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Muscles, Bones, and Movement

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Muscles, Bones, and Movement

Do muscles come in pairs?

This woman is doing a stretch for the muscles on the back of her legs, the hamstrings. She also has a muscles on the front of her legs, directly opposite the hamstrings. These are the quadriceps. The hamstrings and quadriceps work together as a pair to move your legs back and forth while you run.

Muscles, Bones, and Movement

When skeletal muscles contract, bones move. But how do muscles make your bones move? A voluntary muscles usually works across a joint. It is attached to both the bones on either side of the joint by strong cords called tendons. A tendon is a tough band of connective tissue that connects a muscle to a bone. Tendons are similar to ligaments, except that ligaments join bones to each other. Muscles move the body by contracting against the skeleton. When muscles contract, they get shorter. By contracting, muscles pull on bones and allow the body to move.

Muscles can only contract. They cannot actively extend, though they can move or relax back into the non-contracted neutral position. Therefore, to move bones in opposite directions, pairs of muscles must work in opposition. 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 the flexor . The muscle that contracts to cause the joint to straighten is called the extensor . When one muscle is contracted, the other muscle from the pair is always elongated.

For example, the biceps and triceps muscles work together to allow you to bend and straighten your elbow. When you want to bend your elbow, your biceps muscle contracts ( Figure below ), and, at the same time, the triceps muscle relaxes. The biceps is the flexor, and the triceps is the extensor of your elbow joint. Other muscles that work together are the quadriceps and hamstrings used to bend and straighten the knee, and the pectorals and trapezius used to move the arms and shoulders forward and backward. During daily routines we do not use muscles equally. For example, we use our biceps more than our triceps due to lifting against gravity.

The biceps and triceps act against one another to bend and straighten the elbow joint. To bend the elbow, the biceps contracts and the triceps relaxes. To straighten the elbow, the triceps contract and the biceps relax.

Smooth Muscles and Cardiac Muscles

Smooth muscles and cardiac muscles are not attached to bone. Recall that these types of muscles are under involuntary control. Smooth muscle is responsible for the contractility of hollow organs, such as blood vessels, the gastrointestinal tract, the bladder, or the uterus. Like skeletal muscles, smooth muscle fibers do contract together, causing the muscle to shorten. Smooth muscles have numerous functions, including the following.

  • The smooth muscle in the uterus helps a woman to push out her baby.
  • In the bladder, smooth muscle helps to push out urine.
  • Smooth muscles move food through the digestive tract.
  • In arteries, smooth muscle movements maintain the arteries' diameter.
  • Smooth muscle regulates air flow in lungs.
  • Smooth muscle in the lungs helps the airways to expand and contract as necessary.
  • Smooth muscles in arteries and veins are largely responsible for regulation of blood pressure.

Cardiac muscle also contracts and gets shorter. This muscle is found only in the heart. The sudden burst of contraction forces blood throughout your body. When the cardiac muscle relaxes, the heart fills with blood. This rhythmic contraction must continue for your whole life, luckily the heart muscle never gets tired. If your heart beats 75 times a minute, how many times does it beat in an hour? A day? A year? 85 years?


  • extensor : Muscle that contracts to cause a joint to straighten.
  • flexor : Muscle that contracts to cause a joint to bend.
  • tendon : Tough connective tissue that attaches muscle to bones.


  • Muscles move the body by contracting against the skeleton.
  • Muscles work together in pairs to bend or straighten the joint.


Use the resource below to answer the questions that follow.

  1. How does the biceps muscle move when you bend your arm? How does the triceps muscle move when you bend your arm?
  2. How does the triceps muscle move when you straighten your arm? How does the biceps muscle move when you straighten your arm?
  3. Why can muscles only pull and not push? How is this related to muscles working in pairs?


  1. How are skeletal muscles attached to the skeleton?
  2. Explain why many skeletal muscles must work in opposing pairs.
  3. List three functions of smooth muscles.
  4. What happens when cardiac muscle contracts?

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