====== Joint Power Scalar ======

This model-based item represents the joint power for the specified joint as a proper scalar value.

Joint Power is a scalar term computed at:
<code>
Power = [Mx,My,Mz] . [wx ,wy,wz]
Power = Mx.wx + My.wy + Mz.wz
</code>

==== Necessity of a Joint ====

In order for Visual3D to calculate Joint Power there must be a [[visual3d:documentation:kinematics_and_kinetics:joint|Joint]] created because [[visual3d:documentation:pipeline:model_based_data_commands:joint_velocity_and_joint_angular_velocity|Joint Angular Velocity]] is the relative angular velocity between the two segments intersected by the Joint.

It is possible for a [[visual3d:documentation:pipeline:model_based_data_commands:joint_moment|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.

==== Decomposition into Components ====

Although power is properly a scalar term, it has become common in the biomechanics community to resolve power into a segment coordinate system. This can be achieved directly by using the [[visual3d:documentation:pipeline:model_based_data_commands:joint_power|JOINT_POWER]] model-based item.

==== See Also ====

There is also a model-based item that decomposes joint power into a given coordinate system ([[visual3d:documentation:pipeline:model_based_data_commands:joint_power|JOINT_POWER]]). Or return to [[visual3d:documentation:visual3d_signal_types:link_model_based_data_type#kinetic_items|LINK_MODEL_BASED items]].