The Skeletal System and Its Movements 15- Gliding or plane joints contain joint surfaces
that are generally flat or slightly curved in shape.
Examples of gliding joints occur between some of
the carpal bones (intercarpal joints), tarsal bones
(intertarsal joints), and the articular processes of
the vertebrae (facet joints). - Hinge joints are composed of a spool-shaped sur-
face that fits into a concave surface. Examples of
hinge joints are the ankle, elbow, and knee (the
latter is a modified hinge). - Pivot joints are composed of an arch- or ring-
shaped surface that rotates about a rounded or
peg-like pivot. Examples of pivot joints occur in
the upper forearm (upper radioulnar joint) and
between the first and second vertebrae of the
spine (atlantoaxial joint). - Condyloid or ellipsoid joints consist of an oval-
shaped condyle that fits into an elliptical cavity.
An example of a condyloid joint occurs at the
wrist (radiocarpal joint) and knuckles of the
hands (#2–#5 metacarpophalangeal joints) and
feet (metatarsophalangeal joints). - Saddle joints are composed of a saddle-shaped
bone that fits into a socket, which is concave-
convex in the opposite direction. An example of
a saddle joint occurs at the thumb (first carpo-
metacarpal joint). - Ball-and-socket joints consist of a ball-shaped
head that fits into a socket. Ball-and-socket joints
are the most freely movable type of joint in the
body. Examples of ball-and-socket joints occur at
the shoulder joint and hip joint.
FIGURE 1.7 Associated structures of the foot.Body Orientation Terminology.
Before we consider the specific movements allowed
by the synovial joints just described, it is helpful
to learn some basic anatomical terminology. This
terminology can be used to describe the location of
anatomical structures, body segments, or the body as
a whole. Key terminology includes the center of mass,
line of gravity, anatomical position, anatomical direc-
tions, anatomical planes, and anatomical axes.The Center of Mass and Line of Gravity
The center of mass of the body is the single point
of a body about which every particle of its mass is
equally distributed. This can be thought of as the
point at which the body could be suspended or sup-
ported where the body would be totally balanced
in all directions. When studying bodies subject to
gravity (such as human movement on Earth), the
center of mass may also be termed the center of
gravity (CG). During upright standing with the arms
down by the sides, the center of mass or center of
gravity of the body is approximately located just in
front of the second sacral vertebra and at about 55%
of a person’s height (Smith, Weiss, and Lehmkuhl,
1996).
The line of gravity is an imaginary line run-
ning vertically from the center of mass of the body
toward the ground. Gravity is the attraction of the
mass of the earth for the mass of other objects, and
due to the effect of gravity, every particle of the
body has a vertical force vector. However, these
individual force vectors can be simplified into one
force vector for the entire body. This single force
vector acting on the whole body is termed the line
of gravity of the body. Since the line of gravity of
the body must run through the center of mass of the
body, its position in space constantly changes as the
body changes its position and configuration during
movement.
These concepts of the center of mass and line of
gravity can be applied to body segments, as well as
the body as a whole. For example, the center of mass
of the trunk, thigh, leg, and foot segments can be
derived. One can then establish the line of gravity
of each of these segments by dropping a vertical
line from the center of mass of the given segment.
These concepts of the center of mass and line of
gravity are key for the analysis of alignment, forces,
and movement. They will be used in later chapters,
including the calculation of resistance torque when
lifting a dumbbell or another dancer (see figure
2.12 on p. 48).