Structure and function of the musculoskeletal system; types of muscle, muscle contractions (concentric, eccentric, isometric), joint types, fibre types (slow-twitch and fast-twitch)

What this dot point is asking

VCAA wants you to know the structure and function of the musculoskeletal system: the three muscle types, the three types of contraction (concentric, eccentric, isometric) and how to identify them in a movement, the major joint types and the movements they allow, and the slow-twitch versus fast-twitch fibre distinction. The exam expects you to apply this to a named sporting movement using precise anatomical terminology.

The answer

The musculoskeletal system is the structural foundation of movement. Bones, joints and muscles work together to produce the movements your sport demands.

Muscle types

The body has three types of muscle:

• Skeletal muscle. Voluntary, striated, attached to bone via tendons. Produces all voluntary movement. The primary focus of Unit 1.
• Cardiac muscle. Involuntary, striated, found only in the heart.
• Smooth muscle. Involuntary, non-striated, found in walls of blood vessels, gut, bladder, and other organs.

Skeletal muscle accounts for roughly 40-45% of body mass in young adults. Its proportion and distribution affect movement capacity directly.

Muscle contractions

Skeletal muscle produces three types of contraction. Identifying them in movement is a frequent exam task.

Concentric contraction

The muscle shortens while producing force. The classic example is lifting a weight - the muscle generating the force is getting shorter as the limb moves.

In a biceps curl, lifting the weight is the concentric phase: biceps shortens.

Eccentric contraction

The muscle lengthens while producing force. The muscle is opposing the movement, not driving it.

In a biceps curl, lowering the weight under control is the eccentric phase: biceps lengthens while still producing force to control the descent. Without that eccentric force, the weight would fall.

Eccentric contractions produce more force than concentric and cause more muscle damage. This is why downhill running produces more soreness than uphill running of equivalent intensity.

Isometric contraction

The muscle produces force without changing length. The classic example is holding a position.

A plank is isometric. The core muscles are firing to hold position, but they are not changing length. Wall sits, holding a heavy weight overhead without moving, grip holds in climbing - all isometric.

Joint types

Joints are categorised by their structure and the movement they allow.

By structure

• Fibrous joints (e.g., skull sutures). Little to no movement.
• Cartilaginous joints (e.g., between vertebrae, pubic symphysis). Limited movement.
• Synovial joints (e.g., shoulder, hip, knee, elbow). Free movement; the bulk of sporting joints.

Synovial joint sub-types

Synovial joints allow different movements:

• Hinge joints (elbow, knee). Allow flexion and extension only.
• Ball-and-socket joints (shoulder, hip). Allow flexion/extension, abduction/adduction, rotation, and circumduction.
• Pivot joints (atlas-axis at the neck, radioulnar joint). Allow rotation only.
• Ellipsoidal joints (wrist). Allow flexion/extension and abduction/adduction.
• Saddle joints (thumb base). Similar to ellipsoidal with greater range.
• Plane joints (between tarsal bones in the foot, between vertebrae). Allow sliding movements.

The range of motion at any joint is determined by the joint structure plus the surrounding soft tissue (muscles, tendons, ligaments).

Movement terminology

VCAA expects precise movement terms:

• Flexion. Bending a joint (knee bending in a squat).
• Extension. Straightening a joint (knee straightening in stand-up).
• Hyperextension. Beyond normal range of extension (e.g., extending the spine backwards).
• Abduction. Moving a limb away from the body's midline (raising arm out to the side).
• Adduction. Moving a limb toward the midline (lowering arm back to side).
• Rotation. Turning a joint along its axis (head turning side to side).
• Circumduction. Combination producing a circular movement (shoulder circles).
• Plantar flexion. Pointing toes down.
• Dorsiflexion. Pulling toes up.

In an exam response, naming the specific movement (flexion at the knee, abduction at the shoulder) demonstrates the technical precision the study design expects.

Muscle fibre types

Skeletal muscle fibres come in different types with different functional properties.

Slow-twitch (Type I) fibres

• High mitochondrial density.
• High capillary density.
• High myoglobin content (gives them a red appearance).
• Fatigue-resistant.
• Lower peak force production.
• Aerobic energy production dominant.

Slow-twitch fibres are well-suited to endurance activity. Marathon runners typically have 70-80% slow-twitch fibres in their key muscles.

Fast-twitch Type IIa fibres

• Intermediate properties.
• Moderate mitochondrial and capillary density.
• Capable of both aerobic and anaerobic work.
• Moderate fatigue resistance.
• Higher peak force than Type I.

Type IIa fibres are recruited during high-intensity efforts that exceed aerobic capacity but are not maximal.

Fast-twitch Type IIx fibres

• Low mitochondrial density.
• Low capillary density.
• Anaerobic energy production dominant.
• Rapid fatigue.
• Highest peak force production.

Type IIx fibres are recruited for maximal, brief efforts (sprinting, jumping, throwing).

Distribution and training

Fibre type distribution is largely genetic. Elite sprinters tend to have 70-80% fast-twitch fibres in their key muscles; elite marathon runners tend to have 70-80% slow-twitch.

Training can shift Type IIx toward IIa with endurance work (more aerobic-leaning) or shift the other way with sprint and power work. The broad ratio (Type I vs Type II) is set by birth.

How this dot point applies

A typical Unit 1 exam question is "Explain the muscular and skeletal contributions to a movement in a sport of your choice". Strong responses:

1. Name the specific movement using precise terminology.
2. Identify the joints involved and their type.
3. Name the muscles producing the movement and their contraction type.
4. Note any sport-relevant fibre type considerations.

The Unit 3 dot points on energy systems and skill acquisition build on this anatomical foundation.

Examples in context

Example 1. A basketball jump shot. As the player rises, the ankle plantar flexes, the knee extends and the hip extends. At the knee, the quadriceps contract concentrically (shortening) to drive extension; on landing, the same quadriceps contract eccentrically to absorb force and control knee flexion. The explosive take-off recruits fast-twitch Type II fibres through the ATP-PC system, which is why elite jumpers tend toward a high Type II proportion. The knee acts as a hinge joint (flexion and extension), while the shoulder, a ball-and-socket joint, allows the flexion and external rotation of the shooting arm.

Example 2. The eccentric load of downhill running in the Melbourne Marathon. Running downhill, the quadriceps repeatedly contract eccentrically to control knee flexion and decelerate the body against gravity. Eccentric contractions produce more force than concentric and cause more micro-damage to muscle fibres, which is why downhill sections produce more delayed-onset muscle soreness than flat or uphill running of the same intensity. Endurance runners rely heavily on fatigue-resistant slow-twitch Type I fibres, with their high mitochondrial and capillary density supporting sustained aerobic work.

Try this

Q1. Name the three types of skeletal muscle contraction and give a sporting example of each. [3 marks]

• Cue. Concentric (shortening, e.g. lifting phase of a biceps curl); eccentric (lengthening under load, e.g. lowering phase of a squat); isometric (no length change, e.g. holding a plank).

Q2. For a sporting movement of your choice, identify one joint involved, classify it by structure and sub-type, and state two movements it allows. [3 marks]

• Cue. E.g. the shoulder is a synovial ball-and-socket joint allowing flexion/extension, abduction/adduction, rotation and circumduction.

Q3. Compare slow-twitch (Type I) and fast-twitch (Type II) fibres on three characteristics, and state which athlete (marathon runner or sprinter) would be expected to have more of each. [4 marks]

• Cue. Type I: high mitochondrial/capillary/myoglobin density, fatigue-resistant, lower peak force, aerobic, favoured in marathon runners. Type II: low mitochondrial/capillary density, rapid fatigue, high peak force, anaerobic, favoured in sprinters.