Body Movements: How We Move | Class 6 Science NCERT

Understanding Body Movements and the Human Skeleton

Every living creature, from tiny insects to large elephants, performs movements. Movement is essential for survival; it helps animals find food, seek shelter, escape from danger, and even find mates. In humans, movements can be voluntary (like walking or lifting an arm) or involuntary (like our heart beating or blinking). Our body's ability to move depends mainly on our skeletal system and muscles. The human skeleton is like a strong framework made of bones that gives shape and support to our body. Without it, we would be a shapeless mass! It also protects our delicate internal organs. For example, your skull protects your brain, and your rib cage shields your heart and lungs. There are 206 bones in an adult human body. What makes this rigid framework flexible are special places called joints, where two or more bones meet. These joints, along with our muscles, allow us to bend, twist, and turn. Imagine trying to pick up a book without being able to bend your elbow! It would be impossible. So, bones provide structure, joints provide flexibility, and muscles provide the power for movement.

The Incredible World of Joints

Joint

A joint is a place where two or more bones meet in the body. Joints allow different parts of our body to move relative to each other.

Ball-and-Socket Joint

This type of joint allows movement in all directions (up, down, side to side, and rotation). One bone has a 'ball'-like end that fits into a 'socket'-like hollow of another bone. Examples: Shoulder joint, hip joint.

Hinge Joint

Similar to the hinges on a door, this joint allows movement only in one plane, like bending and straightening. Examples: Knee joint, elbow joint, finger and toe joints.

Pivot Joint

A pivot joint allows rotational movement. In this joint, a cylindrical bone rotates in a ring-like bone. Example: The joint where your neck joins your head, allowing you to turn your head left and right.

Fixed Joint (Immovable Joint)

Unlike other joints, fixed joints do not allow any movement between the bones they connect. Examples: The joints between the bones of your skull.

How Muscles Power Our Movements

1. Muscles Work in Pairs — Muscles cannot push bones; they can only pull them. To achieve movement, muscles always work in pairs. When one muscle contracts (shortens and becomes thicker), it pulls the bone it's attached to. At the same time, the other muscle in the pair relaxes (lengthens).
2. Contraction and Relaxation — Imagine your arm. When you bend your arm at the elbow, the biceps muscle (at the front of your upper arm) contracts and pulls your forearm upwards. Simultaneously, the triceps muscle (at the back of your upper arm) relaxes.
3. Opposite Action for Straightening — To straighten your arm, the roles reverse. The triceps muscle contracts, pulling your forearm back down, while the biceps muscle relaxes. This coordinated pulling and relaxing by muscle pairs is how most of our voluntary movements occur.

Fascinating Movements in the Animal Kingdom

• Earthworm: An earthworm has no bones or legs. It moves by extending and shortening its body. It does this using two sets of muscles that work against each other. It also has tiny bristles (called setae) on its underside which help it grip the ground.
• Snail: A snail moves with the help of a strong, muscular foot. It glides along by waves of muscle contractions in its foot, leaving a trail of mucus (slime) behind it, which reduces friction and helps it move smoothly.
• Cockroach: Cockroaches have three pairs of legs for walking and two pairs of wings for flying. Their legs are strong and enable them to run very fast. They can also climb on various surfaces and fly short distances.
• Bird: Birds have hollow bones, which makes their bodies light, and strong breast muscles connected to their wings. They use their wings to flap, creating lift and thrust for flight. Their tail feathers help in steering.
• Fish: Fish swim by rapidly moving their body and tail from side to side. They have a flexible backbone and strong muscles. Their fins help them balance, change direction, and stop. The streamlined shape of their body also helps them move easily through water.
• Snake: Snakes move by looping their body. They have a long backbone and many strong muscles connected to their ribs. They push against the ground with their loops, propelling themselves forward. Different snakes move in different ways, like side-winding or slithering.

Exam Tip: Identifying Joint Types

When studying joints, don't just memorize definitions. Try to feel the movement in your own body! For example, bend your knee – it moves only one way, like a door hinge. Rotate your arm at the shoulder – it moves in many directions, like a ball in a socket. This hands-on understanding will help you correctly identify examples of different joint types in exams.

Practice Questions with Solutions

• Q: What is a joint? Give two examples of where ball-and-socket joints are found in the human body. A: Step 1: A joint is a place where two or more bones meet in the body, allowing for movement. Step 2: Ball-and-socket joints allow movement in all directions. Two examples in the human body are the shoulder joint and the hip joint. Final answer: A joint is where bones meet. Ball-and-socket joints are found in the shoulder and hip.
• Q: Explain how muscles help in moving a bone. Can a muscle push a bone? A: Step 1: Muscles work in pairs to move bones. When one muscle contracts (shortens), it pulls the bone it is attached to. Step 2: Simultaneously, the opposing muscle in the pair relaxes. This coordinated action causes movement. Step 3: No, a muscle cannot push a bone. Muscles can only pull. Final answer: Muscles move bones by contracting and pulling. Muscles cannot push bones.
• Q: Describe how an earthworm moves. A: Step 1: An earthworm moves by using its muscles to extend and shorten its body segments. Step 2: It first extends the front part of its body, keeping the rear part fixed to the ground using tiny bristles (setae). Step 3: Then, it fixes the front part and contracts the body to pull the rear part forward. This process repeats, allowing it to move. Final answer: Earthworms move by alternately extending and contracting their body segments with the help of muscles and bristles.
• Q: Differentiate between a hinge joint and a pivot joint with one example for each. A: Step 1: A hinge joint allows movement in only one plane, similar to a door hinge. An example is the elbow joint or knee joint. Step 2: A pivot joint allows rotational movement. An example is the joint in the neck that allows you to turn your head side to side. Final answer: Hinge joints (e.g., elbow) allow movement in one direction, while pivot joints (e.g., neck) allow rotational movement.

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