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.

Bones

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, permanent bones.

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 flexibility.

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.

Joints

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.

Muscles

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 tissue.

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 striated.

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 appears dark.

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 internal organs.

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 moves.

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 overtraining.

The pages in this section (see menu to the right) discuss disorders of the musculoskeletal system: conditions of the spine, shoulder, joints, and pelvis.