I’m interested in the force-length relationship of the AnyBody 3 elements muscle model. I would like to plot the force-length curve of a muscle on my own, based on this relationship and its parameters, such as FcFast, Jt, Jpe, K1, K2 and PEFactor. However, in the tutorials, I dit not find any equation related to these parameters. In the literature, there are equations related to the Hill muscle model, but they don’t use the same nomenclature as you do. Thus, I am a bit confused.

Knowing this equation would help me to show on which part of the force-length curve each muscle work for a specific movement, such for the middle deltoid during scapular abduction.

I’m reposting this old thread, which has not been answered yet (see preceding question). Also I have another question related to the 3-elements model:

I’m using the rep 1.5 (AMS 5.3), and when I put the 3-elements model with the GaitUniMiami model, I obtain overload muscles. However, when I use the rep 1.6.1 (AMS 6.0), I do not obtain muscle overload. So what did you do between these to versions of the UniGaitMiami to get rid of the muscle overload ?

you are pinpointing two interesting topics here, even though they are not really related.

To the first question: the description of the 3-element muscle is rather short, that is true. Unfortunately, we don’t have a simple set of equations that describes the complete model which we could just write here at the moment, but we are trying to update the reference manual until the next release.

To the second question: There have been two changes in the UniMiami model: The weights for the markers have changed from former unique weights to higher weights of the trunk to allow e.g. head markers to have larger differences. Secondly, the overall strength of the muscles has been increased. This is probably why you don’t see the overloaded muscles anymore.

Its sounds logical to increase the strength of all the muscles to avoid overload. How did you increase them ? Is it the following line ?

[i]AnyVar PCSAfactor = 90*…HumanModel.StrengthParameters.StrengthIndexLeg; // = PCSAfactor in N/cm^2 , the PCSA itself is given in cm^2 so F0 is in Newton`s.

//Klein Horsman used a PCSAfactor of 27 N/cm^2 this is now changed to 90 N/cm^2[/i]

If so, where did you found the value of 90 N/cm^2 and does it simulates a young “musculature” ?

When I switch the 27 N/cm^2 to a 90 N/cm^2 in my model, I still get muscle overload. I looked in the muscle parameter file (musPar.any), and there has been a lot of changes in the muscular parameters from version 1.5 to version 1.6.1. There are some new variables (e.g. TLEM_MORPHOLOGY), so this new file would not fit in the version 1.5. In my other post, I’ve been told to keep the gait model of version 1.5, but now I’m not able to make it work in the 3 element model.

I think you found one place where it was changed. Another is in the file HumanModel.any where the strength index of the leg was now set to 1 (before it was 1.5)

The value of 90 N/cm^2 came up different literature source and also some experience, so I am afraid I cannot give you one reference proposing this value.

I am not sure what you did with the GaitUniMiami model so it is difficult to give you a good advise what could be best for you. Lately, we discussed some issues with the MoCapUniMiami model and decided to do some bug fixes for the next service release. So if it is somehow possible for you to switch to the AMMR 1.6 model, I would recommend that. Otherwise you could stay with the 1.5 model for now and see if the next fix will solve your problems.

Ok nice to know. I found that my model used the uniform scaling, which scaled down the strength of some muscles of the shank, so I just put back the standard scaling. Now I have no more muscle overload.

I tried the new version 1.6.1 and it works, but not for all my models. I have one model with which I had to compute contact nodes on the tibia based on the proximity between the femur and the tibia for all the steps of the simulated gait. When I tried it again with the new kinematics, I did not obtain the same contact points. Did you change the location and orientation of the knee joint in the new release ? One thing that did change was the parameter identification in the first study. Now my tibia segment is less downscaled and my femur segment is less upscaled, so the proportions are more preserved.

Also, I’m going towards the end of my post-doc, so I’m still not sure if this model update worth it or not.

Hi,
I came across this old post, since I didn't found any other comment regarding the change in PCSAfactor I will post my question here. In the post of Daniel, he stated that he couldn't give the reference about the number 90N/cm^2. I think this number is a bit on the high range, so I am very curious about any further explanation about this number.

The PCSA factor is a funny one indeed and a whole range of values have been reported in literature. For example, the paper titled “Evidence that maximum muscle stress is not a constant: differences in specific tension in elbow flexors and extensors.” by Thomas Buchanan reports muscle stresses that could justify 90 N/cm2. If you feel this is too strong for your model, you can use define statements to state values for the BM parameters BM_ARM_STRENGTH_INDEX, BM_LEG_STRENGTH_INDEX and BM_TRUNK_STRENGTH_INDEX to a fractional value “f” which will then make the effective stress 90xf N/cm2.