# Artificial-Hand Technology--Current State of Knowledge in Designing and Forecasting Changes
## 2 Human-Hand Overview and Evaluation  
### Natural Principles  
In order to better understand hand structure and purpose, and to introduce the nomenclature used in this paper, the main body systems are described below. The human hand consists of five fingers: thumb, index, middle, ring and little, which are numbered from 1 to 5, respectively. Its skeletal system is composed of 27 bones, where eight are carpals and five metacarpals. Digits 2 to 5 have three bones each, called the phalanges: proximal, middle, and distal. The thumb differs from the other fingers and has only two bones, the proximal and the distal (Figure 1). Ligaments between the appropriate bones create joints that allow them to move in particular directions and ranges. Three main joint types can be distinguished: proximal interphalangeal (PIP), distal interphalangeal (DIP), and interphalangeal (IP) joints are simple hinge joints, which means that they can move in one axis. Another type is the condyloid, which forms metacarpophalangeal (MCP) joints and allows biaxial movement, i.e., flexion or extension, and adduction or abduction. The third and most important is the saddle joint of the thumb, the carpometacarpal (CMC). Due to specific construction, this joint provides triplane movement, that is, flexion or extension, abduction or adduction, and rotation. It should be noted that the CMC joint is defined as a joint with two degreesof freedom because there is no possibility to independently execute a third move. Nevertheless, as a result of these three actions, the human thumb can perform opposition, which makes it unique [49; 50]. The contentious issue of hand kinematics is the opposition of the little finger. [PERSON] in [51] described the CMC joints of Fingers 2-5 as follows: the second and third may be considered as having 0 DOF, the fourth is the plane synovial joint with 1 DOF (flexion or extension), and the fifth is a 2 DOF saddle joint that permits flexion, extension, some abduction, adduction, and limited opposition. On the contrary, human-hand kinematics in [52; 53] included CMC joints of the ring and the little finger as two biaxial joints. As settlement of this dispute is not in the aims of this article, the overall term of \"little-finger opposition\" is used in the following sections. Muscles are natural one-way actuators, so they mostly work in opposing pairs. A basic unit of muscle fiber is a sarcomere. It consists of thin (actin) and thick (myosin) filaments. The mechanism of contracting is described by muscle-sliding-filament theory where, generally, thick filaments slide past thin ones. As a result, a muscle shortens and its volume simultaneously increases. More than 30 hand muscles could be classified by their location; those in the forearm respond to power grasps, and those in the hand to precise grasps and manipulation [54; 55]. Motion transmission is carried by the tendon system. Elastic connective tissue, connected between muscles and bones, slides in sheaths that not only prevent it from rubbing, but also keep it in the right place close to the digits. There are simple, direct tendon connections as well as complex ones, i.e., the extensor mechanism, otherwise called the tendon network. The structure of the extensor mechanism combines forces from such muscles as interosseous, lumbrical, and extensor digitorum that, depending on the sequence, allow executing different finger moves [49; 56]. The nervous system is the control unit of an organism that transmits signals to and from different parts of the body. As there is no need to explain the sensing-mechanism principles of upper limbs, the most significant are listed as follows: tension of muscle fiber, muscle-contraction velocity, and touch detection of the skin (pressure, slip, vibration, temperature, and pain). Furthermore, processing of these data happens in conjunction with vision, our dominant sense, and proprioceptors provide information about joint angle and body-part orientation [57; 58; 59; 60; 55].