I am currently trying to model the gait of a trans tibial amputee using the GaitUniMiamiTDRightLeg as a basis. I have removed the muscles from the tibia and the motion is done by the thigh muscles and the knee, according to the data from output-euler.txt
What I want next is to create an elastic connection between a segment representing the prosthetic device and a part of the tibia, at a location that can later be modified to investigate other amputees.
So far, I have split the tibia in two parts and they are connected by a rigidized joint, the location of which is determined based on the length of the tibia desired in the model.
Any ideas about adding the elastic element would be greatly appreciated.
The new version 5 has a new feature called force dependent kinematics. This feature allows you to select certain dof in the model where the motion is controlled by the forces. If you have elastic connection between the leg and the prosthetic then motion in this joint could be determined by the force dependent kinematics.
If you are running the model without this feature you will need to drive the motion on this connection.
The elastic connection can be made in many ways the simplest solution would be to create linear and rotational springs in for all six dof between the two segments.
Here is an example of a linear spring
AnyKinLinear Lin ={
Ref=0;
AnySeg &ref1=…Shank;
AnySeg &ref1=…Prosthetic;
};
You get zero forces and moments because the joint deosn’t move. Your ForceLin and ForceRot are function of the position, so because you fixed the joint the linear and rotational measure (Lin and Rot) are zero and so are the forces.
You need some motion here if you want the stiffness to have an effect. At the moment it is the reaction of the joint driver (you set it to “On”) that takes all the load.
Thank you for the explanation, I suspected what you explained. During the walking trial we only used one lateral marker for the whole shank, so the relative motion between the segments is impossible to determine at this time.
I have considered using tendons to take the load between the two segments, but with one tendon, the model cannot find a muscle recruitment solution. Probably using several tendons around the joint would do what I want. However, I am unsure of this approach, so I ask again for a oppinion about this tendon method, or maybe another suggestion for making this work.
I will assume that by tendon you actually mean ligament. Because the tendons are attached to the muscles, and I understand you removed all muscles in that area of the shank.
The ligaments will give the same problem as the joint stiffness: if there is no motion between the two part you wont get forces, because force in the ligament only comes with elongation.
So as long as there is no motion you can as well consider the joint as completly rigid and keep the reaction force. Maybe this is what you want, with the reaction force you will still be able to read the forces and moments acting at this point.
Now if you really want motion in this joint and forces that are the result of the elastic deformation, you have to either record that motion or use the force dependent kinematic that Søren talked about earlier in the post. This last one will calculate the motion in the joint according to external loads and joint stiffness.
So the choice is yours depending on what you want to investigate exactly. Feel free to give more details about it so we can give you better advices.