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Organ Systems Involved

Movement requires a coordinated response of multiple organ systems. When the body engages in regular physical activity, all physiologic systems undergo specific adaptations to increase movement efficiency and exercise capacity.

Musculoskeletal System

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Muscular adaptations to exercise involve changes in muscle fiber composition and function driven by the specific demands of physical activity. These adaptations, alongside skeletal responses such as increased bone mineral density, are essential for optimizing training strategies, enhancing athletic performance, and promoting long-term musculoskeletal health.

Muscular adaptations to exercise

The musculoskeletal system is responsible for regulating the strength, speed, and coordination required to perform physical tasks. Muscle fibers may be classified based on their myosin heavy chain isoforms, which determine their shortening velocities, or by their oxidative capacity, which relates to their metabolism and fatigability.[7]

The composition of a subject’s muscle fibers can impact sports performance. Individuals with a higher proportion of type I fibers tend to excel in longer-duration events. In contrast, individuals with more type II fibers generally perform better in shorter, higher-speed events. Training at slower speeds with higher loads can result in a shift from IIx and IIx/IIa hybrids to a more pure IIa phenotype, with minimal change in the pure Ia phenotype. Conversely, high-speed, high-power training can reduce type Ia fibers and produce a shift toward the faster IIx/IIa phenotype.

Muscle contraction initiates movement by acting on the skeleton. Muscles adapt to increasing loads over time through exercise training, resulting in muscle fiber hypertrophy and increased muscle diameter and volume. Satellite cells are positioned on the outer edge of muscle fibers closely linked to the plasma membrane. These myogenic precursors are essential for supporting the development of skeletal muscle fiber adaptations to loading. These cells also play a vital role in muscle hypertrophy and repair. Exercise, whether through long-distance running or powerlifting, stresses the muscle fibers and bones, causing microtears that activate and mobilize satellite cells to regenerate the damaged muscle tissue.[10][11]

Skeletal adaptations to exercise

Bone remodeling occurs in response to mechanical stimuli and involves an increase in mineral density to manage increasing loads. Mechanical loading during childhood and adolescence enhances bone formation and strength, helping to prevent osteoporosis in later life, with similar effects seen in adulthood.[12]

Cardiovascular System

The cardiovascular system plays a crucial role in maintaining homeostasis during exercise by responding directly to the oxygen requirements of working muscles. To support the increased metabolic activity in skeletal muscle, the circulatory system regulates the transport of oxygen (O2) and carbon dioxide (CO2) and helps buffer the pH decrease in active tissues. Increasing cardiac output—the amount of blood pumped by the heart in 1 minute, calculated as heart rate × stroke volume—and modulating microvascular circulation achieve this adjustment in blood flow. As the workload rises, so does the cardiac output to meet the heightened metabolic demands. Additionally, local vasomediators, such as nitric oxide produced by endothelial cells, ensure sufficient local tissue blood flow as demand increases.

Aerobic exercise training leads to cardiovascular adaptations, including cardiac enlargement, enhanced myocardial contractility, and an increase in total blood volume. These adaptations enable greater ventricular filling and an increased stroke volume, measured in mL/beat. Furthermore, the increased capillary density improves the effective delivery of O2 to the tissues during exercise.[13]

Blood flow is preferentially shunted away from the gastrointestinal and renal systems and toward active muscles through selective constriction and dilation of capillary beds.[14] The increased skeletal muscle blood flow delivers O2 and aids in the removal of CO2. During exercise, the affinity of oxyhemoglobin for O2 decreases due to increased temperature, lower blood pH, and increased CO2 concentration. The reduced affinity enables red blood cells (RBCs) to extract CO2 and release O2 to the working muscles efficiently.[15][16]

The coronary arteries supply the myocardium with O2 and nutrients while removing metabolites. Increased cellular metabolism during exercise leads to increased coronary blood flow through vasodilation and capillary bed recruitment. This adaptation elevates O2 demand during exercise. The major determinants of myocardial O2 consumption (VO2) are heart rate, contractility, and myocardial wall stress, all of which markedly rise during exercise, thereby requiring a substantial increase in coronary blood flow.

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The primary role of RBCs during exercise is to transport O2 from the lungs to the tissues and carry metabolically produced CO2 to the lungs for exhalation. On a mechanical level, senescent RBCs tend to be less compliant and are hemolyzed intraluminally when passing through capillaries in contracting muscles. This activity leads to an average decrease in RBC age since the younger RBCs have more favorable rheological properties. Younger RBCs also show increased O2 release compared to older RBCs. In addition, exercise increases erythropoietin levels, enhancing RBC production. These factors improve the body’s O2 supply, gas exchange, and metabolic capacity over time during exercise.[17]

Plasma

Plasma volume expansion typically occurs after acute endurance exercise and endurance training. Hypervolemia may occur within minutes or hours after stopping exercise, with peak plasma volume expansion typically reached around 2 days after a marathon or similar long-distance race. This expanded volume can persist for up to 2 weeks after initiating such physical activity.

Fluid-regulating hormones, such as aldosterone, arginine vasopressin, atrial natriuretic factor, and increased plasma protein content contribute to hypervolemia. Greater plasma volume can enhance performance by improving muscle perfusion, increasing stroke volume, and maximizing cardiac output. Plasma volume expansion also improves the body’s ability to regulate temperature during exercise by increasing skin blood flow. In most cases, an increase in plasma volume correlates with a lower hematocrit. True anemia results if plasma expansion is accompanied by red cell mass reduction. Relative anemia arises from plasma expansion without red cell mass lowering.[18]

Respiratory System

The respiratory system works in conjunction with the cardiovascular system to provide the tissues with O2. The respiratory system responds to exercise by immediately increasing pulmonary ventilation through the stimulation of the respiratory centers of the brainstem via the motor cortex and the muscle and joint proprioceptors during exercise.

The rise in CO2 production, hydrogen ions, and body temperature during exercise stimulates further increases in respiratory rate. In adults, pulmonary ventilation can increase from approximately 10 liters/minute at rest to more than 100 liters/minute at high-intensity efforts. The pulmonary circuit receives the same cardiac output as the systemic circuit. The available surface area for gas exchange increases in response to the increased cardiac output, resulting in a decrease in the alveolar dead space. Blood gas and pH balance can be maintained with more alveolar surface area available for gas exchange and increased alveolar ventilation due to increased frequency and volume of respiration. [19]

CO2 is one of the metabolic products of muscular activity and is carried away from peripheral active tissue, mostly as bicarbonate. A portion travels as dissolved CO2 in plasma and carbaminohemoglobin when bound to hemoglobin in RBCs. CO2 is readily incorporated into the RBC cytosol, where it is metabolized into carbonic acid by the enzyme carbonic anhydrase. Carbonic acid then spontaneously dissociates into a hydrogen ion and a bicarbonate ion. Once bicarbonate reaches the lungs, carbonic anhydrase catalyzes the reverse reaction to produce CO2, which is exhaled and removed from the body. The decreased alveolar dead space and increased tidal volume sustain the volume of carbon dioxide eliminated per unit of time in exercises of higher intensity.[20]

Endocrine System

Hormones modulate cellular growth. The key anabolic hormones whose function entails cellular growth and repair are discussed below.

Testosterone

Testosterone is an anabolic-androgenic steroid hormone that interacts with androgen receptors, stimulating skeletal muscle protein synthesis and, consequently, muscle hypertrophy. Testosterone levels increase, especially in response to resistance training.

Growth hormone

The pituitary gland releases growth hormone in response to acute and chronic exercise training. This hormone enhances bone and tissue growth.

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Insulin-like growth factor

Insulin-like growth factors (IGFs) are small polypeptide hormones structurally related to insulin produced by the liver in response to growth hormone stimulation. Other tissues also produce IGFs in response to mechanical loading. IGFs play a vital role in the activation and proliferation of satellite cells, leading to an increase in myotube size, the number of nuclei per myotube, and damage repair. Additionally, these factors stimulate protein synthesis and muscle hypertrophy, axonal sprouting, and neuronal myelination.

Glucocorticoids

Glucocorticoids, mainly cortisol, in addition to the anabolic hormones, significantly impact human skeletal muscle. Cortisol increases during exercise. This hormone participates in glycemic regulation by stimulating gluconeogenesis, mainly in the hepatocytes, and inhibiting glucose uptake in myocytes and adipocytes. Cortisol also stimulates lipolysis in adipocytes, increasing the availability of metabolic substrates in skeletal muscle. This hormone also counteracts cellular inflammation and cytokine synthesis, thus aiding in maintaining vascular integrity and decreasing muscular damage.[21]

Cortisol levels follow a circadian rhythm, peaking in the morning, gradually decreasing throughout the day, and reaching their lowest levels around midnight. Systemic regulation occurs through the hypothalamic-pituitary-adrenal axis, while local control involves the action of 11β-hydroxysteroid dehydrogenase enzymes.[22]

Catecholamines

Plasma levels of epinephrine, norepinephrine, and dopamine increase during maximal exercise and return to baseline at rest, reflecting enhanced sympathetic nervous system activation. Catecholamines stimulate the sympathetic nervous system, raising the heart rate and cardiac output while promoting coronary artery vasodilation to enhance blood flow to the working myocardium and meet heightened O2 demands.[23]

Insulin

Insulin sensitivity refers to the amount of insulin needed to achieve 50% of its maximum effect on glucose transport. Reduced insulin sensitivity, or insulin resistance, impairs insulin action on glucose uptake, increasing the risk of developing type 2 diabetes. Improved insulin sensitivity means less insulin is required to achieve 50% of the maximum response.

Insulin sensitivity increases after long-term exercise. Moderate-intensity exercise of at least 30 minutes 3 to 5 days a week is linked to improved glycemic control. Muscle contraction during exercise activates adenosine monophosphate-activated protein kinase, increasing glucose uptake through the translocation of glucose transporter type 4 vesicles into working myocytes. This process is independent of insulin and only affects the muscle fibers undertaking the work during exercise, not the hepatocytes or adipocytes. This increased glucose uptake by the muscles can last for several hours after exercise. Insulin sensitivity increases after the immediate effect of exercise on glucose transport wears off.[24]

Skin

During exercise, the increase in blood flow facilitates skin thermoregulation to help sustain higher core temperatures. Exercise training produces microvascular adaptations that enhance the endothelium-dependent vasomotor functions of the skin. Endurance training shifts the threshold for vasodilation, resulting in increased skin blood flow at lower core temperatures. Thus, higher skin blood flow can be achieved with enhanced heat dissipation, thereby allowing longer and greater effort intensity.[25]

Immune System

The immune system responds to the extent and duration of exercise. The acute immune response to exercise depends on the intensity of the effort. Moderate exercise induces acute increases in interleukin 6 that exert direct anti-inflammatory effects. Exercise also causes a transient increase in white blood cells. Thus, engaging in daily exercise enhances immunity. However, heavy exercise can produce a transient immune dysfunction. For example, acute episodes of intense and prolonged exercise can lower salivary immunoglobulin A, decrease natural killer cell lytic activity, impair T- and B-cell function, and increase the risk of upper respiratory infections during the first 1 to 2 weeks following a race.[26]

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