---
title: "Defining the femur segments"
canonical: "https://help.vicon.com/space/ProCalc16/12648494/Defining%20the%20femur%20segments"
format: markdown
---
> ⚠️ **Important**  
> ⚠️ 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](https://vicon-help.atlassian.net/wiki/spaces/ProCalc16/pages/12650396)). 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:

1. 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**.
2. Repeat Step 1 for the right side.
3. Add a new vector named **LFemurPD** from point A=**LKJC** to point B=**LHJC.**
4. Add a new vector named **LFemurML** from point A=**LKNE** to point B=**LKNM.**
5. 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**.
6. 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:

![image](media://98c7a174-f8d1-4c0e-b880-2e8972e0a12a)