4-17. RELATIVE MOBILITY
A second way of categorizing joints of the human skeleton is according to relative
a. The junctions of some bones are nonmobile, such as a synosteosis.
b. Others are semimobile, as seen with some syndesmoses.
c. Being structured to facilitate motion, synovial articulations (see the next
section) are mobile to various degrees.
4-18. DEGREES OF FREEDOM
The term degrees of freedom refers to the number of planes in which movement
is permitted. This also equals the number of axes around which motion can take place
at a particular joint.
a. One Degree of Freedom. One degree of freedom means that the joint is
uniaxial. Motion can take place in a single plane around one axis only. An example is a
b. Two Degrees of Freedom. Two degrees of freedom mean that the joint is
biaxial. Motion can take place around two different axes.
c. Three Degrees of Freedom. With three degrees of freedom, we say that the
joint is multiaxial. Motion can take place around the three axes in all three planes. An
example is "ball and socket" type joints.
Section VIII. A "TYPICAL" SYNOVIAL JOINT
A synovial joint is structured to facilitate freedom of motion in one or more of the
three planes around the three axes of any given joint. The "typical" synovial joint
(Figure 4-3) is a schematic representation rather than an actual synovial joint, but it
contains the structural features common to all synovial joints.
The synovial articulation is formed between two bones. These bones are parts of
the skeleton. They are levers of motion. To them are attached skeletal muscles, which
provide the forces for motion.