Exercise Physiology

Physical activity places some of the highest metabolic demands on the body. Cardiac output (CO) can increase from a resting 5 L/min up to 25-30 L/min in elite athletes, primarily through increases in heart rate and, to a lesser extent, stroke volume. Blood flow is redistributed away from the GI tract and kidneys toward skeletal muscles via sympathetic-mediated vasoconstriction and local metabolic vasodilation (active hyperemia). Ventilation increases significantly (hyperpnea) to maintain blood gases; during intense exercise, the 'anaerobic threshold' is reached, where lactic acid production exceeds its clearance, leading to an extra drive for CO2 excretion. Energy is supplied sequentially: immediate (ATP and Phosphocreatine), short-term (anaerobic glycolysis), and long-term (aerobic metabolism of glucose and fatty acids). Muscle fiber recruitment switches from Type I (slow-twitch, oxidative) for endurance to Type II (fast-twitch, glycolytic) for power. Chronic exercise leads to adaptations such as eccentric ventricular hypertrophy ('Athlete's heart'), increased capillary density, and improved mitochondrial efficiency, all facilitating a higher VO2 max (maximum oxygen uptake).

At the start of exercise, the 'oxygen deficit' is met by anaerobic pathways until aerobic metabolism catches up. After exercise, 'Excess Post-exercise Oxygen Consumption' (EPOC) occurs to restore PCr stores, clear lactate (via the Cori cycle in the liver), and replenish O2 on hemoglobin/myoglobin. Hemodynamically, the increase in CO is facilitated by the skeletal muscle pump and respiratory pump (increased venous return). Peripherally, locally produced metabolites like K+, H+, Adenosine, and CO2 cause vasodilation of resistance vessels. The 'Lactate Threshold' is the point where blood lactate begins to accumulate exponentially, correlating with the onset of fatigue. Respiratory Exchange Ratio (RER) correlates CO2 produced to O2 consumed; an RER of 0.7 indicates pure fat oxidation, 1.0 indicates pure carbohydrate oxidation. Temperature regulation is maintained by increased skin blood flow and sweating, though this can lead to dehydration and electrolyte loss if not managed.

Exercise is a vital therapeutic intervention for chronic diseases like DM Type 2 (increases GLUT4 expression), Hypertension, and IHD. Cardiopulmonary Exercise Testing (CPET) is the gold standard for preoperative risk stratification and assessing unexplained breathlessness. 'Athlete's heart' may present with physiological bradycardia and ECG changes (e.g., LVH voltage criteria) that must be distinguished from pathology. Red flags during exercise include syncope, chest pain, or sudden cardiac death (often due to Hypertrophic Cardiomyopathy/HOCM).

ECG (resting and stress testing). CPET measures VE/VCO2 and VO2 peak. Pulse oximetry during exercise. Blood lactate levels can be measured in a lab setting to determine training zones.

Exercise prescription ('Exercise as Medicine') is part of NICE guidance for many conditions. For athletes, nutrition (carbohydrate loading) and hydration strategies are key. Management of overtraining syndrome involves rest and physiological monitoring.