How do bones, joints and muscles work together to produce movement?
Option B Human musculoskeletal anatomy: the structure of the skeleton and joints; antagonistic muscle action; the structure of skeletal muscle; the sliding filament theory of contraction; and musculoskeletal injuries.
A focused answer to the Eduqas Component 3 Option B on the human musculoskeletal system. Covers the skeleton and synovial joints, antagonistic muscle action, skeletal muscle structure, the sliding filament theory of contraction, and musculoskeletal injuries.
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What this dot point is asking
Eduqas Option B wants you to describe the skeleton and joints, explain antagonistic muscle action, describe skeletal muscle structure, explain the sliding filament theory of contraction (including calcium and ATP), and describe musculoskeletal injuries. Study this option if your school has chosen it.
The skeleton and joints
The skeleton provides support, protection (the skull protects the brain, the ribs the heart and lungs), movement (a system of levers for muscles), and makes blood cells in the marrow. Bones meet at joints:
Antagonistic muscle action
Muscles can only pull (contract), not push, so they are arranged in antagonistic pairs that pull in opposite directions. At the elbow, the biceps (flexor) contracts to raise the forearm (flexion) while the triceps relaxes; the triceps (extensor) contracts to lower it (extension) while the biceps relaxes.
Skeletal muscle structure
A skeletal muscle is made of many muscle fibres (cells), each containing many myofibrils. A myofibril shows a banded pattern of repeating sarcomeres, the contractile units, built from two protein filaments: thin actin and thick myosin. The fibres also contain many mitochondria (for ATP) and a network called the sarcoplasmic reticulum that stores calcium ions.
The sliding filament theory
Contraction shortens the sarcomere by sliding the filaments past each other:
- A nerve impulse causes the sarcoplasmic reticulum to release calcium ions into the sarcoplasm.
- Calcium binds troponin, which moves tropomyosin off the binding sites on the actin, exposing them.
- Myosin heads bind the exposed sites (forming cross-bridges) and flex (the power stroke), pulling the actin towards the centre of the sarcomere, so it shortens.
- ATP binds the myosin head, causing it to detach; ATP is hydrolysed (by ATPase on the head) to re-cock it, ready to bind again.
The cycle repeats while calcium and ATP are present; when the impulse stops, calcium is pumped back, tropomyosin re-covers the sites, and the muscle relaxes.
Examples in context
Example 1. Why trained muscles have more mitochondria. Endurance training increases the number of mitochondria in muscle fibres, so more ATP is made aerobically to power repeated cross-bridge cycling, linking muscle structure to respiration.
Example 2. A torn ligament. A sprained ankle is a stretched or torn ligament (bone to bone), whereas a pulled muscle (strain) damages the muscle or tendon, a distinction Eduqas may test in an injury question.
Try this
Q1. Name the structures that join muscle to bone and bone to bone. [2 marks]
- Cue. Tendons join muscle to bone; ligaments join bone to bone.
Q2. Explain why muscles must work in antagonistic pairs. [2 marks]
- Cue. Muscles can only pull (contract), not push, so one muscle is needed to move a joint one way and an opposing muscle to move it back.
Q3. State the role of calcium ions in muscle contraction. [2 marks]
- Cue. They bind troponin, moving tropomyosin off the actin binding sites so myosin heads can attach and the filaments slide.
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20196 marksDescribe the sliding filament theory of muscle contraction, including the role of calcium ions and ATP.Show worked answer →
A nerve impulse causes calcium ions to be released from the sarcoplasmic reticulum into the sarcoplasm.
The calcium ions bind to troponin, which moves tropomyosin away from the binding sites on the actin filaments, exposing them.
Myosin heads bind to the exposed sites, forming cross-bridges, and then flex (the power stroke), pulling the actin filaments past the myosin towards the centre of the sarcomere, so the sarcomere shortens.
ATP binds to the myosin head, causing it to detach; ATP is then hydrolysed (by ATPase on the head), which re-cocks the head ready to bind again. This cycle repeats while calcium and ATP are present.
Markers reward calcium release binding troponin and moving tropomyosin, myosin heads binding actin and the power stroke shortening the sarcomere, and ATP causing detachment and re-cocking of the heads.
Eduqas 20214 marksExplain what is meant by antagonistic muscle action, using the example of the muscles that move the forearm at the elbow.Show worked answer →
Antagonistic muscle action means muscles work in pairs that pull in opposite directions, because muscles can only pull (contract), not push.
At the elbow, the biceps and triceps are an antagonistic pair.
When the biceps contracts (and the triceps relaxes), the forearm is raised (the joint flexes); the biceps is the flexor.
When the triceps contracts (and the biceps relaxes), the forearm is lowered (the joint extends); the triceps is the extensor.
Markers reward muscles pulling in opposite directions because they can only contract, and the biceps flexing and triceps extending the elbow as the antagonistic pair.
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Sources & how we know this
- Eduqas A Level Biology Specification (A400) — Eduqas (2015)