The Muscular System

This lesson covers: 

  1. How muscles cause movement
  2. Antagonistic muscles

How muscles cause movement

Our body is comprised of approximately 640 skeletal muscles.

These muscles are linked to bones by strong cords known as tendons.

Diagram showing muscle contraction pulling on a tendon which moves the bone.
  • Muscles contract to create movement, pulling on the tendon which pulls on the bone.
  • This action causes the bone to move in the direction the muscle contracts.

Antagonistic muscles

For the body to move, most muscles that cause movement are arranged in pairs on opposite sides of a joint.

These pairs are known as antagonistic muscles, which are essential for controlling movement.


  • When one muscle contracts, the opposing muscle relaxes.
  • The contracting muscle shortens and pulls on its attached tendon.
  • This tendon pulls on the bone, causing movement at the joint.
  • The relaxing muscle lengthens to allow the movement.

The biceps and triceps are antagonistic muscles

The action of the biceps and triceps allow your arm to move around your elbow joint.

Illustration showing antagonistic muscles with biceps relaxing and triceps contracting.
Illustration showing the biceps contracting and the triceps relaxing to bend the arm.
  • To bend the arms - The biceps contract while the triceps relax.
  • To straighten the arms - The triceps contract, pulling in the opposite direction, while the biceps relax.