Weight scaling with forceplates

Hey everybody,

I do have a problem concerning weight scaling in a MoCap model with forceplates.
Im using the "Simple MOCAP-driven Gait Model" from the AMMR and try to scale it for a child of 10 years. Therefor a rough weight and height scaling is applied before Parameter Identification.
I compared the hip joint forces to the standard model without custom c3d data and custom scaling and they were roughly the same, even though the child only weighs half as much.
To validate that the LENGTHMASSFAT scaling law I applied is active and therefor applying the given body weights to the model, I doubled and halfed the weight of the standard AMMR model (without any custom c3d data or size scaling) and looked at the hip joint forces. The results were not even close to what I expected.
When i doubled the weight of the model from 66kg to 132kg, the hip net force reduced for the occuring peakforces, and increased, when reducing the weight to 33kg. (shown in the attached diagram)
This seems very conuterintuitive to me. Even though the data from the force plates stays the same when the weight is changed, you might expect higher forces, caused by the stabilizing of the ligamentous apparatus. I could also imagine that because the force transmitted to the ground is always the same for the software (defined by the data of the forceplates) it has to distribute the force occurrence weirdly withhin the model.
My question is now, do these reaction to weight scaling seems reasonable to you, or might there be a problem with my model?

Best Regards,

Hi Leo,

I think the reason why you see these forces being reduced for a higher weight is primarily caused by better muscle moment arms. As you write the applied loads stays the same, the length mass fat scaling law will increase the bone thickness and thus also the moment arms, a better moment rm will reduce the joint reaction force.

Concerning the comparison with the standard model, note that this is also a relatively small subject around 60 kg as far as i recall, if you calculate the number in %BW how are you then within reasonable range?

Please also check direction of gravity is correct.

Best regards

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