Reaction of Buckle


i fear, my question is not very much liked and questions to this subject are quite frequent, but this time i have a new question :wink: for which i cannot really find an answer, but i really would like to understand this.

My question refers to the

According to Søren ( i switched off the IAP by lowering the abdominal muscle strength. In the same posting (and in others) it is mentioned that the ReactionForces are some internal quantities that are usually not of interest or should cancel out with the transversus.

But i am not sure what they represent in reality and whether they really should cancel out or should be zero.

In my opinion, there must be some reaction force from the buckle to the vertebrae because rectus abdominis, internal and external oblique pull the buckle to the spine. When the buckle keeps on its position, some horizontal, anterior force must be present, with or without IAP.

Regarding all forces acting at the 5 vertebrae, these disc reaction forces are between nearly 0 and above 20 N (at L4) acting posteriorly. I could not figure out any other force which acts in the opposite direction which could cancel out these reaction forces. (The transversus muscle is nearly inactive, as there is no intra-abdominal IAP pressure. Even when the IAP is acting, these ReactionForces do not zero with the transversus.)

Now my two specific questions:

  1. Am i right about the necessity of reaction forces and there pushing against the vertebrae?

  2. The distribution of these reactions on the vertebrae is rather uneven. While they are only 1 or 2 N at L1, L2, L3, and L5, the force is above 20 N at L4. (For other constellations, the forces at L4 are even higher.) Is this intentionally or a limitation and what is the reason for this?

I use AMMR 1.6.3 with AB 6.03

Thanks for any enlightment,


For illustration i attached two slides from AB.

My first question refers to the upper slide which says: “This reaction force will not carry any load since all forces on the disc lie in the plane of the disc.” I don’t understand this because the force represented by the red arrow pushes the disc in plane and is not compensated at the spherical, according to the figure. Shouldn’t there be a force in opposite direction acting at the Spherical joint?

My second question refers to the second slide. Here all 5 arrows are similar in length. But why are the resultant in plane forces in the model so different for the different disks (if they exist, according to question 1)?

I think, when waiting a few more days i will understand the principle more and more. :o

Are there two components, abdominal pressure and buckle support?

Abdominal pressure is generated by transversus and balances out. Buckle support is generated by oblique and rectus abdominis and does not balance out within each vertebra? The sum of both those forces is acting on each of the MidPoint Nodes of every lumbar vertebra?

The resultant force originating from abdominal pressure is the same for all vertebrae. But why is buckle support force different for the different vertebrae? Is this a bug?

It is not only the antero-posterior component of the Disc?Force acting on the vertebrae but also the Slider?.Slider1Jnt.Constraints.Reaction forces of the Buckle segment which are not similar for the different discs. They are increased for L4 (and L2) compared to L1, L3, and L5 (with and without abdominal pressure).

Any hints, please?

Kind regards,


Hi Thomas,

I am sorry for keeping you waiting on this one, very busy period. We agreed that I would answer this one but I had too many other things going on :wink:

The explanation on why the buckle support is different for the the individual levels must be related to the activation of oblique and rectus muscles.

The buckle is supported by five pushing muscles, and depending on the activation of the oblique and rectus muscles there will be a need for different support on these support muscles.

The recruitment will select the combination of the support that reduces the activation as much as possible, and this might mean that the individual support to the buckle will not be evenly distributed and it may favor one level, if this is the best way to reduce activations.

Hope it helps a little

Best regards

Hi Søren,

thanks for your answer. I interpret your answer in such a way, that i understood the principle of abominal pressure and buckle support correctly. Is this clever?

But is still have two concerns. One is the first question of 04-02-2015.
The second refers to your answer: The Buckle is rigid. Shouldn’t this lead to a monotonic increase/decrease of vertebrae load or to one maximum? From L1 to L5 we have an increase, decrease, increase, decrease of support forces. This makes me wonder.

Thanks a lot and kind regards,


Hi Thomaz,

Here are some more answers, hope it helps.

You are right that the buckle is rigid, and this probably why you see this behaviour in the support forces.

It would have been possible to introduce a linkage of segments with for example revolute joints between to represent a more flexible buckle, this would how we could model this with rigdi bodies. Then we would have needed to drive the motion of these joints to something which would also be an assumption, so we introduced the one seg buckle for mainly that reason i think.

Concerning the other question you had, yes are right, on the figure the reaction in the spherical joint is not displayed and it would act the other way.

Best regards

Hi Søren,

thanks again for your explanations.

I wonder why it is more economic for the model to have those jumps, especially as the buckle is rigid. Probably jumps are convenient for the spinal muscles … (Obviously too complex to understand it easily).

My only remaining concern is, whether in reality there will be such “large” jumps of more than 20 N shear between adjacent vertebrae.

Kind regards,


Hi Thomas,

As you write the activation of the support to the buckle will be made in a way that is most economical for the muscle recruitment.

The buckle is kept in balance by the support muscles and oblique and rectus muscles which together needs to balance the six dof. As you know the model is based on rigid bodies and the current model is a balance of what is practical feasible within this framework, we could have added more complexity but it would also add further assumptions.

Best regards

Hi Søren,

thanks for your explanations.

Kind regards to Ålborg,