Moment from ID do not equal to the sum of the products of muscle forces from SO and moment arms from MuscleAnalysis tool

[Jack Zhao]

Dear All:
I performed inverse dynamics (ID), static optimization (SO) and MuscleAnalysis using gait2392 model and the same motion and ground reaction force of subject01 sample data, while finding that the moment calculated from ID (in "inverse dynamics.sto" file) do not equal to the sum of the products of muscle forces from SO and corresponding moment arms reported from the MuscleAnalysis tool (all of them correspond to the same joint angle/generalized coordinate). The tested coordinates include the knee_flexion_angle, hip and ankle_flexion_angle.
The error between them is often lager than 5Nm, or 10Nm even when the calculated moment value is only 4Nm (often,e.g. one is 25, the other is 30).
It is known that the SO contains the ID procedure, and the muscle forces are computed and optimized strictly based on the constraints (i.e. muscles' total torque equals to the moment from ID). Why does this large error happen in the sample gait2392 data ?

Thanks and best regards
Jack

[Chaofei Zhang]

Hi Jack,

We have found exactly the same problem. I also do ID and SO with Gait2354.osim model, and read moment arm using API 3.3, find these two moments don't match. Please see the details:
viewtopicPhpbb.php?f=91&t=8496&p=0&start=0&view=

I also have used arm26.osim doing the same simulation, and found the moment from ID matched with sum of the products of muscle force and moment arm.

I think the difference is there are translation degrees of freedom for pelvis in Gait2354 except for rotational DOF. since the pelvis is connected to ground, the force and moment of pelvis maybe the residual moment/force to maintain posture.

I am glad to talk with you if you like, we may be working on the similar project.
My email is zhangf14china@gmail.com.

Chaofei

[Marvin Zedler]

Hi folks,

I am having the exact same issue. Muscle forces (from SO) times muscle moment arms (from Muscle Analysis) do not equal the external joint moments (from ID) nor the muscle moments (from Muscle Analysis). I am performing the analysis on a static (no motion!) squat on two force plates. If I sum the muscle forces (SO) times moment arms about the knee joint for instance (MA) I get zero. For a static trial this would be true if external forces are neglected but this is not the case (I checked by running SO w and w/o external loads).

Have you figured out the problem or is there a misunderstanding on my side?

Thanks in advance,
Marvin