Segments must be defined with the origin at the proximal end. Sometimes, biomechanical models use the distal end as the origin for segments (eg, the knee joint center for the femur). For successful VSK generation, you must define all segments using the proximal end as the origin (i.e. the hip joint center for the femur).
You already have the origins for the two femur segments defined: LHJC and RHJC (see Defining the hip model). For the distal end, you need the knee joint centers. These are defined as the half-way point between the lateral and medial knee markers. You can then define the axes that define the femur segments:
- On the Variables tab, with the CGM2.3 scheme selected, add a new variable named LKJC. Define this as a point halfway between LKNE and LKNM.
- Repeat Step 1 for the right side.
- Add a new vector named LFemurPD from point A=LKJC to point B=LHJC.
- Add a new vector named LFemurML from point A=LKNE to point B=LKNM.
- Use these two vectors to define the femur segment, which must be named L_Femur (to match the name of the segment in the VSK file). Add a new segment, name it L_Femur, select Origin A, Z-axis=B, X-Axis=BxC, then A=LHJC (remember, at the proximal end), B=LFemurPD and C=LFemurML.
- Repeat Steps 3-5 for the right side to define R_Femur.
Note that if you define the RFemurML from RKNE to RKNM, this vector points in the opposite direction (left) compared to the left side, which means that the R_Femur will have its Y-axis pointing right and its X-axis pointing backwards. To correct this, either use a factor -1 for the RFemurML in the R_Femur's specification, or flip the RFemurML vector itself.
The following image shows the coordinate system of the R_Femur segment: