Chapter 14: Support and Movement in Humans and Animals

Learning Objectives

By the end of this chapter, you should be able to:

• Compare the three types of skeletons in animals
• Describe the types of joints in the human musculoskeletal system
• Explain antagonistic muscle action in movement and locomotion
• Describe locomotion mechanisms in humans, fish, birds and earthworms
• Explain health issues related to the musculoskeletal system and their prevention

Overview

Support and movement are essential life processes. Animals have evolved different skeletal systems to provide structural support, protect internal organs and enable movement. In humans, the musculoskeletal system — bones, joints and muscles working together — allows a wide range of precise movements. Understanding how antagonistic muscles operate across different animals reveals the unity and diversity of biological design.

14.1 Types of Skeletons

Animals have three fundamentally different types of skeletons:

Exoskeleton

• Protects and supports soft internal tissues
• Covers the entire outer body surface
• Limitation: cannot grow — the organism must moult (shed) its exoskeleton periodically to allow growth

Endoskeleton

• Internal framework that maintains body shape
• Supports soft tissues and protects vital organs
• Can grow continuously with the body — no moulting needed
• Provides attachment points for muscles

Hydrostatic Skeleton

• No rigid material; uses pressurised fluid in the coelom (body cavity)
• Circular and longitudinal muscles contract against the incompressible fluid to change body shape
• Enables locomotion through peristaltic waves

14.2 Musculoskeletal System of Humans

A joint is the point where two or more bones meet. Different joint types allow different ranges of movement.

Types of Joints

Immovable Joints (Fibrous Joints)

• Bones are held together by tough fibrous tissue
• No movement allowed
• Example: sutures between bones of the skull (cranium)

Slightly Movable Joints (Cartilaginous Joints)

• Bones are connected by cartilage
• Limited movement only
• Examples: intervertebral discs between vertebrae; cartilage between the first rib and sternum

Freely Movable Joints (Synovial Joints)

• Allow wide range of movement
• Contain synovial fluid that lubricates and reduces friction

Components of a synovial joint:

Types of synovial joints:

SPM Tip: Know the 3 types of joints with one example each. For synovial joints, be able to name all 5 components and state the function of each.

14.3 Movement and Locomotion

Antagonistic Muscles

Antagonistic muscles are pairs of muscles that produce movement by working in opposite directions. When one muscle contracts, its antagonist relaxes.

Human Arm — Biceps and Triceps

• The biceps is attached to the radius via a tendon; when it contracts it pulls the forearm upward
• The triceps is attached to the ulna; when it contracts it pulls the forearm downward

Human Leg

• Quadriceps femoris (front of thigh) and biceps femoris / hamstrings (back of thigh) form the antagonistic pair for extending and flexing the knee

Locomotion in Different Animals

Fish

• Myotomes (W-shaped muscle blocks along the body) contract alternately on left and right sides
• Creates a wave of undulation from head to tail
• The caudal (tail) fin generates forward thrust
• Paired fins (pectoral, pelvic) provide stability and steering

Birds

• Pectoralis major (largest flight muscle): contracts during downstroke → generates lift and forward thrust
• Pectoralis minor: contracts during upstroke → raises the wing
• The large sternum (breastbone) with keel provides a broad attachment surface for flight muscles

Earthworm

• Circular muscles contract → body becomes long and thin (fluid squeezed longitudinally)
• Longitudinal muscles contract → body becomes short and thick (fluid squeezed radially)
• Setae (tiny bristles) anchor segments to the soil during movement
• Coordinated peristaltic waves propel the worm forward

14.4 Health Issues Related to the Musculoskeletal System

Arthritis

Arthritis is inflammation and degeneration of joints.

Causes:

• Osteoarthritis (most common): wear and tear of cartilage, reduced synovial fluid
• Rheumatoid arthritis: autoimmune disease — the immune system attacks the joint lining

Effects:

• Cartilage becomes thinner → bones rub together
• Ligaments become less elastic
• Joints become swollen, painful and stiff
• More common in women and the elderly

Prevention and management:

• Regular, low-impact exercise (swimming, cycling)
• Maintain healthy body weight to reduce joint stress
• Anti-inflammatory medication; physiotherapy

Osteoporosis

Osteoporosis is a disease where bones become weak and brittle due to loss of bone mineral density.

Causes:

• Body loses too much calcium or produces too little new bone tissue
• Common in women after menopause — low oestrogen reduces calcium absorption
• Risk factors: lack of exercise, insufficient calcium/vitamin D intake, smoking, excessive alcohol

Effects:

• Bones become fragile → fracture easily (hip, spine, wrist)
• Often called a "silent disease" — no symptoms until a fracture occurs

Prevention:

SPM Exam Tip: Be ready to compare arthritis and osteoporosis — both affect the musculoskeletal system but through different mechanisms. Arthritis affects joints (cartilage + synovial fluid); osteoporosis affects bone density. Know at least 2 prevention measures for each.

Practice Questions

1. State the THREE types of skeletons and give one example for each.
2. Explain why an insect must moult but a human does not need to.
3. Describe the role of antagonistic muscles using the biceps and triceps as an example.
4. Explain how an earthworm moves using circular and longitudinal muscles.
5. Compare arthritis and osteoporosis under these headings: cause, effects on the body, prevention.

Summary

• Three skeleton types: exoskeleton (arthropods — must moult), endoskeleton (vertebrates — grows with body), hydrostatic (worms — fluid pressure)
• Three joint types: immovable (skull sutures), slightly movable (vertebrae), freely movable (synovial — hinge, ball-and-socket)
• Antagonistic muscles produce movement by working in opposite directions — one contracts while the other relaxes
• Locomotion mechanisms differ by body plan: human arm (biceps/triceps), fish (myotome undulation), birds (pectoralis major/minor), earthworm (circular/longitudinal + setae)
• Arthritis: joint cartilage degeneration → pain and stiffness; Osteoporosis: bone density loss → brittle bones; both preventable through exercise and good nutrition

Related Topics

• Chapter 6: Cell Division
• Chapter 12: Coordination and Response in Humans
• Chapter 15: Sexual Reproduction, Development and Growth