Bones, joints, and muscles
The musculoskeletal system
The musculoskeletal system consists of two systems: the skeletal system, composed of
bones and joints, and the skeletal muscle system. Bones and muscles work together to
perform functions essential to life such as protecting the brain and internal organs,
providing the support necessary to good posture, forming blood cells through a process
known as hematopoiesis, and storing fat and minerals.
There are a various types of tissues that comprise the musculoskeletal system:
bones, ligaments, cartilage,
skeletal muscles, and finally, tendons.
The average adult skeleton consists of 206 bones, attached to muscles by tendons.
As babies, we are born with 270 soft bones. However, by the time we reach adulthood
at around age 20, these 270 soft bones will have fused together into 206 hard,
Living bone consists of three layers. The periosteum is the outside skin of
the bone. The hard compact bone supports the weight of the body. Less dense than
compact bone, bone marrow occurs at the ends of long bones such as the femur
(thigh bone), sternum (breastbone) or humerus (the bone of the upper arm). The red
marrow plays a key role in the formation of blood cells, while yellow marrow,
found at the center of these larger bones, is used to store fats.
Yes, bone is a living tissue composed of both organic components such as collagen
and inorganic minerals such as calcium, phosphorus, magnesium, and potassium. It is
calcium and phosphorus that give bone its strength and hardness and that allow it to
resist compressive forces. These minerals are "cemented" together with the help of
magnesium, sodium, potassium and other "trace" elements. Were it not for collagen
fibers, our bodies would not move as easily: collagen is what gives bones their
So if bone is living tissue, how does it grow?
Old bone is removed and new bone is produced. This process is
aided by two different types of cells: osteoblasts and osteoclasts.
Osteoclasts are responsible for the breakdown of bone.
Osteoblasts, on the other hand, are responsible for the
production of new bone, and they help maintain the balance of
calcium in both blood and bone. Nature gets this balance right—most
of the time. However, when this balance becomes disrupted, as
in osteoporosis, for example, removal
of bone exceeds bone production. As a result, bones become thin
and brittle and by extension, more prone to fracture.
Bone is bone is bone, right?
Well, no. Not necessarily. The body is made up of five main categories of bones, each
designed for a specific purpose.
Long bones, such as the clavicle (collarbone), humerus
(upper arm), radius and ulna (forearm), femur (thigh bone), tibia
and fibula (shin), metatarsals (foot) and metacarpals (hand) are
longer than they are wide. By acting as interconnected levers,
these bones provide support and allow us to move.
Short bones, such as those of the carpals (fingers)
and tarsals (toes), are different in both function and composition.
Because they are intended to allow movement and to provide flexibility,
elasticity and shock absorption, these bones consist mostly of
marrow covered by a thin layer of compact bone.
Flat bones, such as the ribs, sternum (breastbone) and
scapula (shoulderblade) are flat for a reason: they protect internal
organs by acting as plates and they also provide attachment sites
for the skeletal muscles.
Irregular bones such as the skull, pelvis and vertebrae
do double duty. Not only do they bear weight, but they also protect
the brain and spinal cord while allowing our bodies to move and
providing sites for muscle attachment.
While some bones allow for easy attachment of muscles, it's necessary
in some instances to alter the angle at which a muscle attaches
to a bone. This is where sesamoid bones come in. Sesamoid
bones, such as the patella (kneecap), because they are embedded
within a tendon or joint capsule, change the angle of insertion
of the muscle.
When two or more bones come together, the union is known as a joint.
But there is more to a joint that simply the meeting of two bones.
Ligaments attach bone to bone, providing stability and
strength to the joint. Ligaments also determine range of motion.
The rotator cuff in your shoulder, for example, consists of four
ligaments: one to raise your arm from your side to shoulder level,
one to help you raise your arm above your head, and two to help
you move your arm in a complete circle, as in the front crawl.
Were it not for ligaments, your arm would not be able to move
in this manner.
When you think of the shock your knees must take in sports such
as running or tennis, be thankful that your knee joints are well
designed to absorb shock and to bear your body's weight. This
is all thanks in part to cartilage, a gel-like substance
designed to provide protective cushioning to your body's joints.
Muscles, discussed below, are attached to bones with tendons.
While your skeleton has 206 bones, your body requires a far greater number
of muscles in order to support you in all of your daily activities. Over 600
muscles, served by nerves which connect them to the brain and spinal cord,
help you sit, stand, walk, bend, blink, and breathe.
Just as there are different types of bones, there are three types of muscle
The most abundant tissue in the body, skeletal muscles
(also called voluntary or striated muscles) create
movement. Skeletal muscle fibers are roughly cylindrical and each
is crossed by alternating light and dark bands. Hence the name
Why light and dark? This is because muscle fibers consist of
actin and myosin—two different types of protein
strands. Where these protein strands overlap, the muscle fiber
Connective tissue helps hold these muscle fibers together in bundles.
These bundles, in turn, bind together to form muscles. Skeletal muscles
are therefore complex structures consisting of muscle fibers, nerves,
blood vessels and connective tissues.
Elongated and thin, smooth muscles interlace to form
sheets rather than bundles. It is these sheets of smooth muscle
that line the walls of the digestive tract, bladder, and other
Combining traits and characteristics of both smooth and striated
muscles, cardiac muscle (also called heart muscle)
makes up the heart.
Working in concert: how we move
As we've seen, skeletal muscles are attached to bone by tendons.
The proximal attachment refers to the connecting tendon
closest to the body. This is the origin of the muscle.
The other end of the tendon, known as the distal attachment,
is called the insertion of the muscle. When a muscle
contracts, its origin remains stationary; it's the insertion that
When a joint such as your elbow bends, the force that makes it
bend is always exerted by a pull, or contraction. Contraction
makes the muscle shorter. Reversing the direction in which a joint
bends—straightening your elbow—involves the contraction
of a different set of muscles. Your elbow is a perfect illustration
of how antagonistic muscles work: when one group of muscles
contracts, an antagonistic group of muscles stretches. This creates
an opposing pull, thereby reversing the direction of movement.
Disorders of the musculoskeletal system
The musculoskeletal system is very complex. Proper health of bones, joints
and muscles depends on good posture, good nutrition, and good overall health.
Adequate amounts of calcium and vitamin D are necessary for proper bone health,
while proteins help build and repair muscle.
Exercise is as important as proper nutrition since it helps to minimize
age-related loss of bone (osteoporosis) and muscle mass.
This is particularly important for women, as women are twice as prone to
osteoporosis as men.
Bear in mind that the risk of injury to muscles, tendons and ligaments can
be reduced with proper stretching, training, and equipment and by avoiding
The pages in this section (see menu to the right) discuss disorders
of the musculoskeletal system: conditions of the spine, shoulder,
joints, and pelvis.