T Nation

How Spine Works/Amplification of Compression?


#1

After reading that compressive force on the spine is amplified about 7 times (or more, since the the ratio between the tangential and the normal component of the weight gets lower the closer the spine gets to horizontal; tangential= purely compressive (direction=direction of spine) normal= perpendicular on the tangential component ) , I started wandering how the spine works.

http://www.T-Nation.com/free_online_article/sports_body_training_performance_repair/5_common_technique_mistakes&cr= ) (i.e. squat 1000 N ~ 240 lb = 7000 N compression on the spine)

Can anyone give me a simplified model?
Or is the posted model (although overly simplistic) good enough for educational purposes (i.e. : me understanding how the spine works :slight_smile: )

Thank you, Vlad

As can be seen, I've drawn only the tangential component, in order to make things simpler. And now I see the angle can't be that due to the reason stated in the beginning (changing ratio)


#2

thanks alot guy


#3

You are way too smart to be a powerlifter...i suggest you go hold your breath for 5 minutes and begin to rectify it...you will instantly hit a new squat PR


#4

...so I should be posting in the bodybuilding or body repair forum?
I'm an inquisitive nature (and now I'm doing 5x5 for deads every 4 days)
And I should have a license (slightly above "Bachelor") in structural engineering next fall...

But really, can someone tell me if I got it right?

(Going to a Med Faculty and looking for a bio-mechanics professor is not that easy...plus he might not want to answer)
(and if I'm posting on the T-Nation forum it's because somehow I consider T-mag readers more educated/smarter/having higher IQ and EQ and bigger guns etc etc than readers of...say... BB.com )


#5

And I'm unable to understand why I'm being mocked...the question is not about .0x oz. bananas, or whether I should do concentration curls instead of BB curls, or if I look fab-you-lous and hyooge at 150 etc etc, neither was it a link to a scared-cat-dance deadlift expecting kudos, neither did I ask if there's more stress on the tricipital tendon when doing close grip skull-crushers vs medium grip skull-crushers... etc etc etc


#6

You aren't being mocked. The question you presented is above most on here and is not exactly what people strive for when trying to lift as much as possible. I haven't even attempted anything remotely close to what you are talking about since I was in college - 16 years ago. Sorry, getting this question answered is not impossible, but it is going to take awhile for someone to chime in. That being said, don't be thin skinned about the "other" answers you are getting. :slight_smile:

Even though you didn't ask, the term "et caetera" is abbreviated as etc., not as etc as you have above. Also, it is Latin for "so on and so forth" or "and others." A term that doesn't need to be repeated 3 times, nor does the abbreviation. Once will do. I am pointing this out since we want you to be as well rounded as possible. Being an uber-geek in math and ignorant of things like the term etc. will show a lopsided view of your intelligence and it fails to provide credence to your statements. Just sayin'.


#7

I'd say that given the spine naturally has multiple components with different curvature (different directions and angles) that simplifying a model like that where the spine is modeled on a plane that you'd be missing a lot of the nuance of forces that the spine can handle or that the other smaller musculature is built to alleviate.

Sorry I can't be of more help, I think the guys were just having a bit of fun with you, not at you :slight_smile:


#8

The model is overly simplified. As ozzyaaron pointed out the spine has multiple curvatures, therefore the mathematical model you highlighted would appear simplified. It would be interesting however, if in the article you mention, how the cited articles generated those values. I would suggest pulling those articles from Pub med or some other journal database. These could either be mathematical models or perhaps measurements made from cadaver specimens.

Beef


#9

I'd be happy to help if I could, but i am not really sure what you are asking. do you want to know how the whole spine works in relation to different loads? if so that is way beyond the scope of what anyone would want to answer in a forum. I've taken several upper hours of classes that teaches how the spine works and i still have several more until i feel i'll have a good grasp of it. you can't simplify the spine, its just to complicated.


#10

Do a search for the work of Stuart McGill. He's done a ton of stuff on the spine and back musculature as it relates to safety and load bearing. I'm willing to bet if you get some of his materials many of your curiosities will be answered.


#11

@boldar:

I'm basically asking which is the mechanism by which compression is amplified as the torso gets closer to horizontal.


#12

makes more sense now, but i think you missed something somewhere. Compression force only happens when something is pushing down on the spine(ie compressing). compression happens in squats overhead presses and the likes. unless you have another horizontal force acting on the spine when its horizontal then you will not get compression force. i believe the force that you would get in ur picture would be shearing force(its early i could be wrong). I think Eric was pointing out that compression force increases with load(the weekend warrior vs. highly trained Olifter). I read that article awhile ago and only skimmed it this time but imo i think he saying to stay in the compression bc other forces are a lot harder on your spine. If i got that wrong anyone can correct me. hope this helps somewhat.


#13

I would concur about reading Dr. Stuart McGill, all schools of "back/spine specialists" quote his work at one point or another.

I dropped out of engineering school before I failed out. I worked for over 15 years as an athletic trainer and during that time I took Robin McKenzie's courses parts A-C for diagnosis and treatment of mechanical spine pain. It is not so much about the biomechanics, enough to understand the pathophysiology, but evaluation and treatment.

The spine is very complicated and there are many schools with many theories/opinions on how back pain originates and should be treated. Prevention is pretty straight forward.

When I think of compressive forces I think of two things. There is the "helicopter crash" or "football spear tackle" where a compressive force is transmitted through the spine causing vertebral fractures and then there is vertebral flexion, bending and rounding of the back, that causes compressive forces on the discs. This maybe the most common mechanism of injury resulting in back pain.

As you bend forward, as in a deadlift, each vertebral segment tends to hinge on one another and compresses the discs. If you think of the disc like a jelly donut, the compression squeezes the jelly to the back. This is where people get bulges or herniations. This is why it is important to keep your back rigid or arched, to reduce those compressive forces. If you keep you back arched, you can be almost parallel with the floor and minimize the compression of the discs. A person with really shitty form can round/flex a lot, increase disc compression, and have a short flexion moment arm.


#14

@boldar:

Compression can appear in a truss, for example, despite no compressive forces. 17000 N of compression equal a 1.73 ton weight (3820 lb) on your back, and you sitting perfectly straight for it to be only compressive, and not cause shear etc..

Yes, it does create shear (altough not in "my model"- overly simplistic and maybe completely wrong, but the only I could think of that could so amplify the load).

@danjo228:
Well, if the spine is "perfectly straight" (as in "normal curvature), wouldn't the compression actually force the nucleus in every direction?
(although since the vertebrae aren't perfectly horizontal, the nucleus will be squeezed more in one direction).
But the "if you flex it is squeezed backward/forward" is cool. :slight_smile: Since the disk is non-contractile, simply bending should apply a lot of stress to it (Hooke's law), in order to strengthen the connective tissue (make it adapt),right? (applying the SAID principle, allowing recovery etc. -basically, considering ideal conditions are given).
There are doctors not agreeing with dr. McGill, and Nick Tuminello also noticed that a dead pig spine* flexed to tissue failure is NOT a the spine of a living human, which also regenerates and adapts. BTW, wouldn't this -flexing-extending a dead spine- be equal to having a dead biceps, and loading while passing an electric current through it to simulate concentric and eccentric action, and after many cycles say that exercise will lead to a biceps tear?)

*"First off, upright flexion is not normal pig function. The only time a pig stands upright (on two legs) is when itâ??s trying to hump another pig.

Secondly, pigs donâ??t have any functional movement similarities to humans what so ever. This goes back to what Comerford said above."

Thanks, Vlad

If I got anything wrong, please oh please correct me. No, I'm not joking.


#15

The studies:

Very complicated models were used...but they were used to calculate the force.

I want to know if the overly simplified model is a good ---basic--- explanation of why the force is amplified (logically, NOT for calculating the compressive force). (consider it an intellectual exercise for me and whomsoever would like to answer why "yes" or why "no")

Strength to you, Vlad


#16

The studies:

http://journals.lww.com/acsm-msse/Abstract/1985/10000/Lumbar_spine_loading_during_half_squat_exercises.16.aspx

http://journals.lww.com/acsm-msse/Abstract/1991/10000/Lumbar_spine_loads_during_the_lifting_of_extremely.12.aspx

The model used for calculating the loads was...very complicated.

I want to know if a truss model of the spine (despite being very simplified) gives a basic explanation of how why the force is amplified (at least during tasks where the spine is held ~rigid)

Mental gymnastics... :smiley:

Also, a very cool one: 3000 N of compression...with only 18 lb!!!
http://journals.lww.com/spinejournal/Abstract/1994/10000/Transfer_of_Loads_Between_Lumbar_Tissues_During.13.aspx

Strength to you, Vlad


#17

And, here's a very cool article from Nick Tuminello (not exactly related to my question, but I'm posting so that you like this thread and answer my questions):

http://nicktumminello.com/2010/10/disc-pressure-measurements-should-it-affect-exercise-selection/

(well, probably posting a pic of a 99% naked beautiful woman would've work just as well or better...sigh...)
(I am a little biased on what's cool...since I was grown into believing my spine is way to weak for me to lift anything, and some bad technique, a slight scoliosis and kyphosis-slight, but people not-in-the-know managed to convince me my spine is f****d...and truth be told it still hurts a little every once in a while...Long live Dan John and his suggestion of a few weeks lay-off every year, and every trainer who instructs lifters to take a back off week every 4-8 weeks)


#18

Intervertebral discs do not adapt to stress very well. The annulus fibrosis is cartilage and is poorly vascularized so it will not adapt as well as other tissue to certain amounts of loading.

Robin McKenzie also theorizes that improper loading of the spine can cause mechanical derangement of the disc, further compromising its ability to adapt.


#19

For a short answer your simplified model explains the increased compressive load. I did a similar exercise in an engineering biomechanics class exploring the effect of trunk angle on the loads in the spine. The only additional force added was the resultant force from intra-abdominal pressure. The exercise also showed that increased intra-abdominal pressure reduced compressive loads on the spine, (I'm sure you could verify this result with a simple model). Also I recall all the forces were assumed to be acting parallel to each other causing a resultant couple on the trunk.

Have fun. Pretty much analysis at any joint can be performed by making a "cut FBD" representing joint forces by a shear and a compressive force (tensile force would lead to dislocation) and each muscle by a tensile forces acting in the line of action of the muscle.

edit: one change I just realized you should make in your model is using an angle theta, the angle between the vertebral column and the horizontal, if you want to see the effects of trunk angle on compressive spine loading.


#20

Finally.

Thank you, Pvoosen.(of course: the greater the lean, the greater the component of the force which causes shear and momentum, and also the greater the lean the greater the "lever arm")

Does IAP lessen compression by creating a greater "moment of inertia"? (spine + compressed abdominal cavity take the compression, instead of spine alone?) (IAP "pushes" and creates a momentum which opposes the momentum created by the weight)