Torso mass properties of full-body models

mrrezaie commented 1 year ago

Hi, I think there is a bug in mass properties of torso in full-body models, e.g., Hamner and Rajagopal. The torso and upper limb of Rajagopal model were derived from the model developed by Hamner.

If I'm not mistaken, the gait2392-simbody model is a combination of Delp1990 (pelvis + lower limb) and Anderson1999 (trunk) who modelled the head, arms, and torso (HAT) as a single rigid body. So, the mass properties of gait2392 torso belong to / are based on Anderson1999 HAT.

gait2392-simbody model:

<Body name="torso">
        <mass>34.2366</mass>
        <mass_center>-0.03 0.32 0</mass_center>
        <inertia>1.4745 0.7555 1.4314 0 0 0</inertia>

Unlike gait2392_simbody, the upper limb of the Rajagopal model was modelled separately and not included in the torso. But, only the masses of their torso are different and both models have identical torso mass_center and inertia which doesn't seem to be correct.

Rajagopal model:

<Body name="torso">
        <mass>26.8266</mass>
        <mass_center>-0.03 0.32 0</mass_center>
        <inertia>1.4745 0.7555 1.4314 0 0 0</inertia>

So, I assume only mass properties of gait2392_simbody model and Rajagopal model without upper limb are valid, i.e., mass properties of torso in Rajagopal model with upper limb requires adjustments. I'm looking forward to seeing your comments about this.

Also, in case of lower limb models such as Delp1990 and Arnold2010, i.e., pelvis + lower limb, I'm interested to know how they scaled the bodies' mass. My solution was to use a ratio of [pelvis + lower limb] mass to total body mass (0.464 based on Dempster's regression equations as mentioned in Winter's book) in Scale tool, i.e., 0.464 * total mass. But this ratio in gait2392 model is 0.545. Please let me know how they handled that in their studies.

Thank you.

Ref:

Delp, S. L., Loan, J. P., Hoy, M. G., Zajac, F. E., Topp, E. L., & Rosen, J. M. (1990). An interactive graphics-based model of the lower extremity to study orthopaedic surgical procedures. IEEE Transactions on Biomedical engineering, 37(8), 757-767.
Anderson, F. C., & Pandy, M. G. (1999). A dynamic optimization solution for vertical jumping in three dimensions. Computer methods in biomechanics and biomedical engineering, 2(3), 201-231.
Arnold, E. M., Ward, S. R., Lieber, R. L., & Delp, S. L. (2010). A model of the lower limb for analysis of human movement. Annals of biomedical engineering, 38, 269-279.

carmichaelong commented 1 year ago

Is there anywhere in the documentation this could be useful to have so that other users are aware of this at least?

mrrezaie commented 1 year ago

Hi, please see Torso in table 2: https://simtk-confluence.stanford.edu:8443/display/OpenSim/Lower+Limb+Model+2010

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Torso mass properties of full-body models #3505