Joints & joint actions

Movement is produced at synovial joints. The knee and elbow are hinge joints; the hip and shoulder are ball-and-socket joints allowing movement in several planes. Each joint permits named joint actions:

• Flexion decreases the joint angle (drawing the leg back before a kick); extension increases it (the leg straightening to strike the ball).
• Abduction takes a limb away from the midline (raising the arm sideways); adduction returns it towards the midline.
• At the ankle, plantar-flexion points the foot (a footballer striking with the laces) and dorsiflexion pulls the toes upward.

Analyse a movement: state the joint type, the articulating bones, the action and the agonist. Example: the kicking phase of a football strike = extension at the knee (femur, tibia & patella), agonist the quadriceps group.

Agonist, antagonist & contraction type

Muscles generate force only by pulling, so they operate in antagonistic pairs across a joint. For any movement:

• The agonist (prime mover) shortens or lengthens under tension to control the action.
• The antagonist relaxes to permit the movement at the opposite side of the joint.
• A fixator holds the origin steady so the agonist can work efficiently.

Contraction type describes what the agonist does: concentric (shortens under tension, e.g. the quadriceps driving up out of a squat), eccentric (lengthens under tension while braking a load, e.g. the quadriceps controlling the descent), and isometric (tension with no change in length, e.g. holding a squat position or a wall-sit).

Knee example: on the way down in a squat the quadriceps act eccentrically to control knee flexion; on the drive up they act concentrically to produce knee extension.

Name the contraction

Muscle-fibre types

Skeletal muscle holds a mixture of three fibre types, and the ratio you inherit shapes what you are best suited to:

• Type I — slow oxidative (SO): red, dense in myoglobin, mitochondria and capillaries. Contracts slowly, produces low force but is highly fatigue-resistant — distance events.
• Type IIa — fast oxidative glycolytic (FOG): contracts quickly with a moderate resistance to fatigue, using both aerobic and anaerobic systems — 400 m / middle-distance work.
• Type IIx — fast glycolytic (FG): white, few mitochondria, generates the greatest force but fatigues within seconds — sprinting, throwing and jumping.

Training effect: the underlying fibre-type ratio is largely genetically fixed, but appropriate training can make Type IIa fibres behave more aerobically and improve efficiency within a type.

Which fibre type?

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

Best fibre for the shot put

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 beat is myogenic — it starts within the heart itself. The SA node sets the rhythm, the impulse reaches the AV node, then travels down the bundle of His and out through the Purkinje fibres.

Cardiac output, venous return & the vascular shunt

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

• Stroke volume depends on venous return — the volume of blood coming back to the heart. Starling's law: greater filling stretches the ventricle wall, so it contracts more forcefully and SV rises.
• During exercise the vascular shunt mechanism redistributes blood — vasodilation of arterioles feeding active muscle and vasoconstriction to the gut and skin — while the skeletal-muscle and respiratory pumps boost venous return.

Units: with SV in ml and HR in bpm, Q is in ml per minute; divide by 1000 to convert to 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:

Aerobic training zones sit roughly between 60% and 80% of HR_max. Below this range builds a general aerobic base; pushing above it moves the performer towards their anaerobic / lactate threshold.

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

Your turn — maximum heart rate

Your turn — target zone

Starting the heartbeat

Breathing & gas exchange

Breathing is driven by pressure changes. During inspiration the diaphragm flattens and the external intercostals lift the ribcage up and out, so thoracic volume rises, pressure falls and air is drawn 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, while carbon dioxide moves the opposite way. The vast alveolar surface area and one-cell-thick walls make exchange rapid.

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

The oxyhaemoglobin dissociation curve

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

The Bohr shift: a rise in CO₂, an increase in temperature and a fall in pH (more acidic, from lactic acid) shift the curve right, lowering haemoglobin's affinity for oxygen so it unloads more O₂ to the working muscles.

What causes the shift?

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 supports recovery. The roles of the key components:

• Carbohydrate — the main fuel for moderate-to-high intensity work, stored as glycogen in muscle and the liver.
• Fats — the dominant fuel during prolonged, low-intensity exercise.
• Protein — for the growth and repair of tissue, especially valuable 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 replenishes them and supports the EPOC recovery process.

The big ideas to know

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