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For calibration, the subject variables of interest: Kinematics (Joint Angles) and Kinetic (Forces, Moments and Powers), must be calculated from a joint center point and have a joint axis line to be accurate.
Whereas earlier versions of Vicon Nexus offered only static joint calibration, Nexus 2 provides functional joint calibration, which estimates the center and axis of a joint using joint movement collected in a Range of Motion (ROM) trial.
Because functional calibration requires you to be able to decide whether, for a particular ROM trial, you have collected the minimum amount of angular movement (per joint) needed to obtain a good calibration, it is a good idea to set up a joint range monitor to give you the instant visual feedback that enables you to make this decision quickly (for information on setting up a joint range monitor, see View real-time subject calibration feedback with monitors).
To save time and effort, the new biomechanics workflow enables you to include functional calibration and joint range monitors in your workflow steps, so that you can set up the required procedures once and then reuse them as required. For information on setting up a biomechanics workflow, see Create a biomechanics workflow.
For more information about the distinction between static and functional joint calibration, see the following definitions:
❙ What is static joint calibration?
❙ What is functional joint calibration?
Static calibration uses a single frame of data. The subject is captured in a single pose (body position). Regression equations are used to estimate / calibrate:
❙ The location of a Virtual Joint Center point (relative to a segment)
❙ A line defining the axis of rotation for the Joint
Vicon's Plug-In Gait model uses static joint calibration.
Functional joint calibration uses multiple frames of data where the joint of interest is moving (dynamic).
Using Range Of Motion (ROM) data from a joint enables functional joint calibration to better estimate the true center and axis of a joint.
The process of eliminating errors and finding the best solution for joint centers and axes is sometimes referred to as optimization.
Local optimization takes information from one joint at a time and tries to find the best solution only for that joint.
SCoRE and SARA are local optimization techniques. They are a set of algorithms produced by researchers at the University of Berlin – Julius Wollf Institute for Biomechanics and Musculoskeletal Regeneration.
See also:
