Support and Movement in Humans and Animals

The biceps-and-triceps example of antagonistic muscles is a classic exam question.

Beyond the human arm, content standard 14.3 expects you to relate a skeleton and its muscles to how an animal actually moves through its own habitat. A fish bends a flexible vertebral column into travelling waves by contracting muscle blocks called myotomes along each side of its body, using its tail fin for main thrust and other fins for steering and balance in water.

A bird instead combines a light, largely fused skeleton with hollow bones and a keeled sternum, a bone that anchors the powerful flight muscles pulling the wings downward for lift and upward for recovery. Comparing locomotion across water, air and land is a favourite way examiners test structure-function understanding rather than memorised facts.

Health issues form the final content standard, 14.4, and are usually tested through short structured questions that ask for a cause, an effect and a preventive measure. Osteoporosis develops when the rate of bone breakdown outpaces the rate of bone rebuilding, so density falls; it is linked to low calcium or vitamin D intake, hormonal changes and ageing, and it raises the risk of fracture from even a minor fall.

Arthritis is inflammation of a joint, most often affecting the cartilage that cushions bone ends or the synovial membrane that lines the joint capsule, producing pain, swelling, stiffness and reduced movement. Weight-bearing exercise and a balanced diet help protect against both conditions.

Content standards in this chapter

1. 14.1 Types of Skeleton: Hydrostatic, Exoskeleton and Endoskeleton
2. 14.2 The Musculoskeletal System of Humans
3. 14.3 Movement and Locomotion
4. 14.4 Health Issues Related to the Human Musculoskeletal System

Key concepts

Types of skeleton

Hydrostatic skeleton (e.g. earthworm), exoskeleton (e.g. insects) and endoskeleton (e.g. humans) each support and allow movement.

Musculoskeletal system

Bones give support, joints allow movement, and muscles attached by tendons produce the force.

Joints

A hinge joint (elbow, knee) allows movement in one plane; a ball-and-socket joint (shoulder, hip) allows movement in many directions.

Antagonistic muscles

Muscles work in pairs: when the biceps contracts to bend the arm, the triceps relaxes, and the reverse straightens it.

Movement and locomotion

Muscle contraction pulls on bones across a joint to produce movement; locomotion is movement of the whole body.

Health issues

Arthritis (joint inflammation) and osteoporosis (weak bones) affect the musculoskeletal system.

Locomotion in water

A fish bends its flexible vertebral column into side-to-side waves by contracting myotome muscle blocks arranged along both sides of the body, pushing against the denser medium of water. The tail fin provides most of the forward thrust, while the dorsal, pelvic and pectoral fins give stability, steering and braking as the fish swims.

Locomotion in air

A bird's skeleton is light because many bones are hollow and fused, reducing the number of separate joints and the overall mass that must be lifted. A keeled sternum, a ridge of bone on the chest, anchors large flight muscles that pull the wings down to generate lift and thrust, then relax as smaller muscles raise the wings again.

Bone versus cartilage

Bone is a hard, living tissue reinforced with calcium and phosphate salts, giving the skeleton the rigidity needed for support, movement and protection of organs such as the brain and heart. Cartilage is softer, more flexible connective tissue that cushions joint surfaces, allows smooth movement and forms flexible parts such as the outer ear, the nose tip and parts of the airway.

Skeletal muscle attachment

A skeletal muscle is joined to a bone by a tendon at each end, so contraction pulls the two bones of a joint closer together. Because a tendon cannot push, every movement in one direction needs a second muscle, or muscle group, to pull the joint back the other way once the first muscle relaxes.

Caring for the musculoskeletal system

Regular weight-bearing activity such as walking or jogging stimulates bone to stay dense, while an adequate daily intake of calcium and vitamin D supplies the minerals bone needs and helps the body absorb them. Correct posture when sitting, standing or lifting, together with avoiding repetitive strain, reduces the long-term risk of joint damage and bone loss.

Antagonistic muscles at the knee

At the knee, the quadriceps on the front of the thigh and the hamstrings at the back form another antagonistic pair: the quadriceps contracts to straighten the leg while the hamstrings relax, and the hamstrings contract to bend the leg while the quadriceps relaxes, the same principle as the biceps and triceps at the elbow.

Why an exoskeleton limits growth

Because an exoskeleton is a rigid outer casing, it cannot expand as the animal inside grows. An insect or crustacean must therefore periodically shed its old exoskeleton in a process called moulting, or ecdysis, and quickly harden a new, larger one underneath before the body can increase further in size.

Dislocation and sprain

A dislocation occurs when a bone is forced out of its normal position at a joint, often tearing the surrounding ligaments and requiring the bone to be manipulated back into place. A sprain is a milder injury in which a ligament is overstretched or partially torn, causing pain and swelling but without the bone leaving the joint.

Structure of a long bone

A long bone such as the femur has a tough outer covering called the periosteum, which carries blood vessels and nerves and allows the bone to grow in width; beneath it lies dense compact bone for strength, surrounding a central marrow cavity that produces blood cells and stores fat, with spongy bone at each rounded end.

How this chapter is examined

SPM Biology is assessed over three papers. Paper 1 has 40 objective questions (40 marks) in 1 hour 15 minutes; Paper 2 carries 100 marks across Sections A, B and C in 2 hours 30 minutes; Paper 3 is the practical, testing science process skills. Content from this chapter can appear in any of them, so lessons drill recall for Paper 1 and structured answers for Paper 2.

Common exam angles

• Explaining how antagonistic muscles bend and straighten the arm.
• Comparing the three types of skeleton.
• Labelling a joint and naming its parts.
• Explaining, with reference to myotomes and fins, how a fish's structure suits swimming.
• Explaining how a bird's hollow, fused skeleton and keeled sternum suit flight.
• Stating one cause, one effect and one preventive measure for either arthritis or osteoporosis.

Common mistakes

Study this chapter

Structures in this chapter

• Skeleton and joints, structure and movement

Key terms

Antagonistic musclesTendonLigamentExoskeleton

Frequently asked questions

Source:SRC-DSKP-EN, SRC-FORMAT