Calibrating tendon lengths

Hello all,
I am trying to calibrate tendon lengths to various heights in AnyBody. I had a few questions regarding this procedure.
The first question is related to the “cheap and dirty” method for calibrating tendon lengths as described in the muscle modeling tutorial, lesson 5. The tutorial says that the “cheap and dirty” method will not work if the motion/posture I am analyzing is “outside the typical posture of the joints in the model”. If I am looking at a posture which is outside the typical posture of the model, could I not just pick a posture that IS inside the typical posture, find what the new ligament length is and use those values for the original (untypical) posture?
The second question is related to the more accurate method of calibrating tendon lengths in AnyBody. I am trying to do this for muscles in the shoulder, upper arm, lower arm and wrist. Does anyone know of a reference which might provide either calibrated values of tendon length or a method for determining the postures at which I should calibrate the tendon lengths?
The third question has to do with the calibration section of the “Inverse Dynamics of Muscle Systems” tutorial. In that tutorial, the shoulder and elbow positions used to calibrate the biceps long is -90 and 30, respectively. Can someone explain how the authors of that tutorial chose those values? They said earlier in the tutorial that “ if you are unable to find the information about optimum joint positions you need, then your best choice may be to calibrate the muscle in the joint position where it primarily works”. Can someone tell me some sort of value I could plot for a particular muscle over a range of joint positions which represents how well the muscle is working so that I can determine the joint position at which to calibrate tendon length?
Thanks for any help anyone could provide.
Brandon

Hi Brandon

The first question is related to the “cheap and dirty” method for calibrating tendon lengths as described in the muscle modeling tutorial, lesson 5. The tutorial says that the “cheap and dirty” method will not work if the motion/posture I am analyzing is “outside the typical posture of the joints in the model”.If I am looking at a posture which is outside the typical posture of the model, could I not just pick a posture that IS inside the typical posture, find what the new ligament length is and use those values for the original (untypical) posture?

Yes. You can do that. But you will need to save the result of the calibration study (i.e. the tendon length) and then reload the values for your other study. You can find an example of saving calibration results here:
http://wiki.anyscript.org/index.php/Caching_result_of_muscle_calibration

The second question is related to the more accurate method of calibrating tendon lengths in AnyBody. I am trying to do this for muscles in the shoulder, upper arm, lower arm and wrist.Does anyone know of a reference which might provide either calibrated values of tendon length or a method for determining the postures at which I should calibrate the tendon lengths?

That is a difficult question. The default AnyBody model already has some calibrations studies built in for the arm model here:
‘Main.HumanModel.Calibration’. I am not sure the postures used by the model are good choices in all cases. So it is probably wise to investigate this in more detail. I would suggest you search the scientific papers for sources on which postures the arm muscles attain their optimal fiber length. I can suggest that you start with this review:
http://www.ncbi.nlm.nih.gov/pubmed/27459500

It also has some real insight into other means of calibration. For example a two parameter calibration (optimal fiberlength and tendon slack length) that doesn’t which doesn’t rely on a single posture.

The third question has to do with the calibration section of the “Inverse Dynamics of Muscle Systems” tutorial. In that tutorial, the shoulder and elbow positions used to calibrate the biceps long is -90 and 30, respectively.Can someone explain how the authors of that tutorial chose those values?

Unfortunately not. I have been working myself on validation of the arm models in my PhD, and I quite sure that the values selected by they original author of the model are not the best choices. So you are probably wise in looking into this. If you figure a good solution, please share it.

They said earlier in the tutorial that “ if you are unable to find the information about optimum joint positions you need, then your best choice may be to calibrate the muscle in the joint position where it primarily works”.Can someone tell me some sort of value I could plot for a particular muscle over a range of joint positions which represents how well the muscle is working so that I can determine the joint position at which to calibrate tendon length?

You can try to plot the normalized fiber length (fiber-length/optimal fiber-length) as well the muscles Strength as a function of the joint angle. That should provide you with some insight.

I hope I was able to help a bit. Again, read the paper I linked to above. I think it could help you a bit.

/Morten

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Hello Morten,

Thanks for those suggestions. I will try those and let everyone know if I have any success.

Thanks,
Brandon