Skeletal Muscle Movement
Skeletal muscles, which are the organs of the skeletal muscle system,
generally act by pulling on bones to produce movement. To pull the
bone, muscles need to attach to the bone, either directly or indirectly.
The epimysium of a muscle can attach directly to bone or cartilage to
form a direct attachment. An indirect attachment is formed when the
connective tissue layers, the epimysium, perimysium, and endomysium
form a complex at the end of the muscle.
For connections, muscles require either a tendon or a broader sheet of
connective tissue called an aponeurosis. Muscles connect to muscles
via aponeuroses, and muscles attach to bones via tendons or
aponeuroses. Thus, when a muscle contracts, the force of movement is
transmitted through the attachment, which pulls on the bone to produce
skeletal movement.
Tendons also help to stabilize the joints. Tendons are a common form of
attachment because the collagen fibers are more resistant to tearing
than direct muscle tissue attachment to bone would be, and the compact
form of the tendon requires a small amount of space. Tendons can be
easily seen as the hand is flexed, causing the thick, cordlike tendons of
the forearm to stand out prominently. The calcaneal (Achilles) tendon is
visible from the heel to the calf. A connective tissue layer called a tendon
sheath, which protects the tendon as it moves, also surrounds some
tendons. Fluid-filled sacs called bursae also join to tendons to reduce
friction as the tendon moves. Bursae are present in connective tissue
near bone, where tendons experience friction, but they occasionally
arise in other areas due to stress caused by movement.
EXAMPLE
Bursae can become inflamed, a condition known as bursitis. This is
usually caused by overuse of a joint or by other mechanical stress,
which can result in pain and swelling. Bursitis is common in knees,
elbows, and shoulders.
Keep in mind that the range of motion produced by muscles is restricted
by the anatomy of the bones and other support structures involved in a
particular joint. Consider the movements of the hip and shoulder joints,
both of which are freely moveable. The hip is the attachment point for
the thigh and leg. The shoulder is the attachment point for the arm and
forearm. The shoulder can produce movements that the hip cannot
because of the anatomy of the bones involved and the ligaments
(connective tissue) associated with the joint. Hold your arm out to the
side and move your thumb around the axis of the arm in a 360-degree
circle. Can you do that with your hip and leg? No, you can’t. There are
some people who appear to be “double-jointed"; they are able to push
their joints past the normal limits of movement. The reason they can do
this is because the ligaments of their joints are “loose” compared with
someone who is not “double-jointed.” This ability appears to have a
genetic basis.
For muscles attached to the bones of the skeleton, the location of the
connection determines the force, speed, and type of movement. These
characteristics depend on each other and can explain the general
organization of the muscular and skeletal systems.