# Uniform vs. Lognormal Kinematics in Robots: Perceptual Preferences for Robotic Movements
## 2 Human Arm  
The human arm is made up of bones, joints and muscles. All these structures must work in coordination to produce movements such as those involved in writing. Bones and cartilage are rigid parts that shape and support the entire structure of the arm. The bone of the upper arm is the humerus and its central part is formed by the ulna and radius and the final part consists of the bones of the hand. The shoulder joint connects the scapula to the humerus, while the elbow joint connects the humerus to the ulna and radius. These two bones connect with the carpal bones, forming the wrist. Finally, the last joints are in the hand and join the carpal bones, metacarpals and finger bones [16].  
The joints of the arm are the connections between the rigid parts of the arm and allow its movement through the muscles. The shoulder joins the arm to the shoulder. It is the most flexible joint in the human body and allows free movement of the arm. Under normal conditions, the arm hangs vertically from this joint. The elbow is the central joint of the arm and has a range of action going from 0 to 180\({}^{\circ}\). The carpals and metacarpals, together with the rotation of the wrist, allow the hand to orient itself in all directions. Finally, the joints of the fingers allow one to grasp almost any object.  
Human and Robotic Arm Equivalence  
Due to its great versatility, the human arm has served as the inspiration for most robotic arms. The robot analyzed in the present work is an industrial robot with 6 degrees of freedom, manufactured by ABB, model IRB120. It was chosen because it is a robot based on the PUMA robot (Programmable Universal Manipulation Arm) and has a structure similar to most actual industrial robots. Its appearance can be seen in Figure 1.  
The human arm can be considered as a redundant robot; that is, it has more degrees of freedom than are actually needed to orient and position an object in space. This means that a human arm provides theoretically infinite configurations to grasp a pencil in a certain position and with a certain orientation. This is not the case with industrial robotic arms, in which the number of possible configurations for holding a pencil at a certain position and orientation is limited.  
The human shoulder joint has its reflection on the robot in its first two joints. The first joint of the robot is a rotation of the first link related to the base and on a vertical axis and the second joint is a rotation of the second link related to the first. These two joints allow two movements of the second link with respect to the base, the equivalent of the humerus with respect to the scapula. In any case, the robotic arm has a degree of freedom less than the human shoulder that also allows a turn along the humerus. The third joint is the one that relates the second and third links and is the equivalent of the human elbow. The rotational movement that allows the wrist to rotate with respect to the elbow is obtained in the robot with the rotation of the fourth link with respect to the third. The rotation of the wrist with respect to the forearm is obtained by rotating the fifth link with respect to the fourth and the rotation of the pencil is obtained by rotating the sixth link with respect to the fifth.