RRA adjustments to the centre of mass of the torso

[Rodrigo Mateus]

Hi!

Firstly, I'd like to apologise for the incessant posting in this forum, however I have a lot of doubts regarding the theory behind the analysis of this software.

While performing RRA, I notice that in certain cases, the adjustments for the location of the centre of mass stay the same throughout several passages of RRA. In addition to this, the respective mass adjustments to the bodies of the model do not converge, and the average residuals follow suit.

After the first passage:
************************************************************
* Summary of Mass Adjustments to Reduce Residuals *
************************************************************
* Body adjusted: torso
* Mass Center (COM) adjustment: dx =0.1, dz =-0.00149315
* New COM location: ~[-0.13177,0.33888,0.00149315]
************************************************************
* Recommended mass adjustments:
* Total mass change: -2.17196
* ground: orig mass = 0, new mass = 0
* pelvis: orig mass = 9.87102, new mass = 9.53071
* femur_r: orig mass = 7.79607, new mass = 7.52729
* tibia_r: orig mass = 3.10748, new mass = 3.00035
* talus_r: orig mass = 0.0838161, new mass = 0.0809264
* calcn_r: orig mass = 1.0477, new mass = 1.01158
* toes_r: orig mass = 0.181546, new mass = 0.175287
* femur_l: orig mass = 7.79607, new mass = 7.52729
* tibia_l: orig mass = 3.10748, new mass = 3.00035
* talus_l: orig mass = 0.0838161, new mass = 0.0809264
* calcn_l: orig mass = 1.0477, new mass = 1.01158
* toes_l: orig mass = 0.181546, new mass = 0.175287
* torso: orig mass = 28.6958, new mass = 27.7065
************************************************************
* Note: Edit the model to make recommended adjustments to *
* mass properties. *
************************************************************
* Average residuals before adjusting torso COM:
* FX=29.1115 FY=21.2997 FZ=25.162
* MX=0.420188 MY=0.117644 MZ=71.7149
************************************************************
* Average residuals after adjusting torso COM:
* FX=29.2005 FY=-5.0112 FZ=33.2878
* MX=-8.11268 MY=-8.95822 MZ=52.2856
************************************************************

************************************************************
* Final Average Residuals *
************************************************************
* After torso COM and Kinematics adjustments:
* FX=15.1838 FY=-20.4208 FZ=9.14009
* MX=-9.35382 MY=-5.64544 MZ=53.1986
************************************************************
After the eighth passage:

************************************************************
* Summary of Mass Adjustments to Reduce Residuals *
************************************************************
* Body adjusted: torso
* Mass Center (COM) adjustment: dx =0.1, dz =-0.0620884
* New COM location: ~[-0.83177,0.33888,0.115013]
************************************************************
* Recommended mass adjustments:
* Total mass change: 3.95978
* ground: orig mass = 0, new mass = 0
* pelvis: orig mass = 12.4879, new mass = 13.1083
* femur_r: orig mass = 9.86283, new mass = 10.3528
* tibia_r: orig mass = 3.93128, new mass = 4.1266
* talus_r: orig mass = 0.106035, new mass = 0.111303
* calcn_r: orig mass = 1.32545, new mass = 1.3913
* toes_r: orig mass = 0.229673, new mass = 0.241084
* femur_l: orig mass = 9.86283, new mass = 10.3528
* tibia_l: orig mass = 3.93128, new mass = 4.1266
* talus_l: orig mass = 0.106035, new mass = 0.111303
* calcn_l: orig mass = 1.32545, new mass = 1.3913
* toes_l: orig mass = 0.229673, new mass = 0.241084
* torso: orig mass = 36.3031, new mass = 38.1067
************************************************************
* Note: Edit the model to make recommended adjustments to *
* mass properties. *
************************************************************
* Average residuals before adjusting torso COM:
* FX=-75.7758 FY=-38.8322 FZ=111.837
* MX=22.1042 MY=-43.8537 MZ=93.8738
************************************************************
* Average residuals after adjusting torso COM:
* FX=-71.2481 FY=-72.264 FZ=122.044
* MX=30.0162 MY=-58.5206 MZ=111.474
************************************************************

************************************************************
* Final Average Residuals *
************************************************************
* After torso COM and Kinematics adjustments:
* FX=-16.3816 FY=-116.696 FZ=41.9557
* MX=14.182 MY=-65.4353 MZ=114.528
************************************************************

I searched for topics where this was adressed and i found a post which put forth some recomendations:

"Here are some key considerations I have picked up from other researchers' advice when using the RRA tool.

a) Which model are you using? Does it include the trunk segment?
b) Are the ground reaction forces in the correct coordinate system? Are they applied to the correct body? Is the COP in the correct location?"

I am still trying to wrap my head around why this happens, but still have not come to any particular conclusion. Any thoughts or possible solutions?

On a different, is there any documentation available which explains in detail the theoretical background of RRA, CMC and so on?

Thank you in advance.

Rodrigo Mateus

[jimmy d]

There is a theory page on the OpenSim Wiki; https://simtk-confluence.stanford.edu/d ... +RRA+Works

At the end of the simulation, the average value for each residual actuator is computed. The average values for Mx (the left-right residual torque) and Mz
(the fore-aft residual torque) are used to adjust the torso mass center to correct excessive "leaning" of the model due to inaccuracies in the mass distribution and geometry of the torso in the model. A new model file containing the adjusted torso mass center (specified in the setup file under the <output_model_file> property) is created.

[Rodrigo Mateus]

Thank you it was very helpful!