The Forces calculated by Plug-in Gait and displayed by 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 and Ground Reaction Force X will look similar to Ankle Force Y.

For the tibia this will change as the axis orientation now changes. Z force is therefore compression or tension at the joint, Y force is mediolateral forces at the joint while 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 and 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 a negative force is compression and a positive force, tension, for the Z axis. A positive force for the right side is medial and negative lateral for the Y axis and a positive force is anterior and negative posterior for the X axis.