I am planning on doing a study on lumbar loads during squatting using experimental motion capture and GRF data. I am trying to figure out to what extent I should modify the MoCap full-body model to get the most accurate results and would appreciate some advice. Will exchanging the spinal rhythm with markers on the lumbar vertebrae enhance the modelling or do you think it is a greater "risk" that I introduce some errors by using marker data (the vertebrae might also be quite difficult to palpate and identify)? If I have understood it correctly, having let's say 3 markers on the lumbar spine will not have any effect on the kinematics if I have the spinal rhythm on at the same time?
I am also thinking about things like muscle strength and disc stiffness. The subjects that I will recruit will be experienced in strength training and therefore quite strong. Do you think it is a good idea to change the overall muscle strength index as well as disc stiffness? And if so, how do I know what values might be valid?
Will exchanging the spinal rhythm with markers on the lumbar vertebrae enhance the modelling or do you think it is a greater "risk" that I introduce some errors by using marker data (the vertebrae might also be quite difficult to palpate and identify)?
Yes. I think it will enhance the model. But it is impractical to place enough markers to control all the DOF of the lumbar spine. Luckily there is another solution. You can make the spine-rhythm a soft driver, and just add a few extra markers on the lumbar spine. The model will then mostly follow the markers and the rhythm support the remaining DOFs. This works well if the markers have a much higher weight than the soft drivers for the rhythm.
Remember that the markers errors are in meters while error on the spine rhythm are in radians. So you can not directly compare the magnitude of the marker weights with the weights on the soft rhythm.
I am also thinking about things like muscle strength and disc stiffness. The subjects that I will recruit will be experienced in strength training and therefore quite strong. Do you think it is a good idea to change the overall muscle strength index as well as disc stiffness? And if so, how do I know what values might be valid?
I think you should try with different muscle strength and see if it makes any difference. I don't know if you should also change disc stiffness. I am not an expert in spine modeling, so I would have to ask one of my colleges. @pgalibarov do you have any ideas if disc stiffness would make a difference?
Thanks for your great and clear answer, and excuse me for not replying sooner!
It sounds like I should definitely try to use the soft constraint on the spine rhythm then. Is there a minimum amount of markers that I need to have on the lumbar region to make it work, will one marker be to little? I am asking as in one of the conditions the test subjects will wear a harness where one strap is spanning partly over the lumbar region.
A different question concerning the spine model. Is the model in AMS v 7.2 including the data from the work by Bayoglu et al. 2017 (Twente spine model)?
And a third (quite basic) question. My subjects will perform a barbell squat in one of the conditions, I guess I have to model the barbell itself but I am not quite sure about why I have to do that. As the bar will be placed on the shoulders I thought the load/weight of the barbell will anyway only be picked up by the ground residuals in the thorax, and thus make no difference? I would be happy if you could explain this to me
I just noticed that Morten referred to me about the disc stiffness values. They will not affect the kinematics, only the moments generated by the discs.
I must say I do not know what is the minimum amount of markers, but you want to capture thorax orientation and the curvature of the lumbar spine for best results. I would probably check literature for recommendations.
No, the Twente spine model is not a part of our repository. The model is based on this work.
Regarding the barebell modelling - yes, you are correct. If you have the residual on thorax, then it will absorbed the load applied on the shoulders. The load for the lumbar spine is provided by the GRF measurements. If the residual was on pelvis - you would need to have the barebell.
Regarding the disc stiffness values, I am planning on studying the lumbar compression and shear forces and that's why I asked about the stiffness values. I guess it will have an effect but how can I resonate about it? Should I keep the pre-selected setting or does it make sense to change it, since my subjects are probably more well-trained than the person the model was based on? I would be happy if there are some reading about the topic, explaining the effect in the model, that you could re-direct me to.
Thanks for explaining about the residuals, but I have another follow-up question. You say that I could either have the residuals on thorax or I have them on pelvis. Then, to me it sounds easier to have the residuals on thorax because then I don't have to model the barbell, but is there a drawback or are the two alternatives equally good?
The trained people generally develop stronger ligamentous support for the joints, and on the other side, potentially, have higher risks of rupturing discs due to increased loads (frequently even without knowing it). The disc material properties would probably also undergo some alterations due to the increased loads, but i suspect it is not something that would dramatically change the picture. There are so many variables that you would need to take care of. The model is already highly parameterized - if i were you i would keep the numbers of parameters as low as your research question requires.
Yes, it is indeed easier. There will probably be a difference since applying barebell weight in the model will not be done perfectly realistic - there might be some discrepancies due to the modelling approach. But potentially it could be interesting to try comparing them.
It sounds reasonable as you suggest, I will keep the standard settings. Considering that the focus of my research is on comparing differences between different exercises I think it is not reasonable to put in the tremendous amount of work needed to come as close to reality as possible.
Ok, it would indeed be interesting to compare them. But i guess I will go for the residuals on thorax to a start at least, based on your information
I am playing around with the MoCap model and some new marker data that I have, but I have encountered a few problems. I am trying to add markers to the lumbar segments (in this trial I had markers on the L5 and L1 vertebrae) and after searching some old threads on the forum I was successful in adding a marker on the L5 vertebra. But when I am about to add the L1 marker it seems like the L1Seg has the same reference frame as the L5Seg. This seems a bit strange to me, they should be separate segments, right?
And a second question regarding the markers. I might not be able to place experimental markers on the same lumbar vertebrae for all my subjects due to anatomical variations and use of a harness which has a strap/band covering parts of the lumbar region. Do you think it is still valid to use this method with a soft lumbar rhythm and a few but maybe different markers on various subjects or will the variation be too big and the method too uncertain in that case? For example, if one subject would have markers on L5 and L1, and another subject on L1 and L2.
But when I am about to add the L1 marker it seems like the L1Seg has the same reference frame as the L5Seg. This seems a bit strange to me, they should be separate segments, right?
I think that may be OK. This is cause by the way the model is constructed from the cadaver data. All segments have same common segmental reference frame. Once the joints articulate the coordinates system will deviate of course.
In general the segmental coordinate system can be completely arbitrary. But in many cadaver studies we often see that all bones are measured with respect to common lab reference frame. This then becomes the segmental reference frame. In the TLEM1 legmodel for example has all segments have a common reference frame at the hip joint center.
Do you think it is still valid to use this method with a soft lumbar rhythm and a few but maybe different markers on various subjects or will the variation be too big and the method too uncertain in that case? For example, if one subject would have markers on L5 and L1, and another subject on L1 and L2.
Yes. I think that would be OK. That is the reason we made it easy to enable a soft lumbar rhythm. Only, issue I have seen is that the kinematic solver can be little fragile to the soft rotational drivers in the rhythm. So were large marker errors are usually handled well, the same is not always the case with large errors on rotational drivers. It is not something I have a lot of experience with. So maybe there there is ways to solve should you run into it.
Thank you Morten for the explanations.
I will go ahead and try out the model more as I (hopefully) will be able to collect more data and I will post again if I run in to more problems.
