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.

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

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