The Segment Proximal End Position can be resolved into any segment coordinate system.

For example, to compute the location of the proximal end of the foot relative to the pelvis, set the segment to be the foot and the reference coordinate system to be the pelvis.

To compute the location of the proximal end of the shank relative to the distal end of the thigh and resolve the signal in the thigh coordinate system.

Compute Link Model Based Signals has a model based item property labeled “Seg_Proximal_Joint”. If you select the “Segment”= shank, the “Reference Segment”= thigh, and the “Resolution Coordinate System”= thigh, you will get the answer you are looking for if the origin of the thigh segment coordinate system is at the distal end of the thigh (e.g. adjacent to the proximal end of the shank).

The trick is to create a new thigh segment that satisfies this requirement

A straightforward way to accomplish this is to build a second thigh segment; make it a kinematic only segment because you wouldn't want two thigh segments included in the inverse dynamics calculations. Use the proximal end markers of the original thigh segment as the distal end markers of the new thigh segment. Similarly, use the distal end markers of the original thigh segment as the proximal end markers of the new thigh segment.

At this point you will note that the segment coordinate systems for the two thigh segments are not aligned. In the segment properties tab you can specify the Segment Orientation. If the original thigh segment had A/P Axis=+Y; Distal to Proximal=+Z, the new thigh segment should have A/P Axis=-Y; Distal to Proximal=-Z

Compute Seg_Proximal_Joint using the new thigh segment.