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Muscle Fibers

Muscle Fibers

Changes (white drops) in an astronaut's muscle before and after spaceflight.
Courtesy of NASA.

  • Grades:
  • Length: 60 Minutes


Students learn about muscle structure by comparing yarn and cooked meat.

This activity is from The Science of Muscles and Bones Teacher's Guide, and was designed for students in grades 6–8. Lessons from the guide may be used with other grade levels as deemed appropriate.

Teacher Background

Despite our amazing skeletons, without muscles, we would not be able to stand, balance ourselves or move. Every person has more than 600 muscles throughout his or her body.

Movement happens when muscles contract and become shorter. As seen in the activity, “Arm Model,” the contraction moves the two places of muscle attachment closer together. These types of contractions take place countless times each day in the body.

Skeletal muscles (the ones responsible for movement of the body) are made of bundles of progressively smaller fibers. The largest fiber bundles can be seen with the unaided eye in a piece of muscle tissue or meat. The “strings” that can be teased (pulled) apart are bundles of fibers. Within these large bundles are numerous muscle cells (also called fibers). Each muscle cell is filled with hundreds of even smaller strands (myofibrils). The myofibrils contain the smallest muscle elements of all—tiny units (sarcomeres) that become shorter by sliding one set of protein molecules over another. Added together, all of the minute contractions shorten the length of the entire muscle.

This activity introduces students to the structure of muscles by having them compare and contrast the structure of yarn to the structure they can observe in a cooked piece of beef stew meat or other coarse meat.

Objectives and Standards


  • Muscles are made of fibers within fibers.

  • The structure of muscles makes them strong.

Science, Health and Math Skills

  • Observing

  • Modeling

  • Inferring

Materials and Setup

Teacher Materials

  • Approximately 1/2 pound of stringy or fibrous cut of beef, such as brisket, flank steak or stew meat (see Setup)

Materials per Group of Students (see Setup below)

  • 12-in. section of yarn

  • 4 pairs of disposal safety gloves
  • 4 toothpicks

  • 1-in. cube of prepared stringy beef

  • Plastic knife

  • Plate or tray to work on

  • Disposable plastic gloves

  • Copies of the student page


  1. Cook beef brisket or stew meat in advance for students. Each group should have at least one, 1-inch cube of cooked meat to observe.

  2. Place all materials in a central location for students.

  3. Have students work in groups of two to four.


Please follow all school district and school laboratory safety procedures. It always is a good idea to have students wash hands before and after any lab activity.

Procedure and Extensions

  1. Ask students, Have you ever seen muscle? What does it look like? If necessary, remind students that “meat” is muscle tissue and that many different kinds of muscle are on display at the grocery store. Follow by asking, Which characteristics of muscle help make it strong? Tell students that they will be investigating one aspect of this question.

  2. Give each group of students a length of yarn, toothpicks and a small cube of cooked beef brisket or stew meat.

  3. Have students follow the instructions on the student page to observe the structure of yarn. They should progressively tease apart and test the relative strength of the strands comprising the length of yarn. Have them use a “snap” test, in which they hold the strand between both hands and quickly pull or “snap” it, to estimate the strength of each size of strand.

  4. After students have made their yarn observations, direct their attention to the cooked piece of meat. Have a student in each group slice the meat across the grain using a plastic knife. Students should observe and draw the meat cross section on their sheets. They will note that the muscle looks stringy. The strings are the large fibers of the muscle. They may see white, rubbery tendons attached to the muscle, or fat, which is a source of energy, along with the fibers.

  5. Next, have students tease a section of meat into progressively smaller fibers. Have students observe the fibers using their hand lens and draw the fibers on their student page. Have students explore the strength of the meat by pulling it in two different directions (along the grain and across the grain).

  6. Discuss students’ observations with the class. Ask, In what ways were the yarn and muscle sections similar? Did the fiber-within-fiber design of the yarn make it stronger or weaker? Why? What does this imply for the structure of muscles?

  7. Conclude by discussing how muscles contract. Point out that unlike the yarn fibers, which are not very stretchy, muscle fibers can shorten. To demonstrate, have students extend their arms and feel the muscle (biceps) in their upper arms. Ask them to bend their arms at the elbow and notice any changes that occur in their muscles. Help them understand that muscles become short and fat when they contract. Explain that, unlike yarn, muscles are made of a series of fibers packaged inside each other. The largest fibers were the ones the students were able to observe in class. Inside each larger fiber are smaller and smaller fibers. Finally, inside the smallest fibers are tiny filaments that make the whole muscle change shape. The number of filaments determines how big and strong the muscle is.


Have students compare other meats to the one observed in class. The color of uncooked meat (redder or whiter) depends on the kinds of fibers present. Red or “dark” muscle has more fibers that are specialized for long-term or repetitive activity without fatigue. These muscle fibers release energy from stored fat. White muscle has more fibers specialized for very fast contractions. These fibers, however, provide power for only a short period of time before they become fatigued from lack of oxygen and accumulation of waste products. White muscle uses energy from sugar.

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National Space Biomedical Research Institute

National Space Biomedical Research Institute

This work was supported by National Space Biomedical Research Institute through NASA cooperative agreement NCC 9-58.