In the organism of a living human, joints play a triple role (Davies, 1961): (1) they help to maintain body posture; (2) they contribute to the transposition of body parts in relation to one another; (3) they are organs of locomotion of the body in the environment. This work of the joints is associated closely with the work of muscles. The joints form "gaps" in the rigid skeleton and are a potential source of weakness and instability of the upright and mobile skeleton. This shortcoming is compensated for by the muscles, which even at rest reinforce the joints much as wires reinforce a radio tower. This makes the role of the muscles as active motor organs even more important since without the muscles, the joint, a passive organ of movement, cannot act. Since the conditions for muscular activity in the process of evolution differed, the articulations that formed differed in shape and function. Accord­ing to shape, the articular surfaces can be regarded as segments of geometri­cal rotation bodies: a cylinder rotating on a single axis; an ellipse rotating on two axes; and a sphere rotating on three or more axes. Movements at the joints occur on three main axes. The following types of movements at the joints are distinguished: Movement on the frontal transverse (horizontal) axis: flexion (flexio), i.e. a reduction of the angle formed by the articulating bones, and extension (extensio), i.e. increase of this angle. Movements on the sagittal (horizontal) axis: adduction (adductio), i.e. drawing towards the median plane, and abduction (abductto), i.e. draw­ing away from it. Movements on the vertical axis, i.e. rotation (rotatio), inward and outward or to the right and to the left. Movement in a circular manner, circumduction (circumductio), by changing from one axis to another with one end of the bone describing a circle and the whole bone describing the figure of a cone. Gliding movements of the articular surfaces are possible as well, as they draw apart, for instance, when the fingers are stretched. Movements at joints which have several pivotal axes often- occur not in isolation on one of the axes, but simultaneously on two or three axes in different combinations. For instance, abduction at the shoulder joint on the sagittal axis is combined with flexion of the joint (anteflexio) on the frontal axis. This is usually not noticeable. In some diseases of the joints or disorders of muscle functioning, however, such combination is broken up, and each movement is made separately. In impaired function of the deltoid muscle serving this joint, for instance, to abduct the arm, the patient must first bend it. The character of movement at the joint is determined by the shape of the articular surfaces. The range of movements at the joints depends on the difference in the size of the articulating surfaces. If, for example, the articu­lar fossa forms an arc of 140 degrees and the head forms an arc of 210 degrees, the arc of movements is 70 degrees. The greater the difference in the length of the articulating surfaces, the larger the arc (range) of movements, and vice versa. If both articulating surfaces are equal, no or only slight movement is possible. Movements at the joints are limited not only by a decrease in the difference in the length of the articulating surfaces but also by various types of brakes; some ligaments, muscles, bony projections, and so on, play this role. Since increased physical (force) load can cause work hypertrophy of the bones, ligaments, and muscles and lead to growth of these structures and restriction of movements, different joint flexibility is encountered in ath­letes, depending on the sport. For example, the range of movements at the shoulder joint is greatest in track and field athletes and least in weight lifters and wrestlers. Movements at joints with particularly strongly developed brakes are sharply limited. Such joints are called amphiarthroses (see p. 127). Intra-articular cartilages also have an effect on the range of movements, increasing their number. Thus, the temporomandibular articulation, which belongs to biaxial joints according to the shape of the articular sur­faces, has an intra-articular disc which allows three types of move­ments.