knee coordinates in generic model

[Firooz Salami]

Hello,

I wanted to know why in generic model (e.g. 2392 gait model) the knee joint coordinates are 0 0 0 in both parent and child ?

Regards
Firooz

[Josh Baxter]

Knee translations are a prescribed as a function of knee flexion angle. This function basically shifts the knee down 40 cm (the length of the femur bone). There is an underlying reference buried somewhere in the opensim literature.

Code: Select all

<TransformAxis name="translation2">
									<!--Names of the coordinates that serve as the independent variables         of the transform function.-->
									<coordinates>knee_angle_r</coordinates>
									<!--Rotation or translation axis for the transform.-->
									<axis>0 1 0</axis>
									<!--Transform function of the generalized coordinates used to        represent the amount of transformation along a specified axis.-->
									<function>
										<SimmSpline>
											<x> -2.0944 -1.22173 -0.523599 -0.349066 -0.174533 0.159149 2.0944</x>
											<y> -0.4226 -0.4082 -0.399 -0.3976 -0.3966 -0.395264 -0.396</y>
										</SimmSpline>
									</function>
								</TransformAxis>

[Firooz Salami]

Thanks a lot!
So if I want to define a joint between knee and hip joints ( with one rotational degree of freedom ) I need such a spline function or I can directly put the coordinate in parent and child segment ?

Firooz

[Josh Baxter]

Are you interested in putting a joint in between the hip and knee joint? If this effectively turns the femur into proximal and distal segments, then you'd need to (1) reformulate the knee translation function or (2) just model the knee joint as a pin-joint. After first read of your post, I would be careful about changing the model topology in this way... being able to actuate the model with the current muscles might be challenging or impossible.

[Firooz Salami]

Thanks. It is helpful.
As I understood by defining new joint between hip and knee I need to reformulate knee joint too.