Figure 16.22: There are three types of muscles in the body: cardiac, skeletal, and smooth.
Everyone has the same three types of muscle tissue, no matter their age. ( 15 )
Muscles, Bones, and Movement
Skeletal muscles are attached to the skeleton by tendons. Atendonis a tough band of
connective tissue that connects a muscle to a bone. Tendons are similar to ligaments except
that ligaments join bone to each other. Muscles move the body by contracting against the
skeleton. When muscles contract they get shorter, when they relax, they get longer. By
contracting and relaxing, muscles pull on bones and allow the body to move. Muscles work
together in pairs. Each muscle in the pair works against the other to move bones at the
joints of the body. The muscle that contracts to cause a joint to bend is called theflexor.
The muscle that contracts to cause the joint to straighten is called theextensor.
Forexample,thebicepsandtricepsmusclesworktogethertoallowyoutobendandstraighten
your elbow. Your biceps muscle, shown inFigure16.23, contracts, and at the same time
the triceps muscle relaxes. The contracting biceps pull on the radius bone and the elbow
bends. To straighten the arm, the biceps muscle relaxes and the triceps on the opposite side
of the elbow joint contracts. The biceps is the flexor and the triceps is the extensor of your
elbow joint. In this way the joints of your body act like levers. This lever action of your
joints reduces the amount of energy you have to spend to make large body movements.
Muscles and the Nervous System
Muscles are controlled by the nervous system (see theControlling the Bodychapter). Nerves
send messages to the muscular system from the brain. Nerves also send messages to the brain
from the muscles. Remember that smooth and cardiac muscles are involuntary muscles. This
means that you cannot control the nerve messages that get sent to and from these muscles.