1000lb load on their cervical spine. So, what anatomical structure is injured in the cervical spine?

The spine may be flexed in relation to the ground, but the surfaces of the vertebra that articulate with the disk are not flexed relative t each other. So, my model, with little reading in this field, would predict that intervertebral disk pressure would increase with flexoin of the vertebra on each other and the intervetebral disk pressure would increase with increased load on the spine. If the spine were a tensegrity structure, intervertebral pressures would not increase with increased load on the spine. Anyone want to search the literature?

So, would you drop tensegrity as a paradigm if it was shown that intervertebral disk pressure increased with spinal load. If the disks were compression members held apart by tension members as in a tensegrity structure, then there should be only increased tension in the ligaments with increased load.

I'm saying that flexion of the spine vertebra relative to vertebra will cause increased intervetebral disk pressure. When the extensors fire to prevent flexion of the disks there will be less pressure in the disks than if there was an equal vertical load with the spine flexed. Here is where I make an assumption based on real world observations. Take to equal spines, both loaded with 500 lbs of weight, you flex one without the extensor contracting and you keep the other spine straight with the extensor muscles, I would predict the intervertebral pressure will be greater with the flexed spine than with the straight spine with the extensors firing.

It's been over 20 years since I did any squats in a weight room, but I do recall feeling very uncomfortable if there was any foreward flexion of my spine while doing the squats. It felt "safer" to squat with my spine relatively straight. So to make a model that explains this, all you have to do is alter the numbers to make it so that the intervebral disk pressure is greater. In my previous assumptions I made an estimate of area over which the force is spread. You could just reduce that area while the load is kept constant. After all you have to explain the reality. The assumption is that disks rupture at higher pressures as opposed to lower pressures. In tensegrity, do the tensile elements fail at higher tensile forces?

agrreed it is a good discussion.

Regards,
Eric

 

Hi Eric, you must have forgot the original question.
Originally posted in Post 78 and reiterated in Post 94

With your engineering background can you mathematically show me how the stress on the lumbar spine of squatting 1000 pounds with good technique is less than that of the head on the neck?

We see patients that develop disc pathology of the cervical spine, without lifting anything. They usually have a history of a few lumbar laminectomies. Compare this situation to the person who squats 1000 lbs. Something here is not logical, especially since someone should be able to move the head in all positions without having the cervical disc(s) herniated.

With an INTACT tensegrity system, the disc pressure should not increase in a linear progression. There may be some increase in disc pressure, as the discs are acting as shock absorbers, but not in a predicable progression. Show me your articles so we can review them. Again, we are talking about an INTACT tensegrity system. So this means no back injuries, no equinus, and level ASIS and PSIS, and no scoliosis. If this criteria is not in the paper, then the paper is invalid. Do not show me papers where the disc pressures are measured in people with back injuries, as these people have damage to their tensegrity structure and will behave as you predict. If you find this, I would be willing to drop the tensegrity paradigm.

Eric, I find this so interesting. I remember the long discussion we had about the nutcracker analogy, and how important moments are. My point was that if you move the joint one degree, then we can throw out the whole nutcracker analogy, so position is more important than moments. I see you finally see it my way.
The only thing that you forgot about is an exercise called good mornings, which is used to strengthen the back muscles.
Here is 480 pounds for a set of 8 in a seated position.
http://www.youtube.com/watch?v=yTNVfEuoha0&feature=related

And here is some standing good mornings:
http://www.youtube.com/watch?v=dC4g2B-Qfes&feature=related

And here is the best I found:

http://www.youtube.com/watch?v=VbPWMs-iTR0&feature=related

Explain to me why the intervertebral disc pressure is not greater than the value required for damage with the standard engineering model.

Eric, the reason you feel uncomfortable when you are flexing forward with your spine is the center of mass is forward, and you will fall forward. It is very disconcerting to feel like you are going to fall with 300+ pounds on your back.

Regards,

Stanley