The Plug-in Gait Dynamic pipeline consists of the following components: all individual pipeline operations:
This guide covers only the modeling stage of the process. You can perform modeling on the real marker data independently from the filtering and event detection processes by selecting the appropriate check box in the pipeline.
Plug-in Gait outputs virtual markers that are used for several purposes:
Internally, the modeling stage consists of four interdependent models:
The kinematic models are responsible for the definitions of the rigid body segments, and the calculations of joint angles between these segments.
The kinetic models then apply masses and moments of inertia to the segments, and enable the "reactions" that occur on the segments to be calculated.
To run the models, you must supply the required subject measurements. When you have done this, the stages of the Plug-in Gait modeling process are:
In general, the three axis directions are defined using two directions derived from the marker data.
The outputs that are required from the modeling are then calculated, based on the frame-by-frame positions of the segments.
The kinematic models are run slightly differently for the static trials, to calculate certain static 'calibration' angles that are required for the dynamic modeling. These differences are noted in the descriptions of the models, otherwise it should be assumed that the model is calculated in the same way for both trial types.
When the static modeling is being performed, calculated subject measurements are output to the subject measurements file. This is not done for the dynamic trial, even if new values are calculated internally to enable the model to be run.
To define joint centers, the chord function is used extensively in the Plug-in Gait models. Three points are used to define a plane. One of these points is assumed to be a previously calculated joint center, and a second is assumed to be a real marker, at some known, perpendicular distance (the joint center offset) from the required joint center.
(It's called a chord because by definition, the three points (two joint centers and the joint marker) lie on the periphery of a circle.)
A modified version of the function calculates the required joint center position when the plane definition marker is rotated out of this plane by a known angle round the proposed joint center axis. For an illustration of this, see Dynamic knee joint center calculation.