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Movement of the Human Body

  • Muscles help make all of your daily movements possible.
  • A muscle is an organ that can relax, contract, and provide the force to move your body parts.

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Movement of the Human Body

  • In the process, energy is used and work is done.

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  • Some muscles in your body are always moving.

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Muscle Control

  • Muscles that you are able to control are called voluntary muscles.
  • Your hand, arm, and leg muscles are voluntary.
  • You can choose to move them or not move them.

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Muscle Control

  • Involuntary muscles are muscles you can’t control consciously.
  • They go on working all day long, all your life.
  • Blood gets pumped through blood vessels, and food is moved through your digestive system by the action of involuntary muscles.

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Your Body’s Simple Machines—Levers

  • The action of muscles, bones, and joints working together is like a lever.
  • In your body, bones are rods, joints are fulcrums, and contraction and relaxation of muscles provide the force to move body parts.
  • Levers are classified into three types—first-class, second-class, and third-class, all of which are found in the human body.

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Classification of Muscle Tissue

  • The three types of muscles are skeletal, smooth, and cardiac.
  • The muscles that move bones are skeletal muscles.

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  • They are more common than other muscle types and are attached to bones by thick bands of tissue called tendons.

Classification of Muscle Tissue

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Classification of Muscle Tissue

  • When viewed under a microscope, skeletal muscle cells are striated (STRI ay tud), and appear striped.
  • Skeletal muscles are voluntary muscles.

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  • They tend to contract quickly and tire more easily than involuntary muscles do.

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Classification of Muscle Tissue

  • Cardiac muscle is found only in the heart.
  • Like skeletal muscle, cardiac muscle is striated.

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  • This type of muscle contracts about 70 times per minute every day of your life.

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Classification of Muscle Tissue

  • Smooth muscles are found in your intestines, bladder, blood vessels, and other internal organs.

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  • They are nonstriated, involuntary muscles that slowly contract and relax.
  • Internal organs are made of one or more layers of smooth muscle.

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Working Muscles

  • You move because pairs of skeletal muscles work together.

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  • When one muscle of a pair contracts, the other muscle relaxes, or returns to its original length.

Click image to view movie.

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Working Muscles

  • Muscles always pull. They never push.

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  • When the muscles on the back of your upper leg contract, they shorten and pull your lower leg back and up.
  • When you straighten your leg, the back muscles lengthen and relax, and the muscles on the front of your upper leg contract.

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Changes in Muscles

  • Over a period of time, muscles can become larger or smaller, depending on whether or not they are used.

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  • Some of this change in muscle size is because of an increase in the number of muscle cells.
  • However, most of this change in muscle size is because individual muscle cells become larger.

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Changes in Muscles

  • In contrast, if you participate only in nonactive pastimes, your muscles will become soft and flabby and will lack strength.

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  • Muscles that aren’t exercised become smaller in size.
  • When someone is paralyzed, his or her muscles become smaller due to lack of exercise.

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How Muscles Move

  • Your muscles need energy to contract and relax.

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  • As the muscle contracts, this released energy changes to mechanical energy (movement) and thermal energy (heat).
  • Your blood carries energy-rich molecules

to your muscle cells where the chemical energy stored in these molecules is released.

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How Muscles Move

  • When the supply of energy-rich molecules is used up, the muscle becomes tired and need to rest.

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  • During the resting period, your blood supplies more energy-rich molecules to your muscle cells.
  • The heat produced by muscle contractions helps keep your body temperature constant.

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Section Check

Question 1

The skeletal and muscular system working together can be described as a _______.

A. lever

B. inclined plane

C. screw

D. wheel and axle

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Section Check

Answer

The correct answer is A. In your body, bones are rods, joints are fulcrums, and contraction and relaxation of muscles provide the force to move body parts.

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Section Check

Question 2

What type of muscle makes up this organ?

A. cardiac

B. skeletal

C. smooth

D. rough

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Section Check

Answer

The answer is A. Cardiac muscle is found only in the heart.

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Section Check

Question 3

Describe how muscles increase in size.

Answer

Muscles that are given regular exercise respond quickly to stimuli. Some change in muscle size is due to an increase in the number of muscle cells. However, most increase in size is because individual muscle cells become larger.