Joints & joint actions

Movement is produced at synovial joints. The shoulder and hip are ball-and-socket joints (movement in every plane); the elbow and knee are hinge joints. Each joint permits specific joint actions:

• Flexion decreases the joint angle (a rower drawing the handle towards the chest bends the elbow); extension increases it (the drive phase straightening the legs).
• Abduction moves a limb away from the midline (a gymnast raising the arms sideways into a cross position); adduction brings it back.
• The shoulder also allows horizontal flexion / extension and rotation, giving the widest range of any joint.

Analyse a movement: state the joint type, the articulating bones, the action and the agonist. Example: the upward drive of a shoulder press = extension/abduction at the shoulder (humerus & scapula), agonist the deltoid.

Agonist, antagonist & contraction type

Because muscles can only pull, they are arranged in antagonistic pairs across a joint. In any movement:

• The agonist (prime mover) develops the tension that controls the action.
• The antagonist relaxes on the opposite side of the joint to allow the movement.
• A fixator stabilises the origin so the agonist can act efficiently.

Three contraction types describe what the agonist does: concentric (muscle shortens under tension, e.g. the triceps extending the elbow to lock out a shoulder press), eccentric (muscle lengthens under tension while resisting a load, e.g. the biceps controlling the bar back down), and isometric (tension with no length change, e.g. a gymnast holding an L-sit).

Elbow example: during a triceps dip the triceps act concentrically to extend the elbow on the way up, and eccentrically to control the lowering phase.

Name the contraction

Muscle-fibre types

Every skeletal muscle contains a blend of three fibre types, and the ratio you inherit influences the events you are suited to:

• Type I — slow oxidative (SO): red, rich in myoglobin, mitochondria and capillaries. Slow to contract, low force, but very fatigue-resistant — endurance events.
• Type IIa — fast oxidative glycolytic (FOG): quick to contract with moderate fatigue resistance, drawing on both aerobic and anaerobic systems — 800 m / middle-distance work.
• Type IIx — fast glycolytic (FG): white, few mitochondria, produces the greatest force but fatigues almost immediately — sprinting, jumping and throwing.

Training effect: the fibre-type ratio is largely inherited, but training can shift Type IIa fibres towards more aerobic behaviour and raise efficiency within each type.

Which fibre type?

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

Best fibre for the 800 m

The cardiac cycle & conduction

The heart is a double pump. The cardiac cycle alternates diastole (chambers relax and fill) with systole (chambers contract and eject blood). The rhythm is myogenic — generated within the heart. The SA node sets the pace, the impulse passes to the AV node, then travels down the bundle of His and out along the Purkinje fibres to make the ventricles contract.

Cardiac output, venous return & the vascular shunt

The volume of blood the heart ejects each minute is the cardiac output (Q):

• Stroke volume is governed by venous return — the blood returning to the heart. Starling's law: the more the ventricle fills and stretches, the more forcefully it contracts, so SV increases.
• During exercise the vascular shunt mechanism redirects blood — vasodilation of arterioles to working muscle and vasoconstriction to the gut — while the skeletal-muscle and respiratory pumps aid venous return.

Units: with SV in ml and HR in bpm, Q is in ml per minute; divide by 1000 to give 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 using a simple formula:

Aerobic training zones lie roughly between 60% and 80% of HR_max. Below this range develops a broad aerobic base; working above it takes the performer towards the anaerobic / lactate threshold.

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

Your turn — maximum heart rate

Your turn — target zone

The vascular shunt

Breathing & gas exchange

Breathing is driven by pressure changes. During inspiration the diaphragm flattens and the external intercostals raise the ribcage, increasing thoracic volume so pressure drops and air flows in. At rest expiration is passive (elastic recoil); during exercise the internal intercostals and abdominals actively force air out.

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

a–vO₂ difference: the difference in oxygen content between arterial and mixed venous blood. It widens during exercise as the 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. In the lungs (high pO₂) haemoglobin is almost fully saturated; at the tissues (low pO₂) it readily gives up oxygen.

The Bohr shift: a rise in CO₂, a higher temperature and a lower pH (more acidic, from lactic acid) shift the curve right, lowering haemoglobin's affinity for oxygen so more O₂ is released to the working muscles.

Oxygen extraction

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 drives recovery. The roles of the main components:

• Carbohydrate — the main fuel for moderate-to-high intensity work, stored as glycogen in muscle and the liver.
• Fats — the principal fuel for prolonged, low-intensity exercise.
• Protein — for the growth and repair of tissue, particularly after resistance training.
• Water, vitamins, minerals and fibre — hydration, enzyme and immune function, and gut health.

Timing matters: carbohydrate loading before an endurance event maximises glycogen stores, and refuelling within the recovery window restores them and supports the EPOC recovery process.

The big ideas to know

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