Exercise Physiology: What Happens to Your Body During Exercise

Cardiovascular Response to Exercise

Within seconds of beginning exercise, the cardiovascular system initiates a cascade of adjustments to deliver more oxygen to working muscles. Heart rate increases immediately through withdrawal of vagal (parasympathetic) tone, followed by progressive sympathetic activation as exercise intensity rises. Stroke volume (the amount of blood ejected per heartbeat) increases by 20 to 50% through two mechanisms: the Frank-Starling effect (increased venous return from muscle pump action stretches the ventricle, increasing contractile force) and enhanced contractility from sympathetic stimulation and circulating catecholamines (epinephrine and norepinephrine).

The combined effect of increased heart rate and stroke volume raises cardiac output from a resting value of approximately 5 liters per minute to 20 to 25 L/min in healthy adults and up to 35 to 40 L/min in elite endurance athletes. Simultaneously, blood flow is redistributed by selective vasoconstriction in non-essential vascular beds (kidneys receive 50% less blood, splanchnic organs 80% less) and vasodilation in working muscle (which receives up to 85% of cardiac output during maximal exercise, compared to 15 to 20% at rest). Skeletal muscle blood flow can increase 20 to 25-fold in active muscles, driven by local metabolic vasodilators including nitric oxide, adenosine, hydrogen ions, and potassium ions released by contracting muscle fibers.

Systolic blood pressure rises progressively with exercise intensity (from 120 to 180 to 220 mmHg during maximal effort) because cardiac output increases faster than total peripheral resistance decreases. Diastolic pressure remains relatively stable or decreases slightly because vasodilation in the large muscle mass offsets vasoconstriction elsewhere. Mean arterial pressure increases modestly, driving the higher cardiac output through the expanded vascular network of dilated muscle arterioles. In resistance exercise (weight lifting), both systolic and diastolic pressures spike dramatically (measurements above 300/200 mmHg have been recorded during heavy leg press), because the Valsalva maneuver and external muscle compression raise intrathoracic pressure and compress blood vessels.

Respiratory Adaptations During Exercise

Breathing rate and depth increase in proportion to metabolic demand, with minute ventilation (the volume of air moved per minute) rising from approximately 6 liters per minute at rest to 100 to 150 L/min during maximal exercise in trained individuals. Below the ventilatory threshold (approximately 50 to 60% of VO2max in untrained individuals), ventilation increases linearly with oxygen consumption. Above this threshold, ventilation rises disproportionately as the body compensates for lactic acid production by increasing CO2 exhalation (since bicarbonate buffering of lactic acid releases CO2).

At the alveolar level, gas exchange efficiency actually improves during exercise because cardiac output increases pulmonary blood flow, reducing the amount of time required for gas equilibration and recruiting capillaries in upper lung regions that are underperfused at rest (due to gravity). The transit time of blood through pulmonary capillaries decreases from 0.75 seconds at rest to as low as 0.25 seconds during maximal exercise, but this is still sufficient for nearly complete oxygen loading of hemoglobin in healthy individuals. In elite athletes pushing VO2max above 70 mL/kg/min, transit time can become limiting, causing a slight drop in arterial oxygen saturation (exercise-induced arterial hypoxemia).

Muscle Fiber Recruitment and Energy Systems

Skeletal muscle contains two primary fiber types recruited in an orderly fashion according to Henneman's size principle. Type I (slow-twitch, oxidative) fibers are recruited first at low exercise intensities. These fibers have high mitochondrial density, rich capillary supply, and high myoglobin content (giving them a red color), making them fatigue-resistant but limited in force production. As exercise intensity increases, Type II fibers are progressively recruited: first Type IIa (fast-twitch, oxidative-glycolytic) fibers with moderate fatigue resistance, and finally Type IIx (fast-twitch, glycolytic) fibers capable of high force production but rapid fatigue.

Three energy systems supply ATP (adenosine triphosphate, the immediate energy currency) to contracting muscles, with their relative contributions determined by exercise intensity and duration:

The phosphocreatine system (0 to 10 seconds): Creatine kinase transfers a phosphate group from phosphocreatine to ADP, regenerating ATP almost instantaneously. This system supports maximum-intensity efforts (sprinting, jumping, throwing) but depletes within 6 to 10 seconds because intramuscular phosphocreatine stores are limited to approximately 80 to 100 mmol/kg of muscle.

Anaerobic glycolysis (10 seconds to 2 minutes): Glucose (from muscle glycogen or blood glucose) is broken down to pyruvate in the cytoplasm, yielding 2 ATP per glucose molecule. Under high-intensity conditions where oxygen delivery cannot match demand, pyruvate is converted to lactate (not "lactic acid," as it exists predominantly in its dissociated form at physiological pH). Lactate is not a waste product but a valuable fuel: it is transported to other muscles, the heart, and the liver (where it is reconverted to glucose via gluconeogenesis in the Cori cycle). The "burn" during intense exercise results from hydrogen ion accumulation that accompanies glycolysis, not from lactate itself.

Aerobic oxidative phosphorylation (2 minutes onward): Pyruvate enters mitochondria, is converted to acetyl-CoA, and feeds into the Krebs cycle and electron transport chain, yielding approximately 36 to 38 ATP per glucose molecule (18 to 19 times more efficient than glycolysis alone). Fatty acids undergo beta-oxidation to generate acetyl-CoA, yielding even more ATP per molecule (for example, palmitate yields 129 ATP). At low to moderate exercise intensities (below 60% VO2max), fat oxidation provides the majority of energy. As intensity increases, the fuel mix shifts progressively toward carbohydrates because glycolysis can generate ATP faster than fat oxidation, though fat oxidation continues at all intensities.

Hormonal Responses to Exercise

Exercise triggers a coordinated hormonal response that mobilizes fuel, maintains blood glucose, and supports cardiovascular function. Catecholamines (epinephrine and norepinephrine) increase 2 to 5-fold during moderate exercise and up to 15 to 20-fold during maximal efforts, stimulating glycogen breakdown in liver and muscle, fat mobilization from adipose tissue, and increased cardiac contractility. Cortisol rises during prolonged exercise (beyond 60 to 90 minutes), promoting gluconeogenesis and protein catabolism to maintain blood glucose as glycogen stores deplete.

Growth hormone secretion increases 5 to 10-fold during high-intensity exercise, peaking about 30 minutes after exercise onset, stimulating fat mobilization and protein synthesis. Insulin secretion decreases during exercise (mediated by alpha-adrenergic inhibition of pancreatic beta cells) despite stable or rising blood glucose, because muscle contraction independently stimulates glucose uptake via GLUT4 transporter translocation without requiring insulin. This insulin-independent glucose uptake explains why exercise effectively lowers blood glucose in diabetic patients and why post-exercise insulin sensitivity remains elevated for 24 to 72 hours.

Testosterone rises acutely during resistance exercise (by 15 to 30% in men), particularly during heavy compound movements using large muscle groups. This acute elevation, combined with the mechanical tension, metabolic stress, and muscle damage from resistance training, triggers the signaling cascades (particularly mTOR pathway activation) that drive muscle protein synthesis and hypertrophy over weeks of consistent training. Women produce the same percentage increase in testosterone but from a baseline 10 to 20 times lower, which partially explains (along with differences in absolute muscle mass, muscle fiber size, and neural activation patterns) why women typically gain muscle mass more slowly than men.

Thermoregulation During Exercise

Exercising muscles convert approximately 75 to 80% of metabolic energy into heat rather than mechanical work, creating a significant thermal challenge. During vigorous exercise, metabolic heat production can exceed 1,000 watts (compared to 80 to 100 watts at rest), sufficient to raise core body temperature by 1 degree Celsius every 5 to 7 minutes if no heat dissipation occurred. The thermoregulatory system responds by redirecting blood flow to the skin (up to 6 to 8 L/min at skin capillaries) for radiative and convective heat loss, and by activating eccrine sweat glands that can produce 1.5 to 2.5 liters of sweat per hour in heat-acclimated individuals.

Core temperature typically rises 1 to 2 degrees Celsius during exercise before stabilizing at a new equilibrium where heat production equals heat dissipation. However, in hot and humid environments, or during prolonged intense exercise, the equilibrium may not be achievable, and core temperature continues rising toward dangerous levels. Heat exhaustion (core temperature 38.5 to 40 degrees Celsius) produces heavy sweating, weakness, nausea, and dizziness. Heat stroke (core temperature above 40 degrees Celsius) is a medical emergency in which thermoregulation fails, sweating may cease, and organ damage (particularly to the brain, liver, and kidneys) can be fatal without immediate cooling. Dehydration (losing 2 to 3% of body mass through sweat) impairs thermoregulation by reducing blood volume available for both muscle perfusion and skin blood flow.

Training Adaptations: How the Body Improves

Regular exercise produces structural and biochemical adaptations that improve performance capacity and health outcomes. Endurance training (consistent aerobic exercise over 8 to 12 weeks) increases VO2max by 15 to 25% in previously sedentary individuals through central adaptations (increased left ventricular volume, lower resting heart rate from 70 to 50 beats/min, increased blood volume by 10 to 15%, and increased stroke volume) and peripheral adaptations (increased mitochondrial density in muscle by 50 to 100%, increased capillary density, enhanced fat oxidation capacity, and improved oxygen extraction).

Resistance training produces neural adaptations within the first 2 to 4 weeks (improved motor unit recruitment, firing rate, and synchronization, increasing strength by 10 to 15% without measurable hypertrophy), followed by muscular hypertrophy over 6 to 12 weeks (individual muscle fiber diameter increases of 20 to 30%, increasing total muscle cross-sectional area). Muscle protein synthesis is elevated for 24 to 48 hours after each training session, and net muscle growth occurs when cumulative protein synthesis exceeds protein breakdown over training weeks, requiring both mechanical stimulus (training) and substrate availability (dietary protein intake of 1.6 to 2.2 g/kg/day for optimal hypertrophy).

Bone responds to mechanical loading by increasing mineral density and adjusting architecture along lines of stress (Wolff's law). Weight-bearing exercise stimulates osteoblast activity while suppressing osteoclast-mediated resorption, with site-specific effects (running increases hip and spine density but not arms, while tennis players show 10 to 15% greater bone density in their racket arm). This adaptation is most critical during adolescence and young adulthood when peak bone mass is being established, with higher peak bone mass providing greater reserve against age-related decline and osteoporosis.