VCE Physical Education (Year 11) · Unit 1 — The Human Body in Motion · 14m read

The Muscular System — How Muscles Produce Movement

Muscles turn chemical energy into movement. Understanding how they work in pairs, contract in different ways and differ in fibre type explains why sprinters and marathon runners look so different.

Learning objective: Explain how muscles create movement, including muscle roles, contraction types and fibre types.

By the end of this lesson you can:

• I can name major muscles and their actions.
• I can explain agonist, antagonist and stabiliser roles.
• I can compare muscle fibre types.

Muscles are rowers who can only pull. A rower pulls the oar — they can't push it. Muscles can only pull (contract); to move a joint back, the opposite muscle pulls. That's why muscles work in agonist–antagonist pairs.

The Muscular System — How Muscles Produce Movement

MUSCLE TYPES:
• Skeletal (voluntary), cardiac (heart), smooth (organs).

MUSCLE ROLES:
• AGONIST — the prime mover.
• ANTAGONIST — relaxes to allow movement.
• STABILISER — holds a bone still.
• Example: biceps curl up — biceps agonist, triceps antagonist.

CONTRACTION TYPES:
• ISOTONIC — CONCENTRIC (shortens) and ECCENTRIC (lengthens under tension).
• ISOMETRIC — tension without length change (plank).
• ISOKINETIC — constant speed (specialised machines).

FIBRE TYPES:
• TYPE I (slow twitch) — aerobic, fatigue resistant, endurance.
• TYPE IIa (fast twitch oxidative) — power with some endurance.
• TYPE IIx (fast twitch glycolytic) — maximal power, fatigue quickly.

SLIDING FILAMENT THEORY:
• Myosin heads attach to actin, pull the filaments together, shortening the sarcomere; requires ATP and calcium.

| Term | Role | Bicep curl (up) |
|---|---|---|
| Agonist | Prime mover | Biceps brachii |
| Antagonist | Relaxes/lengthens | Triceps brachii |
| Stabiliser | Holds joint steady | Rotator cuff, deltoid |
| Concentric | Shortens under tension | Biceps lifting |
| Eccentric | Lengthens under tension | Biceps lowering |
| Isometric | No length change | Holding mid-curl |

Key ideas at a glance

• Muscles only pull: Pairs create movement.
• Match fibre type to activity: Type I endurance, Type II power.
• Name contraction type: Concentric, eccentric, isometric.
• Motor units follow all-or-none: Recruit more for more force.

Worked example (Medium)

Identify the agonist and contraction type for the quadriceps when lowering into a squat.

1. Movement. Knee flexion under load.
2. Muscle. Quadriceps control the lowering.
3. Contraction. Eccentric — lengthening under tension.

Answer: Quadriceps act eccentrically as the controlling agonist.

Worked example (Hard)

Identify the agonist and contraction type for the quadriceps during the downward phase of a squat.

1. Movement. Knee flexion under load.
2. Muscle. Quadriceps lengthen while controlling the descent.
3. Answer. Quadriceps are the agonist working eccentrically.

Answer: Quadriceps, eccentric contraction.

Common mistakes

• Saying the hamstrings lower you in a squat Gravity lowers you; the quadriceps control it eccentrically. Fix: Identify which muscle is under tension.
• Confusing fibre type with muscle type Fibre types are within skeletal muscle. Fix: Use Type I / IIa / IIx.
• Saying the hamstrings lower you in a squat Gravity + eccentric quads control it. Fix: Identify which muscle is under tension.

Sliding filament theory

Muscle fibres contain myofibrils made of sarcomeres. During contraction, myosin heads attach to actin, forming cross-bridges, and pull actin towards the centre of the sarcomere using ATP.

The filaments slide past each other — they don't shorten themselves — so the whole muscle shortens.

Practice questions

1. Holding a plank is an example of:

   1. Concentric
   2. Eccentric
   3. Isometric
   4. Isokinetic

   (1 marks)

2. Why would a marathon runner have a high proportion of Type I fibres?

   (2 marks)

3. Fibre type best suited to a marathon:

   1. Type IIx
   2. Type I (slow-twitch)
   3. Type IIa only
   4. Smooth muscle

   (1 marks)

4. Holding a plank is which contraction?

   1. Concentric
   2. Eccentric
   3. Isometric
   4. Isokinetic

   (1 marks)

5. Explain the all-or-none principle.

   (2 marks)

Summary

• Muscles work as agonists, antagonists and stabilisers.
• Concentric, eccentric, isometric contractions.
• Fibre types suit different events.
• Agonist, antagonist, stabiliser; concentric, eccentric, isometric.