Fig 2.

The reference frame (zero position) of the left forelimb rig in (A) left lateral and (B) dorsal views. The bones have been made semi-translucent to enhance visualization of the three Joint Coordinate Systems (JCS) that comprised the virtual shoulder, elbow, and radioulnar joints. All angles and translations are recorded in relation to the reference frame. Note that the reference frame is not intended to be a natural orientation: in fact, in this orientation the radius is unnaturally rotated so that it is entirely supinated relative to the ulna, a posture impossible for a rat. However, this reference posture provides a point against which the degree of pronation can be quantified. To account for body movements, the shoulder JCS has the sternal manubrium centered on the caudal humeral head. Each JCS was based on an Euler angle ZYX rotation order that followed the right-hand rule. In this way, movement of the Z-axis also moved the Y- and X-axes as well as the bone model; movement of Y-axis also moved the X-axis and the bone model; and movement of the X-axis only moved the bone model.