--- module: 104-01 language: en chapter: 104 title: "Exercise, Adaptation, Thermoregulation, Altitude, and Extreme Environments" module_title: "Exercise energetics and integrated physiological response" source_sha256: 937011da9e5805ee0fdaacb25850f6e00238de3efc8030826dcbb266d54486c4 --- # Exercise energetics and integrated response ## Energy systems ### ATP stores sustain maximal contraction for seconds ### Phosphocreatine buffers abrupt demand #### Creatine kinase donates phosphate to ADP ### Anaerobic glycolysis makes ATP rapidly #### Pyruvate to lactate regenerates oxidised cofactor ### Oxidative phosphorylation: slower, far greater capacity #### Carbohydrate, fat, some amino acids ### Systems work simultaneously, not in sequence #### Sprint: high phosphagen and glycolytic flux #### Prolonged moderate: mitochondria, glycolysis continues ### Lactate does not mean absent oxygen #### Reflects glycolytic flux and redox balance #### Oxidised by muscle and heart, or hepatic glucose ## Cardiac output and blood flow ### Oxygen uptake: cardiac output x a-v O2 difference ### Sympathetic activation, vagal withdrawal raise heart rate ### Pumps, venoconstriction, redistribution raise return ### Stroke volume rises, then often plateaus #### Endurance-trained may keep increasing ### Metabolic vasodilation in active muscle #### Functional sympatholysis opposes constriction ### Sympathetic constriction in less active beds ### Skin flow later rises for heat loss ### Mean arterial pressure rises moderately #### Cardiac output rises more than resistance falls ## Resistance exercise haemodynamics ### Contracting muscle compresses vessels ### Very high transient pressure with breath holding #### Valsalva alters return, pressure, baroreflex ### Breathing, technique, load modify risk ### Acute pressor response is not chronic hypertension ## Ventilation and gas exchange ### Immediate rise: central command, sensory feedback ### Then follows carbon dioxide and acid-base demand ### Arterial gases near normal at moderate exercise ### Tidal volume first, frequency at high intensity ### Beyond ventilatory threshold: disproportionate rise #### Bicarbonate buffering adds carbon dioxide ### Capillary recruitment raises diffusion, perfusion ### Arterial desaturation when reserve is exceeded #### Short transit, mismatch, diffusion limitation ## Oxygen transport and extraction ### Content set by haemoglobin and saturation #### Anaemia demands higher cardiac output #### Carbon monoxide reduces carriage, distorts readings ### Heat, carbon dioxide, acidity aid unloading ### Extraction: perfusion, gradients, myoglobin, mitochondria ### Delivery and utilisation are interdependent #### Mitochondria cannot offset severe flow limits #### Flow cannot help without functional mitochondria ### Maximal uptake reflects a specific test mode ## Recruitment and substrates ### Low-threshold fatigue-resistant units first ### Larger faster units added as force rises ### Fibre phenotypes lie on adaptable continua ### Order altered by ballistic tasks, stimulation, injury ### Carbohydrate: faster ATP, more per oxygen #### Muscle glycogen local, liver sustains blood glucose ### Fat dominates at low intensity, long duration ### Carbohydrate use rises with intensity #### Crossover shifted by training, diet, hormones ## Hormones and fluid ### Catecholamines: heart, glycogenolysis, lipolysis ### Insulin falls, contraction drives glucose uptake ### Glucagon, catecholamines support hepatic output ### Cortisol, growth hormone in long exercise ### Graded by relative intensity #### Smaller stress response after training ### Plasma volume loss causes cardiovascular drift #### Stroke volume falls, heart rate rises ### Dehydration and hypotonic overdrinking both dangerous ## Efficiency, perception, and fatigue ### Efficiency: external work over energy expended #### Most energy becomes heat ### Economy improves via technique, elastic energy ### Perceived exertion integrates many signals #### Not a disguised heart rate or lactate ### Deception about distance alters output ### Peripheral fatigue: metabolites, coupling, ions ### Central fatigue: reduced effective neural drive ## Testing and integration ### Protocols link workload to responses ### Interpret mode, effort, medication, stop reason ### Peak may be limited before physiological maximum ### Submaximal thresholds locate the constraint ### Match ATP demand to oxygen, substrate, heat removal ### Disease shifts the bottleneck ### Exercise as a stress test of reserve