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
Skeletal muscles are attached to the skeleton by 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 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.
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.
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.
- How does the biceps muscle move when you bend your arm? How does the triceps muscle move when you bend your arm?
- How does the triceps muscle move when you straighten your arm? How does the biceps muscle move when you straighten your arm?
- Why can muscles only pull and not push? How is this related to muscles working in pairs?
- How are skeletal muscles attached to the skeleton?
- Explain why many skeletal muscles must work in opposing pairs.