Step 5 - Define reactions

Thus far, we have defined connections between the force plate and feet, and established the inertial properties for the other segments that comprise the lower limbs. We can use this information to calculate the kinetics at other joints in the body. We do this by calculating reactions.

A reaction is a force and a moment relative to a point of application. The concept of a reaction was first introduced in the modeling software Vicon BodyBuilder. In its user guide, it says that reactions are used to describe the kinetic interactions between segments, and segments and force plates.

Remember that we set up a reaction earlier to describe the interaction of the left foot with the force plate (LeftFootReaction). We now need to calculate the reactions further up the kinetic chain – to the ankle, knee, hip, etc.

To calculate the reaction at the ankle:

1. Create a new variable called LeftAnkleReaction.
2. For the function type, specify Kinetics, and for the function select Reaction at point A from reaction B applied to kinetic segment C.
3. For the Input Variable Types make sure that A is set to Segment, B is set to Reaction, and C is set to KinematicSegment.

4. For Input Variables, select LTI for A, LeftFootReaction for B, and LeftFootKinetic for C

   

5. Copy and Mirror for the right ankle.

This calculation takes the input reaction (LeftFootReaction), subtracts the weight of the foot segment, and calculates any forces and moments at the ankle contributing to the acceleration (linear and rotational) of the foot segment, which are then used to adjust to the new point (LTI, which is the distal end of the tibia segment as calculated by Plug-In Gait). The result is the ankle reaction – the forces and moments caused by the ground reaction force (GRF) and the foot segment's inertial properties at the ankle joint.