CBSE Class 6 Science Chapter 8 Notes: Body Movements

Important Terms & Glossary

Skeleton

The internal framework of bones and cartilage that provides shape, support, and protection to the internal organs of a human or animal body.

Joint

The point or location where two or more bones meet, enabling movement and flexibility in different directions.

Cartilage

A tough, semi-transparent, and elastic tissue that is softer than bone. It covers bone ends at joints to reduce friction and is also found in the upper ear and nose tip.

Ligament

A strong band of fibrous connective tissue that connects one bone to another bone at a joint.

Gait

The specific pattern or manner of walking and movement of limbs during locomotion in animals.

Streamlined Body

A body shape that is narrow at both ends and broader in the middle, designed to minimize friction or drag while moving through air or water (e.g., in fish and birds).

The Human Skeletal Framework

The human body consists of a sturdy, protective framework called the human skeleton. At birth, a baby has around 305 bones, which fuse together as they grow, leaving an adult with 206 bones.

The skeleton provides shape, protects delicate organs (like the brain inside the skull and the heart inside the rib cage), and works with muscles to assist in body movements. Key structures include the skull, backbone (vertebrae), ribs, shoulder bones, and pelvic bones. Since bones are rigid and cannot bend, movement is made possible by structures called joints where bones meet. These joints are supported by cartilage to prevent wear and tear, and ligaments to hold the bones in place.

Types of Joints in the Human Body

How Muscles Move Bones

1. — Muscles always work in pairs (e.g., biceps and triceps) to move a single bone. A muscle can only pull, it cannot push.
2. — When one muscle of the pair contracts, it becomes shorter, stiffer, and thicker, and pulls the bone in that direction.
3. — While the first muscle contracts, the other muscle of the pair relaxes and stretches out.
4. — To move the bone back, the relaxed muscle contracts (pulls), while the first muscle relaxes to return the bone to its original position.

Gait of Animals: How Different Creatures Move

• {"title":"Earthworm Movement","description":"Earthworms do not have bones. They have a liquid skeleton and body rings with muscles. They move by repeatedly extending the front part while keeping the rear fixed to the ground using tiny hair-like bristles (setae), and then pulling the rear forward."}
• {"title":"Snail Movement","description":"Snails slide along using a strong, muscular foot. The foot produces a slimy substance (mucus) to reduce friction, allowing it to move over rough surfaces with a wavy motion."}
• {"title":"Fish Movement","description":"Fish have a streamlined body. During swimming, muscles make the front part of the body curve to one side and the tail part swing towards the opposite side, creating a jerk that pushes the fish forward."}
• {"title":"Cockroach Movement","description":"Cockroaches can walk, climb, and fly. They have an outer skeleton, three pairs of legs for walking, and two pairs of wings attached to breast muscles for flying."}

Must-Remember Key Points for Exams

• Bones are hard structures that cannot bend; movement occurs only at the joints.
• An adult human skeleton has 206 bones, while a baby has over 300 bones.
• Cartilage is a soft, flexible bone-like structure found in the external ear and nose tip, which also cushions bones at joints.
• The rib cage consists of 12 pairs of bent chest bones protecting the heart and lungs.
• The backbone is made up of 33 small bones called vertebrae, which allow us to bend forward and backward.
• Muscles move bones by contraction (pulling); they never push.
• Snakes do not have legs; they slither in S-shaped loops, pushing against the ground to move fast.
• Birds fly because of hollow and light bones, modified forelimbs (wings), and powerful breast muscles.
• Fixed joints inside our skull do not allow any movement except for the lower jaw (mandible).

Exam Tips & Pitfalls

1. The 'Muscle Push' Trap: Never write that muscles push a bone back. Remember: Muscles can only pull. A pair of muscles is always required to move a bone back and forth.
2. Cartilage Locations: Examiners love asking for locations of cartilage. Remember to list the upper ear, nose tip, and between vertebrae/joints.
3. Diagram Practice: Practice sketching a simplified block diagram of a ball-and-socket joint and hinge joint for scoring full marks in 3-mark or 5-mark questions.

Practice Questions with Solutions

• Why can we bend our elbow backward only up to a point, but rotate our shoulder completely? The elbow has a hinge joint which allows only back-and-forth (180-degree) movement, whereas the shoulder has a ball-and-socket joint which permits rotation in all directions (360-degree).
• What is the function of the rib cage? The rib cage protects delicate internal organs like the heart and lungs, and joins the chest bone to the backbone.
• How does an earthworm fix itself to the ground while moving? An earthworm uses tiny, hair-like structures called bristles (setae) projecting from under its body to grip the ground.
• What makes a bird's body suitable for flight? Birds have hollow and light bones, streamlined bodies, forelimbs modified into wings, and strong breast muscles to flap wings.

Frequently Asked Questions