Parts of the System
The musculoskeletal system is made of the skeleton (bones and cartilage), the joints where bones meet, and the skeletal muscles attached to bones. Together they support the body, protect organs, store minerals such as calcium, and produce movement. Muscles are joined to bones by tough cords called tendons, while bones are held to each other at joints by ligaments.
Joints
A joint is a place where two or more bones meet. Movable (synovial) joints let bones move smoothly:
- Hinge joint (elbow, knee): allows movement in one plane, like a door hinge.
- Ball-and-socket joint (shoulder, hip): allows movement in many directions.
At synovial joints, the bone ends are covered with smooth cartilage and lubricated by synovial fluid, which reduces friction and wear.
Key idea
Muscles can only PULL, not push. To move a bone one way and then back again, muscles must work in opposing pairs.
Antagonistic Muscles
Because a muscle can only pull when it contracts, muscles are arranged in antagonistic pairs that produce opposite movements. At the elbow, the biceps and triceps are such a pair. When the biceps contracts (and the triceps relaxes), the forearm is raised (the arm bends, flexion). When the triceps contracts (and the biceps relaxes), the forearm is lowered (the arm straightens, extension).
Example
Lifting a book to your shoulder: the biceps contracts and shortens, pulling the forearm up, while the triceps relaxes. To lower the book, the triceps contracts and the biceps relaxes.
Remember
Tendons join muscle to bone; ligaments join bone to bone. Antagonistic muscles work in opposite pairs.