Forearm rotational efficiency (E_rot) as a function of pronation-supination and flexion-extension angles.

<p><b>A:</b> Three-dimensional surface showing efficiency values at each forearm and elbow angles. <b>B:</b> Projection of the three-dimensional surface on XY plane showing efficiency ranges at each forearm and elbow angles. The points connected by a continuous line indicate the forearm positions where efficiency is maximal for each elbow angle. The dashed line indicates the neutral position of the forearm. The dotted line shows a trajectory followed by the forearm to reach an object with the elbow almost fully extended (170°) and the forearm supinated (60°) (asterisk S) and to bring it close to the face (50° of elbow flexion and −5° of pronation) (asterisk P). This trajectory, which direction is indicated by the arrow, is the shortest and entails an increase of about 190% in the efficiency value.