Plug-in Gait lower body forces and moments

The forces calculated by Plug-in Gait and displayed by Vicon Nexus Insight or Polygon are in the local coordinate 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

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