Body Movements

Animal locomotion:

• Earthworm: No limbs or bones; moves by alternately contracting and relaxing circular and longitudinal muscles; the body's setae (tiny bristles) grip the ground. Earthworms aerate and enrich soil — called "nature's ploughs" (Darwin)
• Snail: Moves on a single muscular foot using wave-like contractions; secretes slime to reduce friction
• Cockroach: 3 pairs of legs; 2 pairs of wings; exoskeleton (hard external skeleton of chitin)
• Fish: Streamlined body; fins for steering and stability; tail fin (caudal fin) provides propulsive thrust; swim bladder controls buoyancy
• Bird: Wings (forelimbs modified); hollow bones (reduce weight for flight); strong chest muscles (pectorals) power wingbeats; streamlined shape
• Snake: No limbs; S-shaped body; scales grip ground; lateral undulation using backbone and ribs

Antagonistic muscles: Two muscles that work in opposite directions across the same joint. When one contracts (shortens), the other relaxes (lengthens passively).

Classic example — Elbow joint (hinge joint):

Other antagonistic pairs:

• Knee: Quadriceps (front thigh) vs Hamstrings (back thigh) — extension vs flexion
• Hip: Hip flexors (iliopsoas) vs Gluteus maximus — walking forward vs backward push
• Wrist: Flexors vs Extensors (forearm muscles)

Why this matters: Understanding antagonistic pairs explains why:

• Muscle cramps are painful (both muscles seize simultaneously)
• Sports injuries often involve one muscle being stronger than its antagonist (hamstring tear in sprinters)
• Paralysis prevents movement even if bones/joints are intact — nerve signals to muscles are lost

Bone is a composite material — two components working together:

Analogy: Bone is like reinforced concrete — the mineral (hydroxyapatite) = concrete (hard, compression-resistant); the collagen = steel rebar (flexible, tension-resistant). Neither alone is as strong as the combination.

When composition changes:

• Osteoporosis: Calcium is lost from bones → mineral component decreases → bones become porous and brittle → fracture risk increases dramatically. Affects postmenopausal women and elderly (reduced calcium absorption, Vitamin D deficiency, hormonal changes)
• Rickets (children) / Osteomalacia (adults): Vitamin D deficiency → impaired calcium absorption → bones soft and weak → bowing of legs in children
• Old bones vs young bones: In children, higher collagen % → bones more flexible (bend before breaking). In old age, collagen decreases → bones more brittle

Bone marrow — two types:

• Red bone marrow: In flat bones (skull, sternum, ribs, pelvis, vertebrae) and the ends (epiphyses) of long bones → produces all blood cells (hematopoiesis): red blood cells (RBCs), white blood cells (WBCs), platelets
• Yellow bone marrow: In the shafts (diaphysis) of long bones in adults → stores fat (adipose tissue); can convert back to red marrow in severe anaemia

Intervertebral discs: Pads of fibrocartilage between each pair of cervical, thoracic, and lumbar vertebrae. Function: shock absorption; allow spine to bend. When a disc ruptures or bulges outward under pressure, it presses on spinal nerves → intervertebral disc prolapse ("slipped disc") — common cause of lower back pain and sciatica in India, especially among manual labourers, truck drivers, and construction workers.

Spinal cord: Runs through the vertebral canal (a channel formed by the aligned holes in successive vertebrae). The vertebral column's primary evolutionary function is to protect the spinal cord while allowing flexible movement.