Model Based Item: JOINT POWER

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Joint Power is a scalar term computed at:

Power = [Mx,My,Mz] . [wx ,wy,wz]
Power = Mx.wx + My.wy + Mz.wz


but it has become common in the biomechanics community to resolve Power into a segment coordinate system, but note that this does not result in Joint Power being a vector. Visual3D inherited many decisions made in the NIH MOVE3D software including this one. It is straightforward for the user to compute the magnitude of the power signal, so we decided to leave the calculation as 3 components. If we had elected to compute Joint Power as a scalar, we would make it very difficult for those users that want it resolved into components.

Joint Power requires a "Joint" to be created because Joint Angular Velocity is the relative angular velocity between the two segments intersected by the Joint.

Trouble Shooting

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, the common reason for not having a shoulder joint identified is that 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.

In model builder mode 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).

Check boxes for -1

You definitely don't want to multiply any of the power terms by minus 1. Power is not a vector, so there is no need to reflect the signals to between right and left to give them anatomical meaning. In fact, if you multiply a term by -1 you have eliminated the anatomical meaning (a negative power indicates an eccentric contraction).

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