Plug-in Gait lower body forces and moments

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

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