# Crutch Simulation Inverse Dynamics Trouble

Hello,

I was trying to simulate a person using crutches. Since I had no marker data I used video references and translated the motions to functions that I could use in Anybody (“DriverJoints.any”). For the crutches, I used simple segments with nodes (“Segment.any”). I inserted drivers so the body model would keep attached to the crutches (“HandFix.any” and “ShoulderFix.any”). The kinematics run fine but when I try to do Inverse Dynamics I get a whole lot of problems with muscle overload. Any help is appreciated. Thanks.

Hi

Please describe what is in the model in some detail.

Is it correctly understood that you are trying to make a model using normal gait data and use it on a person using crutches?

If so, there are several issues with that e.g. motion is different … the forces applied on the crutches needs to be “invented/predicted” and applied, how did you do this?

Best regards
Søren

Thanks for the reply, sorry I took so long to respond. I wasn’t trying to use normal gait data, I watched a video of a person moving with crutches and “replicated” that in Anybody with drivers. For example, I measured the angle between the hip and the right leg in different frames and put that in a driver as a polynomial function. I do not have the reaction forces on the crutch, I just put reaction.type on on the drivers because I was under the impression that anybody would give me those forces. That’s not correct, is it? For a correct analysis would I need to input those forces?

Hi

Yes anybody can be used to predict the force at the ground but only if you have the full body motion described in a good way, otherwise the force predictions will be wrong. The force predictions works by finding the forces at the interface between human and environment which can balance the model. If the motion/dynamics is wrong this will be seen instantly as unrealistic forces at ground because the accelerations impose by the motion may be difficult to balance.

So to be able to predict forces you need reliable full body motion, otherwise i think it will be difficult.

What you could do maybe would be to make a static model is taking different postures this would be much easier than dealing with the dynamics when this is not measured. To do that i would use the standing model in the repository and add the crutch and reactions forces in the tip and to the hand/elbow .

Best regards
Søren

I’ll do that then, thanks for your help!