Fig 3. Joint coordinate systems illustrated.

Each joint coordinate system (JCS) of the forelimb as well as the JCS of the skull and sternum utilized in the gold standard accuracy test are isolated and illustrated to show greater detail. Each JCS was based on an Euler angle ZYX rotation order that followed the right-hand rule. For all JCS, the Z-axis is blue, the Y-axis is green, and the X-axis is red. For all forelimb JCS, the Y-axis measures abduction/adduction and the X-axis measures long-axis rotation (LAR). At the shoulder JCS (A) shown in oblique left cranial view, the Z-axis measures protraction/retraction of the humerus relative to the sternum. At the elbow JCS (B) and the radioulnar JCS (C) (both shown in oblique caudodorsal view), the Z-axis measures flexion/extension of the ulna relative to the humerus or of the radius relative to ulna, respectively. To determine the body midline frame of reference, the skull and sternum were linked together as a separate rig (D). A skull-sternum JCS was created for the gold standard to test how accurately manual registration of the skull allowed us to determine the midline of the body using the sternum as a reference. The skull and sternum rig is shown in left lateral view and the bones are semi-translucent to facilitate visualization of the skull-sternum JCS. For the skull-sternum JCS (D), the pitch (Z-axis), yaw (Y-axis), and roll (X-axis) of the sternum was measured relative to the skull.