---
title: "Plug-in Gait lower body forces and moments"
canonical: "https://help.vicon.com/space/Nexus215/11376207/Plug-in%20Gait%20lower%20body%20forces%20and%20moments"
format: markdown
---
The forces calculated by Plug-in Gait and displayed by Vicon Polygon are in the local co-ordinate frame of the distal segment in the hierarchical kinetic chain.

This means that the Ankle joint forces are recorded in the Foot segment axis system. Therefore:

- Ground Reaction force Z will look similar to Ankle Force X
- Ground Reaction Force Y will look similar to Ankle Force Z
- Ground Reaction Force X will look similar to Ankle Force Y

For the tibia this changes, as the axis orientation now changes:

- Z force is therefore compression or tension at the joint
- Y force is mediolateral forces at the joint
- X force is anteroposterior forces at the joint

The positive force acts in the positive direction of the axis in the distal segment on which it acts. A negative force acts in the negative direction along the axis.

In Plug-in Gait, we use an external moment and force description. That means that:

- For the Z axis, a negative force is compression and a positive force, tension
- For the Y axis, a positive force for the right side is medial and negative lateral
- For the X axis, a positive force is anterior and negative posterior

> Macro (scroll-pagebreak)

The following table lists all the lower body forces and moments with their positive and negative descriptors.

> Macro (scroll-tablelayout)

| **Description** | **Segment axes** | **Force or moment component** | **+ve descriptor** | **-ve descriptor** |
| --- | --- | --- | --- | --- |
| Ankle Force X | Foot | Compression/Tension | Tension | Compression |
| RAnkle Force Y | Right Foot | Medial/Lateral | Medial | Lateral |
| Ankle Force Z | Foot | Anterior/Posterior | Anterior | Posterior |
| Knee Force X | Tibia | Anterior/Posterior | Anterior | Posterior |
| RKnee Force Y | Right Tibia | Medial/Lateral | Medial | Lateral |
| LKnee Force Y | Left Tibia | Medial/Lateral | Lateral | Medial |
| Knee Force Z | Tibia | Tension/Compression | Tension | Compression |
| Hip Force X | Thigh | Anterior/Posterior | Anterior | Posterior |
| RHip Force Y | Right Thigh | Medial/Lateral | Medial | Lateral |
| LHip Force Y | Left Thigh | Medial/Lateral | Lateral | Medial |
| Hip Force Z | Thigh | Tension/Compression | Tension | Compression |
| Ankle Moment X | Foot | Dorsi/Plantar flexion | Dorsiflexion | Plantar flexion |
| Ankle Moment Y | Foot | Abduction/Adduction | Adduction | Abduction |
| Ankle Moment Z | Foot | Rotation | Internal | External |
| Knee Moment X | Tibia | Flexion/Extension | Flexion | Extension |
| Knee Moment Y | Tibia | Varus/Valgus | Varus | Valgus |
| Knee Moment Z | Tibia | Rotation | Internal | External |
| Hip Moment X | Thigh | Flexion/Extension | Flexion | Extension |
| Hip Moment Y | Thigh | Abduction/Adduction | Adduction | Abduction |
| Hip Moment Z | Thigh | Rotation | Internal | External |