Exercise Physiology
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Direct answer
In maximal exercise, cardiac output climbs from 5 to 20-25 L/min in an untrained adult — up to 35 L/min in endurance athletes — driven initially by stroke volume (70 rising to about 100-110 mL, plateauing near 40-50 per cent of maximal effort) and thereafter almost entirely by heart rate, which rises from 75 toward a maximum near 220 minus age. Oxygen consumption goes from a resting 250 mL/min to about 3 L/min in moderate work and 4-5 L/min at maximum for untrained individuals, with trained athletes reaching 70 mL/kg/min; the arteriovenous oxygen difference widens from 4-5 to as much as 15 mL/dL as extracting muscle beds recruit capillaries. Systolic pressure rises to 150-180 mm Hg while diastolic stays flat or falls in dynamic exercise, minute ventilation accelerates from 6 to 100 L/min or more, and the anaerobic threshold — where lactate accumulates — sits near 55-65 per cent of VO2 max in untrained people. The oxygen debt contracted at the start is repaid after exertion, when consumption remains elevated.
What you must remember
- Cardiovascular numbers: output 5 to 20-25 L/min (35 in athletes); stroke volume rises first and plateaus at 40-50 per cent of VO2 max; heart rate carries the rest, with the age-predicted maximum near 220 minus age.
- VO2 max: resting 250 mL/min oxygen consumption; untrained maximum 3-4 L/min; trained endurance athletes reach 4-5 L/min or 70 mL/kg/min — the single best index of cardiorespiratory fitness.
- Extraction: arteriovenous oxygen difference widens from 4-5 to about 15 mL/dL; muscle blood flow rises from 1 L/min at rest (15-20 per cent of output) to 20 L/min (about 80 per cent).
- Pressure behaviour: dynamic exercise raises systolic to 150-180 mm Hg with unchanged or falling diastolic; static (isometric) exercise raises both, which is why isometrics are discouraged in hypertension.
- Respiratory numbers: minute ventilation 6 L/min at rest to 100-150 L/min at maximum, rising linearly with carbon dioxide production.
- Oxygen debt and EPOC: at onset, anaerobic glycolysis covers the gap; after stopping, consumption stays raised to resynthesise ATP and creatine phosphate, clear lactate (mostly reconverted to glucose in the liver, the Cori cycle) and repay roughly 11.5 L of debt in exhausting work.
- Training adaptations: resting bradycardia of 40-50 per minute from higher vagal tone, a bigger stroke volume, higher blood volume, mitochondrial density and capillarisation, and physiological eccentric hypertrophy distinct from hypertrophic cardiomyopathy.
From rest to maximal effort
In the anticipatory first seconds, central command withdraws vagal tone — heart rate jumps before any metabolic signal. As effort builds, the muscle pump drives venous return upward; stroke volume rides the Frank-Starling curve, accounting for output growth to about half of maximal capacity. Beyond that the sarcomeres sit near their optimal length, the stroke volume plateaus, and every further litre comes from rate. Sympathetic outflow redistributes the output: splanchnic and renal shares shrink, skin flow dumps heat, and resting-closed muscle capillaries open — extraction nearly triples. Lactate appears near 55-65 per cent of VO2 max untrained (75 per cent or more trained), marking the anaerobic threshold where ventilation climbs disproportionately. In recovery the oxygen debt — up to about 11.5 litres after exhausting exercise — is repaid, restoring creatine phosphate and clearing lactate.
The distinction examiners probe
The screening paper likes static versus dynamic. Dynamic (isotonic) exercise — running, cycling — is a volume challenge: large stroke volumes, high output, modest pressure rise, left ventricular cavity enlargement with eccentric hypertrophy. Static (isometric) exercise — a hard handgrip, weightlifting — is a pressure challenge: compressed vessels during sustained contraction raise total peripheral resistance, so both systolic and diastolic pressures spike and afterload, not preload, drives ventricular work; the reason isometrics are curtailed in hypertensive patients. A second favourite is the trained heart at rest: sinus bradycardia of 40-50 with a left ventricular mass increase from cavity dilatation — physiological, and distinguishable from hypertrophic cardiomyopathy's asymmetric thickening with a small cavity.
Frequently asked questions
What limits further rise in cardiac output during graded exercise?
Stroke volume plateaus at roughly 40-50 per cent of VO2 max, after which heart rate alone raises output until it approaches its maximum near 220 minus age.
What is VO2 max and its typical values?
The maximum oxygen consumption per minute — about 250 mL/min at rest, 3-4 L/min maximal in untrained adults, and up to 4-5 L/min (around 70 mL/kg/min) in elite endurance athletes.
Why does diastolic pressure not rise during dynamic exercise?
Vigorous vasodilation in exercising muscle lowers total peripheral resistance enough to offset the rise in cardiac output, so diastolic pressure stays flat or even falls, unlike in static exercise.
What is the anaerobic threshold?
The oxygen consumption (roughly 55-65 per cent of VO2 max untrained) beyond which lactate accumulates in blood as anaerobic glycolysis contributes, with ventilation rising disproportionately.
Why do endurance athletes have resting bradycardia?
Chronic training increases vagal tone and stroke volume, allowing a heart rate of 40-50 per minute to maintain the resting 5 L/min output.