Residual Forces and Squat Exercise

[John De Witt]

I have been using OpenSim for about a year now, and have been analyzing squat exercise using motion capture and GRF data. I have been able to run the Inverse Dynamic Tool with no problem. However, I cannot get the Static Optimization Tool or the RRA Tool to solve. I was using Gait2354 model, but have now switched to the most recent 2 legs & HAT model (v2.1) made by Arnold.

I am wondering if part of the problem for the SO or RRA is that because subjects have a bar on their back, there are very large pelvis tx,ty, and tz forces. If this were a bodyweight squat, we would not expect these forces to be large, but since the squat is being performed with a load, these foreces are very high (in the Y direction they are about 350 N, the subject + load wt is actually about 1200 N).

1) Are the residual forces unrealistically high due to the load on the back, or are they worng?
2) Are these high residual loads related to the SO not solving?
3) Should I be using another approach, such as placing an external load on the shoulders?

John

[Rudolf Klemetti]

Hi John,

If you haven't taken the bar into account in modeling, it definitely causes high residuals. And yes, that effects on SO as well. Adding an external force would provide better results, however, it's not an appropriate way to fix the problem since you need more inertia. You want to modify mass, mass center, and moment of inertia of the torso so that the bar is taken into account. Or you could add a new body representing the bar.

Hope this helps,

Rudi