Muscles work in pairs Skeletal muscles produce movements by pulling on tendons, whic...

Muscles work in pairs Skeletal muscles produce movements by pulling on tendons, which in turn pull on bones. Usually, a muscle is attached to two bones via a tendon on each end of the muscle. When the muscle contracts, it moves one bone toward the other. The other bone remains in nearly the original position. The point where a muscle tendon is attached to the stationary bone is called the origin. The point where the other muscle tendon is attached to the movable bone is called the insertion. The origin is like the part of a door spring that is attached to the doorframe. The insertion is similar to the part of the spring that is attached to the movable door.

During movement, bones act as levers and joints act as axes of rotation for these levers. Most movements require several skeletal muscles working in groups, because a muscle can only exert a pull and not a push. In addition, most skeletal muscles are arranged in opposing pairs at joints. Muscles that bring two limbs together are called flexor muscles (such as the biceps muscle in the upper arm in Figure 7.26). Those that cause the limb to extend outward are called extensor muscles (such as the triceps muscle in the upper arm). The flexor muscle is used when you hold a heavy object in your hand; the extensor muscle can be used, for example, to extend your arm when you throw a ball.

Why is it easier to hold a heavy object using a bent arm than a straight arm?

(a) More flexor muscles are involved.

(b) The distance from the joint to the place where gravitational force is exerted by Earth on the object is smaller.

(c) The distance from the joint to the place where force is exerted by the object on the hand is smaller.

(d) There are two possible axes of rotation instead of one.