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This information is for Vicon Nexus 2.9. For up-to-date help, see the latest Nexus documentation.

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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.

DescriptionSegment axesForce or moment component+ve descriptor-ve descriptor
Ankle Force XFootCompression/TensionTensionCompression
RAnkle Force YRight FootMedial/LateralMedialLateral
Ankle Force ZFootAnterior/PosteriorAnteriorPosterior
Knee Force XTibiaAnterior/PosteriorAnteriorPosterior
RKnee Force YRight TibiaMedial/LateralMedialLateral
LKnee Force YLeft TibiaMedial/LateralLateralMedial
Knee Force ZTibiaTension/CompressionTensionCompression
Hip Force XThighAnterior/PosteriorAnteriorPosterior
RHip Force YRight ThighMedial/LateralMedialLateral
LHip Force YLeft ThighMedial/LateralLateralMedial
Hip Force ZThighTension/CompressionTensionCompression
Ankle Moment XFootDorsi/Plantar flexionDorsiflexionPlantar flexion
Ankle Moment YFootAbduction/AdductionAdductionAbduction
Ankle Moment ZFootRotationInternalExternal
Knee Moment XTibiaFlexion/ExtensionFlexionExtension
Knee Moment YTibiaVarus/ValgusVarusValgus
Knee Moment ZTibiaRotationInternalExternal
Hip Moment XThighFlexion/ExtensionFlexionExtension
Hip Moment YThighAbduction/AdductionAdductionAbduction
Hip Moment ZThighRotationInternalExternal