13 – 6 What is the muscular
system?
Key Terms
flexor: muscle that bends joint
extensor: muscle that straightens a joint
Muscles More than 600 muscles make up
the muscular system. Muscles are tissues
that can shorten along their length. Without
muscles, the bones of the skeletal system
could not move the body. Muscles are
attached to bones by tendons. A tendon is a
strong elastic band tissue. Tendons make
movement possible. When a muscle
contracts, or shortens, it pulls on the tendon,
which makes the bone move.
Teamwork Most muscles must work in
teams of two. The muscles that bend and
straighten the arm are examples of muscles
working together. These muscles are called
biceps and triceps. Biceps are flexors. They
bend the arm at the elbow. Triceps are
extensors. They straighten the arm at the
elbow. As you bend your arm, the biceps
contract, and the triceps relax. As you
straighten your arm, the biceps relax while
the triceps contract and pull the arm straight.
.
Muscle Actions
Muscles only move
bones when the muscles contract. For this
reason, muscles can only pull bones. They
cannot push bones. For example, there are
muscles that bend, or flex, your knee joint.
These muscles are called flexors. There are
other muscles that straighten, or extend, your
knee joint. These muscles are extensors.
CHECKING CONCEPTS 1-3
1. What is a tendon?
2. Are muscles relaxed or contracted when they move bones?
3. What is the main function of the muscular system?.
4. What do extensor muscles do?
5. Why must muscles work together?