Joints & joint actions

Movement happens at synovial joints — the shoulder and hip are ball-and-socket, the elbow and knee are hinge. Each allows named joint actions:

• Flexion — decreasing the angle at a joint (bending the elbow); extension — increasing it (straightening).
• Abduction — moving a limb away from the body's midline; adduction — moving it back towards the midline.
• Rotation, circumduction, and at the ankle plantar-flexion (pointing toes) and dorsiflexion (pulling toes up).

Analyse a movement: name the joint, the articulating bones, the action, and the agonist producing it. Example: the upward phase of a biceps curl = flexion at the elbow (radius/ulna & humerus), agonist biceps brachii.

Agonist, antagonist & the muscle pair

Muscles can only pull, so they work in antagonistic pairs. For any action:

• The agonist (prime mover) contracts to create the movement.
• The antagonist relaxes and lengthens to allow it.
• A fixator stabilises the origin so the agonist can act efficiently.

Contraction type matters too: concentric (muscle shortens under tension, e.g. biceps on the upward curl), eccentric (muscle lengthens under tension, controlling the lowering phase), and isometric (tension with no length change, e.g. a plank).

Elbow example: flexion — biceps brachii is agonist (concentric), triceps brachii is antagonist. Extension reverses the roles: triceps agonist, biceps antagonist.

Name the agonist

Muscle-fibre types

Skeletal muscle contains a mix of three fibre types. The proportion you inherit helps decide what you are suited to:

• Type I — slow oxidative (SO): red, rich in myoglobin, mitochondria and capillaries. Contracts slowly, resists fatigue — endurance work (marathon).
• Type IIa — fast oxidative glycolytic (FOG): contracts fast, moderate fatigue resistance, uses both aerobic and anaerobic energy — middle-distance / 800 m.
• Type IIx — fast glycolytic (FG): white, few mitochondria, large force, fatigues quickly — sprints and power (100 m, shot put).

Training effect: endurance training makes Type IIa fibres behave more aerobically. Fibre type ratios are largely genetic, but efficiency within a type can improve.

Which fibre type?

Tap a property, then tap the fibre type it belongs to.

Best fibre for the marathon

The heart & cardiac cycle

The heart is a double pump. The cardiac cycle alternates diastole (chambers relax and fill) and systole (chambers contract and eject blood). It is myogenic — the SA node sets the rhythm, the impulse passes to the AV node, down the bundle of His and Purkinje fibres.

Cardiac output, venous return & the vascular shunt

The volume of blood the heart pumps per minute is cardiac output (Q):

• Stroke volume rises with venous return (blood returning to the heart). Starling's law: the more the ventricle fills and stretches, the harder it contracts, so SV increases.
• During exercise the vascular shunt redirects blood — vasodilation of arterioles to working muscles, vasoconstriction to the gut — helped by the venous pump (skeletal-muscle and respiratory pumps) aiding venous return.

Units: if SV is in ml and HR in bpm, Q comes out in ml per minute; divide by 1000 for litres per minute.

Your turn — cardiac output

Your turn — find stroke volume

Maximum heart rate & training zones

Coaches estimate a performer's maximum heart rate from a simple formula:

Karvonen-style aerobic training zones sit at roughly 60–80% of HR_max. Working below this builds an aerobic base; above it edges into anaerobic work near the lactate/anaerobic threshold.

Remember: 220 − age is an estimate. It ignores individual variation, but AQA still expects you to use it to set target zones.

Your turn — maximum heart rate

Your turn — target zone

Starling's law

Breathing & gas exchange

Breathing is driven by pressure changes. On inspiration the diaphragm flattens and the external intercostals lift the ribs, increasing thoracic volume so pressure falls and air flows in. At rest expiration is passive (elastic recoil); during exercise the internal intercostals and abdominals force it.

At the alveoli, gases move by diffusion down partial-pressure gradients: oxygen from alveolar air (high pO₂) into blood, carbon dioxide the other way. The huge alveolar surface area and thin walls make this efficient.

a–vO₂ difference: the difference in oxygen content between arterial and venous (mixed) blood. It widens during exercise as muscles extract more oxygen from each unit of blood.

The oxyhaemoglobin dissociation curve

Plotting % saturation of haemoglobin against pO₂ gives an S-shaped (sigmoid) curve. At the lungs (high pO₂) haemoglobin is nearly fully saturated; at the tissues (low pO₂) it readily releases oxygen.

The Bohr shift: raised CO₂, higher temperature and lower pH (more acidic, from lactic acid) shift the curve right, reducing haemoglobin's affinity for oxygen so it releases more O₂ to hard-working muscles.

Why shift right?

Match each term to its meaning

Tap a term on the left, then its matching definition on the right.

Diet & nutrition for performance

A balanced diet fuels training and aids recovery. Key fractions of the seven components:

• Carbohydrate — the main fuel for moderate–high intensity work, stored as glycogen in muscle and liver.
• Fats — a major fuel for low-intensity, long-duration exercise.
• Protein — for growth and repair of tissue, important after resistance training.
• Water, vitamins, minerals and fibre — hydration, enzyme function and gut health.

Timing matters: carbohydrate loading before endurance events maximises glycogen stores; refuelling within the recovery window replenishes them and supports the EPOC recovery process.

The big ideas to know

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