Ergoniza Intelligent

15.1.1 How Ergoniza Intelligent calculates humeral and femoral rotation

Once again, the explanation focuses on humeral rotation; the method is equivalent for the femur. The method geometrically defines humeral rotation as a dihedral angle—a relative rotation—about the humerus's longitudinal axis:

  1. Axis of rotation (humeral axis). The upper arm's longitudinal axis is defined by the line connecting the shoulder and elbow. Humeral rotation is interpreted as rotation about this axis.

  2. Trunk-based reference plane. A reference plane is constructed to represent the “base” orientation of the upper arm relative to the trunk. Conceptually, it is the plane formed by the humeral axis and the trunk's longitudinal axis. This plane provides the relative “zero” for measuring rotation about the humerus, and its reference is fixed to the trunk—that is, it follows the trunk's orientation.

  3. Upper-arm–forearm plane. A second plane representing the instantaneous orientation of the upper-arm–forearm assembly is constructed from the shoulder, elbow, and wrist points. This plane captures how the forearm is oriented around the humeral axis.

  4. Dihedral-angle calculation. Humeral rotation is obtained as the signed angle between the two planes, measured about the humeral axis. This angle distinguishes, for example, a position in which the elbow points forward from one in which it points outward, even when the upper arm has the same elevation.

This approach is consistent with a common biomechanical interpretation: humeral rotation appears as a change in the forearm's orientation around the upper-arm axis once a trunk-based reference has been established.

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