Te importance of joints is obvious: Te skeleton’s ability to pro-
tect other organs and to move smoothly reﬂects their presence. Now
that we are familiar with joint structure and with the movements
that joints allow, we are ready to consider how the muscles attached
to the skeleton cause body movements by acting across its joints.
Check Your Understanding
What is the effect of regular exercise on joint health and
For answers, see Appendix H.
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Joints, or articulations, are sites where bones meet. Teir
functions are to hold bones together and to allow various degrees
of skeletal movement.
Classiﬁcation of Joints
Joints are classiﬁed structurally as ﬁbrous, cartilaginous,
or synovial. Tey are classed functionally as synarthrotic,
amphiarthrotic, or diarthrotic.
Fibrous joints occur where bones are connected by ﬁbrous tissue;
no joint cavity is present. Nearly all ﬁbrous joints are synarthrotic.
Te major types of ﬁbrous
joints are sutures, syndesmoses, and gomphoses.
In cartilaginous joints, the bones are united by cartilage; no joint
cavity is present.
Cartilaginous joints include
synchondroses and symphyses. Synchondroses are synarthrotic;
all symphyses are amphiarthrotic.
Most body joints are synovial joints, all of which are diarthrotic.
All synovial joints have a joint cavity enclosed by a ﬁbrous layer
lined with synovial membrane and reinforced by ligaments;
articulating bone ends covered with articular cartilage; and
synovial ﬂuid in the joint cavity. Some (e.g., the knee) contain
ﬁbrocartilage discs that absorb shock.
Bursae and Tendon Sheaths
Bursae are ﬁbrous sacs lined with synovial membrane and
containing synovial ﬂuid. ±endon sheaths are similar to bursae
but are cylindrical structures that surround muscle tendons.
Both allow adjacent structures to move smoothly over one
Factors Inﬂuencing the Stability of Synovial Joints
Articular surfaces providing the most stability have large surfaces
and deep sockets and ﬁt snugly together.
Ligaments prevent undesirable movements and reinforce the joint.
Te tone of muscles whose tendons cross the joint is the most
important stabilizing factor in many joints.
Movements Allowed by Synovial Joints
When a skeletal muscle contracts, the insertion (movable
attachment) moves toward the origin (immovable attachment).
Synovial joints diﬀer in their range of motion. Motion may be
nonaxial (gliding), uniaxial (in one plane), biaxial (in two planes),
or multiaxial (in all three planes).
Tree common types of movements can occur when muscles
contract across joints: (a) gliding movements, (b) angular
movements (which include ﬂexion, extension, abduction,
adduction, and circumduction), and (c) rotation.
Special movements include supination and pronation, inversion
and eversion, protraction and retraction, elevation and
depression, opposition, dorsiﬂexion and plantar ﬂexion.
Types of Synovial Joints
Te six major categories of synovial joints are plane joints
(nonaxial movement), hinge joints (uniaxial), pivot joints
(uniaxial, rotation permitted), condylar joints (biaxial with
angular movements in two planes), saddle joints (biaxial, like
condylar joints, but with freer movement), and ball-and-socket
joints (multiaxial and rotational movement).
Selected Synovial Joints
Te knee joint is the largest joint in the body. It is a hinge joint
formed by the articulation of the tibial and femoral condyles
(and anteriorly by the patella and patellar surface of the femur).
Extension, ﬂexion, and (some) rotation are allowed. Its articular
surfaces are shallow and condylar. C-shaped menisci deepen the
articular surfaces. Te joint cavity is enclosed by a capsule only
on the sides and posterior aspect. Several ligaments help prevent
displacement of the joint surfaces. Muscle tone of the quadriceps
and semimembranosus muscles is important in knee stability.
Te shoulder joint is a ball-and-socket joint formed by the
glenoid cavity of the scapula and the humeral head. Te most
freely movable joint of the body, it allows all angular and
rotational movements. Its articular surfaces are shallow. Its
capsule is lax and poorly reinforced by ligaments. Te tendons of
the biceps brachii and rotator cuﬀ muscles help to stabilize it.
Te elbow is a hinge joint in which the ulna (and radius)
articulates with the humerus, allowing ﬂexion and extension. Its
articular surfaces are highly complementary and are the most
important factor contributing to joint stability.
Te hip joint is a ball-and-socket joint formed by the acetabulum
of the hip bone and the femoral head. It is highly adapted for