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Table of Contents
JOINT POWER
This model-based item represents the joint power for the specified joint resolved into three separate components.
Properly speaking, Joint Power is a scalar term computed as:
Power = [Mx,My,Mz] . [wx ,wy,wz] Power = Mx.wx + My.wy + Mz.wz
It has become common in the biomechanics community, however, to resolve power into a segment coordinate system and this is what this model-based item achieves.
Note: This does not result in Joint Power being a vector! If you want the scalar item, see JOINT_POWER_SCALAR.
Necessity of a Joint
In order for Visual3D to calculate Joint Power there must be a Joint created because Joint Angular Velocity is the relative angular velocity between the two segments intersected by the Joint.
It is possible for a joint moment to be non-zero and the joint power to contain No Data. This occurs when there is no “joint” identified by Visual3D, and no joint means no joint angular velocity. For example, a shoulder joint is commonly not identified because the distance from the head of the humerus (origin of the upper arm segment) to the distal end of the torso is greater than the distal radius of the torso.
You can verify this in model builder mode, where you should see a yellow sphere at the proximal end of the humerus segment. You may have to check the box in the settings to display the joint.
Preserving Anatomical Meaning
Even though this Joint Power item is resolved into three components it is not a vector! Because of this, there is never a reason to multiply any of the power terms by minus 1.
Because power is not a vector there is no need to reflect the signals to between right and left to give them anatomical meaning. In fact, multiplying a term by -1 eliminates the anatomical meaning since a negative power indicates an eccentric contraction.