The Skeletal and Muscular System
The Skeletal System
The skeleton is the framework of bones that gives your body its shape. An adult human skeleton has 206 bones. Bones are living tissue — they grow, repair themselves when broken, and contain nerves and blood vessels.
The skeleton has four main jobs:
- Support — it holds the body up and gives it shape, like the frame of a building
- Protection — hard bone surrounds delicate organs
- Movement — bones act as levers that muscles pull on
- Making blood cells — the soft bone marrow inside some bones produces red and white blood cells
Bones That Protect Organs
| Bone | Organ protected |
|---|---|
| Skull (cranium) | Brain |
| Ribcage | Heart and lungs |
| Backbone (spine) | Spinal cord |
| Pelvis | Lower digestive and reproductive organs |
The spine (backbone) is made of many small bones called vertebrae. It supports the upper body, protects the spinal cord, and lets you bend and twist.
Joints
A joint is where two or more bones meet. Joints allow the skeleton to move. Different joints allow different kinds of movement:
| Joint type | Example | Movement |
|---|---|---|
| Hinge | Elbow, knee | Back and forth in one direction |
| Ball and socket | Shoulder, hip | Round in almost all directions |
| Pivot | Neck (top of spine) | Rotating / turning |
| Fixed | Skull bones | No movement — locked together |
At a moveable joint:
- Cartilage covers the ends of the bones — smooth and tough, it cushions them and stops them rubbing together
- Ligaments hold the bones together and keep the joint stable
- Synovial fluid lubricates the joint so it moves smoothly
The Muscular System
Muscles are tissues that produce movement by getting shorter, or contracting. There are more than 600 muscles in the human body.
Muscles are attached to bones by strong cords called tendons. When a muscle contracts, the tendon pulls on the bone, and the bone moves at a joint.
The key rule to remember: muscles can only pull, they cannot push. A muscle can contract to pull a bone, but it cannot push it back. This is why muscles must work in pairs.
Antagonistic Muscle Pairs
Because a muscle can only pull, bones are moved by antagonistic pairs — two muscles that pull in opposite directions. While one contracts (pulls), the other relaxes.
The best example is in your upper arm:
- To bend the arm: the biceps contracts (pulls the forearm up) while the triceps relaxes
- To straighten the arm: the triceps contracts (pulls the forearm down) while the biceps relaxes
The muscle that contracts to cause the movement is the agonist; the one that relaxes is the antagonist. They swap roles for the opposite movement.
How Movement Happens — Step by Step
- The brain sends a nerve signal to a muscle
- The muscle contracts and becomes shorter
- The tendon pulls on the bone it is attached to
- The bone moves at the joint
- To reverse the movement, the opposite muscle in the pair contracts
Tendons vs Ligaments
These sound similar but do different jobs — a common exam trap:
| Tissue | Connects | Job |
|---|---|---|
| Tendon | Muscle to bone | Passes the muscle's pull to the bone |
| Ligament | Bone to bone | Holds a joint together and keeps it stable |
Looking After Your Bones and Muscles
- Calcium (from dairy, leafy greens) keeps bones strong
- Vitamin D (from sunlight) helps the body absorb calcium
- Exercise makes muscles stronger and keeps bones dense
- Protein helps repair and build muscle tissue
- Warming up before exercise reduces the risk of pulling a muscle
Key Terms
- Skeleton — the framework of bones that supports and protects the body
- Joint — a place where two or more bones meet
- Cartilage — smooth tissue that cushions the ends of bones
- Ligament — connects bone to bone at a joint
- Tendon — connects muscle to bone
- Contract — when a muscle gets shorter to create a pull
- Antagonistic pair — two muscles that work against each other (e.g., biceps and triceps)
- Vertebrae — the small bones that make up the spine
Common Mistakes
- Saying muscles push — muscles can only pull (contract)
- Mixing up tendons (muscle to bone) and ligaments (bone to bone)
- Thinking one muscle can both bend and straighten a joint — it takes an antagonistic pair
- Forgetting that bones are living tissue that can grow and repair
Tips and Tricks
- Muscles pull, never push — that one rule explains why they come in pairs
- Tendon joins muscle to bone; Ligament ties bone to bone
- Biceps bends, triceps straightens — while one works, the other rests
- Hinge = one direction (elbow/knee); ball and socket = all directions (shoulder/hip)