Glossary of Terms

The dark region of a sarcomere that spans the entire length of thick (myosin) filaments, including areas where they overlap with thin (actin) filaments.

The region within a sarcomere where the A band (containing thick myosin filaments) meets the I band (containing thin actin filaments).

The total volume of oxygen consumed by the body per unit of time, expressed in liters per minute (L/min), without adjusting for body weight.

The process by which an individual organism adjusts to a change in its environment—such as temperature, altitude, or humidity—over a short period of time.

The process by which an organism adjusts to changes in its environment over time, improving its ability to function under new environmental conditions.

A physiological condition characterized by an excessive accumulation of hydrogen ions (H⁺) in body fluids, leading to a decrease in blood pH below the normal range (7.35–7.45).

A globular protein that plays a central role in the structure and function of muscle cells and many other types of cells.

A rapid, temporary change in the electrical membrane potential of a neuron or muscle cell that allows the transmission of signals along the cell membrane. It occurs when a stimulus causes the membrane potential to reach a threshold, triggering the opening of voltage-gated ion channels. This results in a sequence of depolarization (influx of sodium ions) and repolarization (efflux of potassium ions), followed by a return to the resting potential.

A nucleotide composed of: adenine (a nitrogenous base), ribose (a five-carbon sugar), and two phosphate groups.

A nucleotide composed of three components: adenine (a nitrogenous base), ribose (a five-carbon sugar), and one phosphate group.

The primary energy carrier in all living organisms. It is a nucleotide composed of: adenine (a nitrogenous base), ribose (a five-carbon sugar), and three phosphate groups linked by high-energy bonds.

The outer region of the adrenal gland, responsible for producing steroid hormones. It consists of three zones: the zona glomerulosa (produces aldosterone), zona fasciculata (produces cortisol), and zona reticularis (produces androgens). These hormones regulate metabolism, immune function, blood pressure, and sexual development.

A pair of endocrine glands located above each kidney. Each adrenal gland consists of two distinct regions: the adrenal cortex, which produces steroid hormones such as cortisol, aldosterone, and androgens; and the adrenal medulla, which secretes catecholamines like adrenaline (epinephrine) and noradrenaline (norepinephrine). These hormones help regulate metabolism, immune response, blood pressure, and the body’s response to stress.

The inner region of the adrenal gland, responsible for producing and secreting catecholamines, primarily epinephrine (adrenaline) and norepinephrine (noradrenaline). These hormones are released in response to stress and help initiate the "fight-or-flight" response by increasing heart rate, blood pressure, and energy availability. The adrenal medulla functions as part of the sympathetic nervous system.

A peptide hormone produced and secreted by the anterior pituitary gland in response to corticotropin-releasing hormone (CRH) from the hypothalamus. ACTH stimulates the adrenal cortex, particularly the zona fasciculata, to produce and release glucocorticoids, primarily cortisol.

The process of generating ATP (energy) through the oxidation of carbohydrates, fats, and, to a lesser extent, proteins in the presence of oxygen.

A steroid hormone produced by the zona glomerulosa of the adrenal cortex. Aldosterone plays a key role in maintaining blood pressure and electrolyte balance by promoting sodium reabsorption and potassium excretion in the kidneys. Its release is primarily stimulated by angiotensin II, elevated potassium levels, and ACTH.

A fundamental property of excitable cells stating that once the threshold stimulus is reached, an action potential is generated and propagated along the membrane at full amplitude without decreasing in size. If the stimulus does not reach threshold, no action potential occurs.

Enzymes whose activity is regulated by the binding of molecules (called effectors or modulators) at a site other than the active site, known as the allosteric site. This binding causes a conformational change in the enzyme, which can either increase (activation) or decrease (inhibition) its catalytic activity.

A class of G protein-coupled receptors that respond to catecholamines such as norepinephrine and epinephrine. These receptors are primarily involved in vasoconstriction, pupil dilation, and other sympathetic nervous system responses.

Specialized immune cells located within the alveoli of the lungs. They play a critical role in the respiratory system’s defense by engulfing and digesting airborne particles, pathogens, and cellular debris through a process called phagocytosis.

Tiny, balloon-like air sacs located at the ends of the bronchioles in the lungs.

Organic molecules that serve as the building blocks of proteins.

The synthesis of complex molecules from simpler ones, which requires energy.

The process of generating energy (ATP) without the use of oxygen, primarily through the breakdown of glucose or glycogen.

A group of steroid hormones that regulate the development and maintenance of male characteristics. Although produced in greater amounts by the testes, androgens such as testosterone are also secreted by the adrenal cortex and ovaries. They influence muscle mass, libido, and secondary sexual traits.

An inactive precursor formed when renin cleaves angiotensinogen. Angiotensin I is converted into the active hormone angiotensin II by the enzyme angiotensin-converting enzyme (ACE), which plays a key role in blood pressure regulation.

A potent vasoconstrictor formed from angiotensin I through the action of angiotensin-converting enzyme (ACE). Angiotensin II increases blood pressure by narrowing blood vessels and stimulates the release of aldosterone from the adrenal cortex. It also promotes thirst and the release of antidiuretic hormone (ADH), contributing to fluid retention and blood pressure regulation.

An enzyme primarily found in the lungs and vascular endothelium that converts angiotensin I into angiotensin II, a potent vasoconstrictor. ACE also degrades bradykinin, a vasodilator, thereby contributing to increased blood pressure.

A plasma protein produced by the liver that serves as the precursor to angiotensin I. When acted upon by renin, angiotensinogen is converted into angiotensin I, beginning a cascade that leads to vasoconstriction and increased blood pressure.

A portion of the pituitary gland that produces hormones such as growth hormone (GH), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), and prolactin.

Also known as vasopressin, it is a peptide hormone produced by the hypothalamus and released by the posterior pituitary gland. It regulates water balance in the body by increasing water reabsorption in the kidneys, thereby reducing urine output and conserving body fluids.

The difference in oxygen content between arterial blood and mixed venous blood, reflecting the amount of oxygen extracted by tissues during circulation. It is a key indicator of tissue oxygen utilization and, along with cardiac output, determines oxygen consumption according to the Fick principle.

Blood vessels that carry oxygenated blood away from the heart to the tissues and organs of the body. They have thick, elastic walls composed of three layers (tunica intima, tunica media, and tunica externa) that allow them to withstand and regulate high blood pressure generated by the heart’s contractions.

Small-diameter blood vessels that branch from arteries and lead to capillaries. They play a critical role in regulating blood flow and blood pressure by adjusting their diameter through vasoconstriction and vasodilation, which control resistance within the circulatory system.

An enzyme that catalyzes the hydrolysis of ATP (adenosine triphosphate) into ADP (adenosine diphosphate) and inorganic phosphate (Pi), releasing energy that can be used to power various cellular processes.

A cluster of specialized cardiac cells located in the interatrial septum near the tricuspid valve. The AV node receives electrical impulses from the sinoatrial (SA) node and delays them briefly before transmitting them to the bundle of His and Purkinje fibers. This delay ensures that the atria contract and empty blood into the ventricles before ventricular contraction begins.

Heart valves located between the atria and ventricles that prevent backflow of blood into the atria during ventricular contraction. There are two AV valves: the tricuspid valve on the right side of the heart and the bicuspid (mitral) valve on the left side.

The division of the peripheral nervous system that regulates involuntary physiological functions, including heart rate, blood pressure, digestion, and respiratory rate. The ANS operates largely without conscious control and is divided into three branches: the sympathetic nervous system (prepares the body for “fight or flight”), the parasympathetic nervous system (promotes “rest and digest” functions), and the enteric nervous system (controls gastrointestinal activity).

A class of G protein-coupled receptors that respond to catecholamines such as epinephrine and norepinephrine. These receptors are involved in the regulation of heart rate, smooth muscle relaxation, and metabolic processes.

The catabolic process by which fatty acids are broken down in the mitochondria (or peroxisomes) to generate acetyl-CoA, which then enters the TCA cycle for energy production.

The study of how energy flows through living systems, particularly how organisms acquire, convert, store, and use energy to perform biological work.

The mechanics of biological and especially muscular activity (as in locomotion or exercise).

The force exerted by circulating blood on the walls of blood vessels, typically measured in the arteries.

Describes how changes in blood pH and carbon dioxide concentration influence hemoglobin’s affinity for oxygen. Specifically, a decrease in pH (more acidic) or an increase in CO₂ causes hemoglobin to release oxygen more readily—a rightward shift in the oxygen-hemoglobin dissociation curve.

A fundamental principle in gas physics that describes the inverse relationship between the pressure and volume of a gas at constant temperature.

The number of breaths taken per minute.

Refers to the branching system of airways within the lungs that conducts air from the trachea to the alveoli

One of the two main pathways (right and left bundle branches) that conduct electrical impulses from the atrioventricular (AV) bundle (bundle of His) down the interventricular septum toward the Purkinje fibers. These branches ensure coordinated contraction of the right and left ventricles by rapidly transmitting the action potential to the ventricular myocardium.

A specialized group of cardiac muscle fibers located in the interventricular septum that conducts electrical impulses from the atrioventricular (AV) node to the right and left bundle branches. The bundle of His is a critical component of the heart’s conduction system, ensuring that the ventricles receive the signal to contract after the atria have contracted.

A structural and regulatory protein located in the thick filament region of the sarcomere, near the M-line. It helps stabilize thick filaments and modulates cross-bridge formation between actin and myosin, influencing the speed and strength of muscle contraction.

A unit of energy commonly used to quantify the amount of energy provided by food and expended by the body. In nutrition, the term “calorie” typically refers to a kilocalorie (kcal), which equals 1,000 small calories and represents the amount of energy required to raise the temperature of 1 kilogram of water by 1°C.

The smallest and thinnest blood vessels in the circulatory system, forming networks that connect arterioles to venules. Their walls consist of a single layer of endothelial cells, allowing efficient exchange of gases, nutrients, and waste products between blood and surrounding tissues.

Organic molecules composed of carbon (C), hydrogen (H), and oxygen (O), typically in a ratio close to 1:2:1. They are one of the main classes of biomolecules and serve as a primary source of energy for living organisms.

The volume of carbon dioxide generated by the body per unit of time, typically expressed in liters per minute (L/min). VCO₂ reflects the rate of metabolic processes that produce CO₂ as a byproduct.

The sequence of mechanical and electrical events that occur during one complete heartbeat, including atrial contraction (atrial systole), ventricular contraction (ventricular systole), and relaxation of all chambers (diastole).

A specialized type of striated, involuntary muscle found only in the walls of the heart. Cardiac muscle fibers are branched, interconnected, and typically contain a single nucleus.

The volume of blood pumped by a ventricle per minute, typically expressed in liters per minute (L/min). It is calculated as the product of stroke volume (SV) and heart rate (HR): Q=SV×HR.

The breakdown of molecules to release energy.

A group of related hormones and neurotransmitters that include epinephrine, norepinephrine, and dopamine. These compounds are derived from the amino acid tyrosine and are involved in the body's response to stress, regulating heart rate, blood pressure, and metabolism.

The reduction in voluntary muscle activation caused by processes within the central nervous system (CNS), rather than within the muscle itself. It limits the ability to fully recruit motor units during sustained or intense exercise.

The part of the nervous system consisting of the brain and spinal cord. The CNS integrates sensory information, processes signals, and coordinates responses by controlling voluntary and involuntary activities throughout the body.

A method of measuring oxygen consumption (VO₂) in which the subject breathes from a sealed system containing a known volume of oxygen. The system absorbs exhaled carbon dioxide, and the decrease in oxygen volume over time is used to calculate metabolic rate.

Small, organic, non-protein molecules that assist enzymes in catalyzing biochemical reactions.

A type of isotonic muscle contraction in which the muscle shortens while generating force, typically occurring when lifting a load or moving against resistance.

The portion of the respiratory system responsible for transporting air to the lungs but not directly involved in gas exchange. It includes the nose, nasal cavity, pharynx, larynx, trachea, bronchi, and bronchioles (up to the terminal bronchioles). These structures serve to warm, humidify, and filter incoming air, ensuring that it reaches the respiratory zone in optimal condition for gas exchange.

A critical metabolic step that links glycolysis to the TCA cycle (citric acid cycle; Krebs cycle) in aerobic respiration. This process occurs in the mitochondrial matrix.

A glucocorticoid hormone produced by the zona fasciculata of the adrenal cortex. Cortisol plays a vital role in the body's response to stress by increasing blood glucose levels, enhancing metabolism of fats, proteins, and carbohydrates, and suppressing inflammation and immune responses. Its secretion is regulated by the hypothalamic-pituitary-adrenal (HPA) axis and follows a diurnal rhythm, peaking in the early morning.

Pairs of chemical reactions that occur together, where one reaction releases energy (exergonic) and the other requires energy (endergonic).

Also called phosphocreatine, it is a high-energy compound found primarily in muscle cells. It serves as a rapid energy reserve for the regeneration of ATP (adenosine triphosphate) during short bursts of intense activity, such as sprinting or heavy lifting.

The repetitive sequence of events during muscle contraction in which myosin heads bind to actin, perform a power stroke, release, and reset.

Also known as the Law of Partial Pressures, this states that the total pressure exerted by a mixture of gases is equal to the sum of the partial pressures of each individual gas in the mixture.

A phase in which the membrane potential of a cell becomes less negative (moves toward zero) compared to the resting membrane potential.

An intermediate filament protein found in muscle cells that forms part of the cytoskeleton. It links adjacent myofibrils at the Z-disks and connects them to the cell membrane, providing structural integrity and maintaining alignment during contraction.

A dome-shaped sheet of skeletal muscle that separates the thoracic cavity from the abdominal cavity. It plays a crucial role in respiration

The phase of the cardiac cycle during which the heart muscle relaxes after contraction, allowing the chambers (atria and ventricles) to fill with blood.

A method of measuring energy expenditure by directly quantifying the amount of heat produced by the body in a controlled environment, such as a calorimeter chamber.

Two monosaccharides linked together (e.g., sucrose, lactose).

A type of isotonic muscle contraction in which the muscle lengthens while generating force, typically occurring when lowering a load or resisting an external force.

The percentage of blood ejected from a ventricle during systole relative to its end-diastolic volume (EDV). EF is calculated as (stroke volume/EDV) × 100% and is a key indicator of ventricular function; normal values for the left ventricle are typically 50–70%.

Differences in electric charge across a membrane or between two points, creating a voltage that can drive the movement of ions.

A diagnostic test that records the electrical activity of the heart over time using electrodes placed on the skin. The ECG produces a waveform with characteristic components (P wave, QRS complex, and T wave) that represent atrial depolarization, ventricular depolarization, and ventricular repolarization, respectively. It is commonly used to assess heart rhythm, detect arrhythmias, and evaluate cardiac health.

A series of protein complexes and associated molecules located in the inner mitochondrial membrane that drive oxidative phosphorylation, the final stage of aerobic respiration.

The volume of blood in a ventricle at the end of diastole, just before ventricular contraction. EDV reflects the heart’s preload and is a key determinant of stroke volume and cardiac output according to the Frank–Starling mechanism.

The volume of blood remaining in a ventricle at the end of systole, after ventricular contraction. ESV reflects the heart’s afterload and contractility and, together with end-diastolic volume, determines stroke volume and ejection fraction.

Chemical reactions that require an input of free energy to proceed.

The innermost layer of the heart wall, composed of a thin layer of endothelial cells and connective tissue. It lines the heart chambers and covers the heart valves, providing a smooth surface that minimizes friction and reduces the risk of blood clot formation during cardiac function.

A delicate layer of connective tissue that surrounds each individual muscle fiber within a fasciculus.

A type of physical activity typically involving aerobic activities, aimed at improving the efficiency and capacity of the cardiovascular and respiratory systems to sustain prolonged physical activity.

The total amount of energy, measured in calories or kilojoules, that an organism uses to maintain basic physiological functions, perform physical activity, and process food. EE is typically divided into three components: basal metabolic rate (BMR), the thermic effect of food (TEF), and energy used during physical activity.

A measure of disorder or randomness in a system. In thermodynamics and biology, it reflects how energy is distributed and how much of it is available to do useful work.

Biological catalysts, typically proteins (and sometimes RNA molecules called ribozymes), that speed up chemical reactions in living organisms without being consumed in the process.

The outermost layer of the heart wall, forming part of the pericardium. It consists of a thin layer of connective tissue and fat covered by mesothelium, providing a protective layer and reducing friction between the heart and surrounding structures during contraction.

A dense layer of irregular connective tissue that surrounds an entire skeletal muscle.

Also known as adrenaline, epinephrine is a catecholamine hormone and neurotransmitter produced by the adrenal medulla. It plays a central role in the body's "fight-or-flight" response by increasing heart rate, dilating airways, enhancing blood flow to muscles, and promoting the breakdown of glycogen to glucose for quick energy.

The measurement of work output during controlled exercise, typically using specialized equipment called ergometers (e.g., cycle ergometer, treadmill). Ergometry is used to assess physical performance, energy expenditure, and physiological responses under standardized conditions.

The most potent and predominant form of estrogen in non-pregnant, reproductive-age females. Produced mainly by the ovaries, estradiol plays a central role in regulating the menstrual cycle, maintaining reproductive tissues, and supporting bone and cardiovascular health.

The weakest of the three major estrogens, estriol is produced in significant amounts during pregnancy by the placenta. It plays a role in maintaining pregnancy and preparing the body for childbirth, but has minimal effects outside of gestation.

A class of steroid hormones primarily produced by the ovaries, with smaller amounts secreted by the adrenal glands and other tissues. Estrogens regulate the development of female secondary sexual characteristics, reproductive function, and menstrual cycle. The most prominent estrogen is estradiol.

A weaker form of estrogen that is primarily produced in adipose (fat) tissue and the adrenal glands. Estrone becomes the dominant estrogen after menopause and can be converted into estradiol or estriol depending on physiological needs.

The elevated oxygen uptake that persists after exercise has ended, compared to resting levels. It reflects the body’s effort to restore physiological balance and recover from the metabolic stress of exercise.

The physiological process linking the electrical excitation of a muscle fiber (action potential) to its mechanical contraction. It involves calcium release from the sarcoplasmic reticulum, which enables actin–myosin interaction and sarcomere shortening.

Small, temporary depolarizations of the postsynaptic membrane that occur when excitatory neurotransmitters bind to receptors, causing positively charged ions (such as Na⁺) to enter the cell. EPSPs increase the likelihood that the postsynaptic neuron will reach threshold and generate an action potential.

Temporary depolarizations of the postsynaptic membrane caused by the flow of positively charged ions (typically sodium, Na⁺) into the neuron following the activation of excitatory neurotransmitter receptors. EPSPs increase the likelihood that the postsynaptic neuron will reach the threshold to fire an action potential. They are graded potentials, meaning their strength depends on the amount of neurotransmitter released and the number of receptors activated.

A field that investigates the effect of exercise on the function and structure of the body.

The ratio of mechanical work output to the total energy expended during exercise, usually expressed as a percentage. It reflects how effectively the body converts metabolic energy into external work. Higher efficiency means less energy is wasted as heat for a given workload.

The study of how the body reacts to physical exercise, in both the long and short term, and how the body adapts to ongoing exercise and any changes to a routine.

A multidisciplinary field that explores the science of movement and the body's responses and adaptations to physical activity. It encompasses various sub-disciplines, including Biomechanics, Exercise and Sport Physiology, Kinesiology, Sports Psychology, and Sports Sociology.

Chemical reactions that release free energy into the surroundings.

A group of skeletal muscles located between the ribs that play a key role in the mechanics of breathing, particularly during inhalation

A broad, flat muscle located on the lateral and anterior parts of the abdomen. It contributes to forced exhalation by compressing the abdominal cavity, which pushes the diaphragm upward and helps expel air from the lungs.

The exchange of gases between the air in the alveoli of the lungs and the blood in the pulmonary capillaries.

A continuous band or sheet of connective tissue, primarily composed of collagen, that surrounds, stabilizes, and separates muscles, organs, and other internal structures.

Bundles of skeletal muscle fibers grouped together within a muscle, surrounded by a connective tissue sheath called the perimysium.

A type of lipid, a class of hydrophobic (water-insoluble) molecules primarily composed of carbon (C), hydrogen (H), and a small amount of oxygen (O). They serve as a major source of long-term energy storage, provide insulation and protection for organs, and are essential components of cell membranes.

A carboxylic acid with a long hydrocarbon chain, which can be either saturated (no double bonds) or unsaturated (one or more double bonds). Fatty acids are the building blocks of many lipids, including triglycerides and phospholipids, and play a critical role in energy storage, membrane structure, and signaling.

A principle used to calculate oxygen consumption (VO2) based on cardiac output (Q) and the arterial–venous oxygen difference (a-vO₂ difference).

This describes the rate at which a gas moves across a membrane. It states that the rate of gas transfer is directly proportional to the surface area of the membrane and the difference in partial pressures across it, and inversely proportional to the thickness of the membrane.

A redox-active coenzyme derived from riboflavin (vitamin B₂).

A physiological principle stating that the strength of ventricular contraction increases with greater end-diastolic volume (EDV), within physiological limits. This relationship ensures that the heart pumps out the volume of blood it receives, maintaining balance between venous return and cardiac output.

The amount of energy in a system that is available to do useful work. The most commonly used form is Gibbs Free Energy (G), defined by the equation: ΔG=ΔH−TΔS
Where:
ΔG = change in free energy
ΔH = change in enthalpy (total energy)
T = temperature in Kelvin
ΔS = change in entropy (disorder)

A simple sugar (monosaccharide) with the molecular formula C₆H₁₂O₆, like glucose, but with a different structural arrangement.

Guanine nucleotide-binding proteins, are molecular switches that play a critical role in transmitting signals from cell surface receptors to intracellular effectors. They are activated when a signaling molecule (e.g., a hormone or neurotransmitter) binds to a G protein-coupled receptor (GPCR), causing the G protein to exchange GDP for GTP. This activation triggers downstream signaling pathways that regulate various cellular responses.

A monomeric form of actin that polymerizes to form filamentous actin (F-actin) in muscle and non-muscle cells.

The precision with which a control system maintains homeostasis.

A peptide hormone secreted by the alpha cells of the pancreas. Glucagon raises blood glucose levels by stimulating the liver to convert stored glycogen into glucose (glycogenolysis) and to produce glucose from non-carbohydrate sources (gluconeogenesis). It acts as a counter-regulatory hormone to insulin.

Steroid hormones produced by the adrenal cortex that influence metabolism, immune response, and stress adaptation. The primary glucocorticoid is cortisol, which increases blood glucose levels, suppresses inflammation, and helps the body respond to physical and emotional stress.

The metabolic pathway that synthesizes glucose from non-carbohydrate precursors, ensuring a continuous supply of glucose during fasting, prolonged exercise, or low-carbohydrate intake. It primarily occurs in the liver (and to a lesser extent in the kidney cortex).

A simple sugar (monosaccharide) with the molecular formula C₆H₁₂O₆.

The primary storage form of glucose in animals and humans, mainly found in the liver (for maintaining blood glucose levels) and skeletal muscles (for energy during activity).

The biochemical process of breaking down glycogen into glucose molecules to provide energy for cellular metabolism. It primarily occurs in the liver (to maintain blood glucose levels) and skeletal muscle (to supply energy during muscle activity).

A metabolic pathway that breaks down glucose (C₆H₁₂O₆) into two molecules of pyruvate, producing a small amount of energy in the form of ATP and NADH.

Specialized proprioceptive sensory receptors located at the junction between muscle fibers and tendons. They detect changes in muscle tension and force rather than length. When tension becomes too high, GTOs send inhibitory signals to the spinal cord to reduce muscle contraction, protecting muscles and tendons from potential damage. They play a key role in reflexes that regulate muscle force and maintain posture.

Localized changes in the membrane potential of a neuron that vary in size (amplitude) depending on the strength of the stimulus. Unlike action potentials, graded potentials are not all-or-none; they can be depolarizing (excitatory) or hyperpolarizing (inhibitory) and diminish in strength as they spread from the point of origin. They occur mainly in the dendrites and cell body and play a key role in initiating action potentials when summed at the axon hillock.

Also known as somatotropin, is a peptide hormone secreted by the anterior pituitary gland. It plays a vital role in stimulating growth, cell reproduction, and regeneration. GH promotes the growth of bones and muscles, increases protein synthesis, and mobilizes fat stores for energy. It also influences metabolism by increasing blood glucose levels and stimulating the production of insulin-like growth factor 1 (IGF-1) in the liver and other tissues.

The central region of the A-band in a sarcomere that contains only thick (myosin) filaments and no thin (actin) filaments.

The phenomenon where deoxygenated hemoglobin has a greater capacity to bind carbon dioxide (CO₂) than oxygenated hemoglobin. This effect facilitates the transport of CO₂ from tissues to the lungs.

A behavioral social science that draws from the biological, environmental, psychological, physical, and medical sciences to promote health and prevent disease, disability, and premature death through education-driven voluntary behavior change activities.

The physiological variation in the time interval between consecutive heartbeats, typically measured as fluctuations in the R-R intervals on an electrocardiogram (ECG). HRV reflects autonomic nervous system activity, with higher variability generally indicating better cardiovascular health and adaptability, while reduced HRV is associated with stress, fatigue, and certain disease states.

A complex iron-containing protein found in red blood cells that is essential for oxygen transport in the body

Molecules that store and transfer energy within cells through phosphate bonds that release a large amount of free energy when hydrolyzed.

The process by which a biological system maintains internal stability despite changes in the external environment. It involves a dynamic balance of physiological variables—such as body temperature, pH, blood glucose levels, and water balance—within a narrow, optimal range.

Chemical messengers produced by endocrine glands and released into the bloodstream to regulate various physiological processes

A change in membrane potential that makes the inside of the cell more negative than the resting membrane potential.

A small but crucial region of the brain located below the thalamus and above the pituitary gland. It serves as a major control center for the autonomic nervous system and the endocrine system, maintaining homeostasis by regulating body temperature, hunger, thirst, sleep, and emotional activity.

The light region of a sarcomere that contains only thin (actin) filaments and spans across two adjacent sarcomeres, intersected by the Z-disk.

A method of estimating energy expenditure by measuring oxygen consumption (VO₂) and carbon dioxide production (VCO₂) during respiration. These values are used to calculate metabolic rate and substrate utilization, based on the principle that energy metabolism is linked to gas exchange.

A temporary hyperpolarization of the postsynaptic membrane that occurs when inhibitory neurotransmitters bind to receptors, causing negatively charged ions (such as Cl⁻) to enter the cell or positively charged ions (such as K⁺) to leave. IPSPs decrease the likelihood that the postsynaptic neuron will reach threshold and generate an action potential.

The number of muscle fibers controlled by a single motor neuron within a motor unit. It reflects the degree of fine motor control: muscles requiring precise movements (e.g., eye muscles) have a low innervation ratio (few fibers per neuron), while muscles generating large, powerful movements (e.g., quadriceps) have a high innervation ratio (many fibers per neuron).

A free phosphate ion (PO₄³⁻) that is not bound to an organic molecule.

A peptide hormone produced by the beta cells of the pancreatic islets (Islets of Langerhans). It plays a central role in regulating blood glucose levels by promoting the uptake of glucose into cells, especially in muscle and adipose tissue, and by stimulating glycogen synthesis in the liver.

A peptide hormone primarily produced by the liver in response to stimulation by growth hormone (GH). IGF-1 plays a key role in promoting cell growth, differentiation, and tissue repair, especially during childhood and adolescence. It mimics some of the actions of insulin and is essential for mediating many of the growth-promoting effects of GH.

How hard the body is working during physical activity.

Specialized junctions between cardiac muscle cells that contain desmosomes and gap junctions. These structures provide strong mechanical attachment and allow electrical impulses to pass rapidly between cells, enabling the heart muscle to contract as a coordinated unit (functional syncytium).

Located between the ribs, deep to the external intercostals. They assist in forced exhalation by pulling the ribs downward and inward, which decreases the volume of the thoracic cavity and helps push air out of the lungs. These muscles are especially active during vigorous breathing or physical exertion.

A muscle that lies just beneath the external oblique and runs in the opposite direction. Like the external oblique, it aids in forced exhalation by compressing the abdominal contents and assisting in diaphragm elevation.

The exchange of gases between the blood in systemic capillaries and the body’s tissues.

A type of muscle contraction in which the muscle changes length at a constant speed throughout the entire range of motion, typically achieved using specialized equipment that controls the velocity of movement while accommodating resistance.

A type of muscle contraction in which the muscle generates force without changing its length, resulting in no visible movement of the joint. This occurs when the force produced equals the external load, such as holding a weight stationary.

Muscle contractions in which tension is generated without a change in muscle length, resulting in no visible joint movement. These occur when the force produced equals the external resistance, such as holding a weight in a fixed position.

A type of muscle contraction in which the muscle changes length while producing constant tension, resulting in movement of a load. It includes concentric contractions (muscle shortens) and eccentric contractions (muscle lengthens).

The study of the principles of mechanics and anatomy in relation to human movement

The exercise intensity at which blood lactate concentration begins to rise significantly above resting levels, typically around 2–4 mmol·L⁻¹. It represents the point where lactate production exceeds clearance, signaling a shift toward greater reliance on anaerobic metabolism.

The metabolic process by which triglycerides (stored fats) are broken down into glycerol and free fatty acids. This occurs primarily in adipose tissue and is triggered when the body needs energy, such as during fasting, exercise, or low-carbohydrate intake.

A glycoprotein hormone secreted by the anterior pituitary gland that plays a key role in regulating reproductive function.

The central region of the A-band in a sarcomere where thick (myosin) filaments are linked together by supporting proteins. It helps stabilize the arrangement of thick filaments and maintains sarcomere structure during contraction.

The maximum rate at which an individual can consume oxygen during intense, whole-body exercise, expressed in liters per minute (L/min) or milliliters per kilogram of body weight per minute (mL·kg⁻¹·min⁻¹). It reflects the integrated capacity of the cardiovascular, respiratory, and muscular systems to deliver and utilize oxygen for energy production.

The average pressure in a patient’s arteries during one cardiac cycle. It represents the perfusion pressure delivered to organs and tissues.

A standardized unit used to estimate the energy cost of physical activities, expressed as a multiple of resting metabolic rate. One MET is defined as the energy expenditure at rest, approximately 3.5 mL of oxygen per kilogram of body weight per minute (mL·kg⁻¹·min⁻¹). Activities are assigned MET values to indicate how many times more energy they require compared to resting conditions.

A series of interconnected biochemical reactions within a cell, where the product of one reaction serves as the substrate for the next.

The sum of all chemical reactions that occur within a living organism to maintain life.

A class of steroid hormones produced by the adrenal cortex, primarily involved in regulating electrolyte and fluid balance. The most well-known mineralocorticoid is aldosterone, which promotes sodium retention and potassium excretion by the kidneys, helping to maintain blood pressure and volume.

Membrane-bound organelles found in most eukaryotic cells, often referred to as the "powerhouses of the cell" because they produce the majority of the cell’s usable energy in the form of ATP (adenosine triphosphate).

The series of biochemical processes that occur within the mitochondria of eukaryotic cells to produce energy in the form of ATP (adenosine triphosphate).

Simple sugars (e.g., glucose, fructose).

The component of the peripheral nervous system responsible for transmitting signals from the central nervous system (CNS) to muscles and glands, initiating movement and regulating bodily functions. It is divided into the somatic nervous system (controls voluntary skeletal muscle movements) and the autonomic nervous system (controls involuntary functions such as heart rate, digestion, and glandular activity).

A specialized region of the muscle fiber’s sarcolemma (cell membrane) at the neuromuscular junction where the motor neuron communicates with the muscle.

An automatic, involuntary response of a muscle or group of muscles to a specific stimulus, mediated by the nervous system without conscious control. Motor reflexes are typically organized through reflex arcs, which involve sensory receptors, afferent neurons, interneurons (in some cases), efferent neurons, and effectors. Examples include the stretch reflex (knee-jerk) and withdrawal reflex.

Learned abilities that enable the execution of coordinated movements involving muscles and the nervous system.

A functional unit of the neuromuscular system consisting of a single motor neuron and all the skeletal muscle fibers it innervates.

Also known as the cross-bridge cycle, it is the repeating sequence of events that occurs during muscle contraction at the molecular level.

The decline in the ability of a muscle to generate force or power during sustained or repeated activity. It is a complex, multifactorial phenomenon involving both peripheral and central mechanisms.

A single, elongated, multinucleated cell that makes up skeletal muscle tissue. Muscle fibers contain myofibrils, which are composed of repeating units called sarcomeres—the basic contractile units of muscle.

An increase in the size of skeletal muscle fibers resulting from resistance training or other stimuli that promote protein synthesis and muscle growth. Hypertrophy occurs primarily through the enlargement of individual muscle fibers rather than an increase in fiber number.

Specialized proprioceptive sensory receptors located within skeletal muscles that detect changes in muscle length and the rate of length change. They consist of intrafusal fibers wrapped by sensory nerve endings. When a muscle is stretched, muscle spindles send signals to the central nervous system to trigger reflexive muscle contraction (stretch reflex), helping maintain muscle tone and posture.

The thick, muscular middle layer of the heart wall composed primarily of cardiac muscle tissue. It is responsible for the contractile force that pumps blood throughout the body. The myocardium varies in thickness, being thickest in the left ventricle to generate the high pressure needed for systemic circulation.

A long, cylindrical organelle found in muscle fibers, composed of repeating units called sarcomeres that contain actin and myosin filaments.

A small, oxygen-binding protein found primarily in muscle tissue. Myoglobin stores and facilitates the transport of oxygen within muscle cells, enabling sustained energy production during periods of high metabolic demand. It consists of a single polypeptide chain and a heme group that binds oxygen molecules.

A structural protein located in the M-line of the sarcomere that crosslinks thick (myosin) filaments and titin molecules. It provides mechanical stability to the sarcomere and helps maintain the alignment of thick filaments during muscle contraction.

A motor protein that plays a central role in muscle contraction and various types of cell movement.

A large structural protein associated with thin (actin) filaments in skeletal muscle. It runs along the length of the actin filament, acting as a molecular ruler to regulate filament length and contribute to sarcomere stability.

A regulatory mechanism in which a change in a system triggers a response that counteracts or reduces that change, helping to maintain stability or equilibrium.

The specialized synapse between a motor neuron and a skeletal muscle fiber where nerve impulses trigger muscle contraction. It includes the motor neuron terminal, synaptic cleft, and motor end plate, and relies on acetylcholine as the primary neurotransmitter.

A coenzyme found in all living cells that plays a critical role in redox reactions.

Also called noradrenaline, norepinephrine is a catecholamine that functions both as a hormone and a neurotransmitter. It is produced by the adrenal medulla and sympathetic nerve endings. Norepinephrine increases blood pressure by constricting blood vessels and is involved in alertness, arousal, and the stress response.

A heart rhythm originating from the sinoatrial (SA) node, characterized by a regular rate of 60–100 beats per minute in adults, with consistent P waves preceding each QRS complex and a normal PR interval. NSR indicates normal electrical conduction through the heart.

A fundamental principle in electricity stating that the current (I) through a conductor between two points is directly proportional to the voltage (V) across the two points and inversely proportional to the resistance (R).

The exercise intensity at which blood lactate concentration reaches approximately 4 mmol·L⁻¹, marking a significant shift toward anaerobic metabolism.

A method of measuring oxygen consumption (VO₂) and carbon dioxide production (VCO₂) in which the subject breathes ambient air, and exhaled gases are collected and analyzed.

A chronic condition characterized by decreased bone mass and deterioration of bone tissue, leading to increased fragility and risk of fractures. It occurs when bone resorption outpaces bone formation, often due to aging, hormonal changes (especially reduced estrogen levels in postmenopausal women), nutritional deficiencies, or lack of physical activity.

The female gonads that produce ova (eggs) and secrete female sex hormones, including estrogens and progesterone. Located on either side of the uterus, the ovaries contain follicles at various stages of development and are responsible for regulating the menstrual cycle, fertility, and the development of female secondary sexual characteristics.

The volume of oxygen used by the body per unit of time, typically expressed in liters per minute (L/min) or milliliters per kilogram per minute (mL·kg⁻¹·min⁻¹).

The amount of oxygen consumed by the body to perform a specific activity or produce a given amount of work, typically expressed in milliliters of O₂ per kilogram of body weight per minute (mL·kg⁻¹·min⁻¹) or per unit of work. Oxygen cost reflects the energy demand of an activity and is commonly used to assess exercise efficiency and metabolic requirements.

(Now more commonly referred to as Excess Post-Exercise Oxygen Consumption, EPOC) is the elevated oxygen uptake that persists after exercise has ended, compared to resting levels. It represents the body’s effort to restore homeostasis and repay the “oxygen deficit” incurred at the start of exercise.

The difference between the oxygen required for a given exercise intensity and the actual oxygen uptake (VO₂) during the initial phase of exercise. It occurs because mitochondrial respiration takes time to fully activate, so energy demand is temporarily met by anaerobic pathways (ATP-PC system and anaerobic glycolysis).

The rate at which the body consumes oxygen during rest or exercise, expressed in liters per minute (L/min) or milliliters per kilogram of body weight per minute (mL·kg⁻¹·min⁻¹).

A peptide hormone produced by the hypothalamus and released by the posterior pituitary gland. It plays a key role in reproductive and social behaviors.

The first deflection on an electrocardiogram (ECG) representing atrial depolarization, which triggers atrial contraction. It reflects the electrical activity as the impulse originates in the sinoatrial (SA) node and spreads through the atria.

A dual-function gland located in the upper abdomen, behind the stomach. It has both exocrine and endocrine roles.

A branch of the autonomic nervous system that promotes “rest and digest” functions, conserving energy and maintaining homeostasis during restful states. It decreases heart rate, stimulates digestion, and supports processes such as glandular secretion and nutrient absorption.

Four small endocrine glands located on the posterior surface of the thyroid gland. They produce parathyroid hormone (PTH), which plays a critical role in calcium homeostasis.

The outer layer of the pleural membrane that lines the inner surface of the thoracic cavity, including the chest wall, diaphragm, and mediastinum

Refers to the pressure exerted by a single gas in a mixture of gases. It is a measure of how much of that gas is present and contributes to the total pressure of the mixture.

The highest rate of oxygen consumption measured during an exercise test, regardless of whether a true VO₂max is achieved. VO₂peak is often used as an indicator of cardiorespiratory fitness when maximal effort or physiological criteria for VO₂max cannot be met, such as in clinical populations or submaximal testing.

A thin, triangular muscle located beneath the pectoralis major in the upper chest. It attaches from the ribs (usually the 3rd to 5th) to the coracoid process of the scapula.

The relative exercise intensity expressed as a percentage of an individual’s VO₂max.

The relative load used in resistance training expressed as a percentage of the maximum weight an individual can lift for one complete repetition of a given exercise (1RM).

A sheath of connective tissue that surrounds and binds together bundles of muscle fibers, known as fasciculi, within a skeletal muscle.

Decline in muscle performance caused by processes occurring within the muscle fibers themselves, rather than in the nervous system. It is a major component of overall muscle fatigue and is typically associated with metabolic and ionic changes during intense or prolonged exercise.

The part of the nervous system located outside the brain and spinal cord, consisting of cranial nerves, spinal nerves, and associated ganglia. The PNS connects the central nervous system (CNS) to the rest of the body and is divided into the somatic nervous system (controls voluntary movements) and the autonomic nervous system (regulates involuntary functions such as heart rate and digestion).

A class of lipids that are major components of cell membranes. They are amphipathic molecules, meaning they have both water-loving (hydrophilic) and water-fearing (hydrophobic) components.

Often called the “master gland,” is a small, pea-sized endocrine gland located at the base of the brain, beneath the hypothalamus. It regulates many vital body functions by secreting hormones that control other endocrine glands.

A double-layered serous membrane that surrounds each lung and lines the chest cavity.

A thin space filled with pleural fluid that reduces friction during breathing and allows the lungs to expand and contract smoothly.

Also called "peptide hormones", polypeptide hormones are composed of short chains of amino acids that are water-soluble and typically bind to receptors on the surface of target cells, triggering intracellular signaling cascades rather than directly altering gene expression. Peptide hormones regulate a wide range of physiological functions including growth, metabolism, and reproduction.

Long chains of monosaccharides (e.g., starch, glycogen, cellulose).

A regulatory mechanism in which a change in a system triggers a response that amplifies or reinforces that change, rather than reversing it.

A portion of the pituitary gland that stores and releases oxytocin and antidiuretic hormone (ADH), which are produced by the hypothalamus.

Power (muscular power) reflects how quickly force can be applied to produce movement, making it a key measure of performance in activities requiring speed and strength.

The force-generating step of the cross-bridge cycle in which the myosin head pivots after releasing inorganic phosphate, pulling the actin filament toward the center of the sarcomere. This movement shortens the sarcomere and contributes to muscle contraction.

The time interval on an electrocardiogram (ECG) from the beginning of the P wave to the start of the QRS complex. It represents the period of atrial depolarization and the delay in the atrioventricular (AV) node before ventricular depolarization begins. Normal duration is typically 0.12–0.20 seconds.

The difference in pressure between two regions, which drives the movement of gases or fluids from areas of higher pressure to areas of lower pressure.

A steroid hormone primarily produced by the corpus luteum in the ovary after ovulation, and in smaller amounts by the adrenal glands and placenta during pregnancy. Progesterone prepares the endometrium (lining of the uterus) for potential implantation of a fertilized egg, supports early pregnancy, and helps regulate the menstrual cycle. It also plays a role in modulating immune responses and maintaining pregnancy.

A principle of exercise training that involves gradually increasing the stress placed on the body’s musculoskeletal and cardiovascular systems to stimulate adaptation and improve performance.

Specialized sensory receptors located in muscles, tendons, and joints that detect changes in body position, movement, and muscle tension. They provide the central nervous system with information about limb position and movement (proprioception), enabling coordination, balance, and posture control.

Large, complex biomolecules made up of chains of amino acids linked by peptide bonds.

A subatomic particle found in the nucleus of an atom. It carries a positive electric charge of +1 elementary charge and has a mass of approximately 1 atomic mass unit (1.67 × 10⁻²⁷ kg).

The process by which gases are exchanged between the alveoli in the lungs and the blood in the pulmonary capillaries.

A lipoprotein substance that reduces surface tension within the alveoli. This surfactant prevents alveolar collapse during exhalation and helps maintain lung compliance, making breathing more efficient.

The process of moving air into and out of the lungs. It involves two phases: inhalation (inspiration), where air is drawn into the lungs, and exhalation (expiration), where air is expelled from the lungs.

The total volume of air moved in and out of the lungs per minute. It is calculated as the product of tidal volume (VT) and breathing frequency (f): VE = VT × f

The difference between systolic and diastolic blood pressure. It reflects the force the heart generates with each contraction and is an indicator of arterial compliance. Normal pulse pressure is typically around 40 mmHg in healthy adults.

Specialized conductive fibers located in the inner ventricular walls of the heart. They rapidly transmit electrical impulses from the bundle branches to the ventricular myocardium, ensuring a coordinated and efficient contraction of both ventricles during systole.

A series of deflections on an electrocardiogram (ECG) representing ventricular depolarization, which triggers ventricular contraction. It typically consists of a small downward deflection (Q), a large upward spike (R), and a subsequent downward deflection (S).

The time interval from the start of the QRS complex to the end of the T wave on an ECG. It represents the total time for ventricular depolarization and repolarization. The QT interval varies with heart rate and is clinically important for detecting arrhythmia risk.

The time between two consecutive R waves on an electrocardiogram (ECG), representing one complete cardiac cycle. It is commonly used to calculate heart rate and assess rhythm regularity; shorter intervals indicate faster heart rates, while longer intervals indicate slower rates.

A neural mechanism in which the activation of a muscle (agonist) is accompanied by the simultaneous inhibition of its opposing muscle (antagonist) to allow smooth and coordinated movement. This process is mediated by spinal interneurons within reflex arcs, such as during the stretch reflex, where contraction of the quadriceps is paired with relaxation of the hamstrings.

Reactions (redox) that involve the transfer of electrons between substances.

The volume of oxygen consumed per unit of body weight per unit of time, typically expressed in milliliters per kilogram per minute (mL·kg⁻¹·min⁻¹). Relative VO₂ accounts for body size, allowing for fair comparisons of aerobic capacity and exercise intensity between individuals of different weights.

An enzyme secreted by the juxtaglomerular cells of the kidneys in response to low blood pressure, reduced sodium levels, or sympathetic nervous system stimulation. Renin initiates the renin-angiotensin-aldosterone system (RAAS) by cleaving angiotensinogen into angiotensin I, ultimately helping regulate blood pressure and fluid balance.

The process by which the membrane potential of a cell returns to its resting negative value after depolarization. In neurons and muscle cells, repolarization occurs primarily when voltage-gated potassium (K⁺) channels open, allowing K⁺ ions to exit the cell, restoring the negative internal environment.

The amount of air remaining in the lungs after a maximal exhalation. This air cannot be voluntarily expelled and serves to keep the alveoli open, maintaining gas exchange between breaths. RV typically accounts for about 1.2 liters in a healthy adult and is a critical component of total lung capacity (TLC).

(Also known as strength training or weight training) is a form of physical exercise designed to improve muscular strength, endurance, and size by working against a force or resistance. This resistance can come from: Free weights (e.g., dumbbells, barbells), Resistance bands, Weight machines, Body weight (e.g., push-ups, squats).

The thin barrier through which gas exchange occurs between the air in the alveoli and the blood in the pulmonary capillaries

The portion of the respiratory system where gas exchange occurs. It includes the respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli. These structures contain thin walls and are closely associated with pulmonary capillaries, allowing oxygen to diffuse into the blood and carbon dioxide to diffuse out.

The electrical potential difference across the plasma membrane of a cell when it is not actively sending signals, typically around –70 mV in neurons.

The amount of energy expended by the body at rest to maintain essential physiological functions such as breathing, circulation, and cellular processes. RMR is typically measured under less strict conditions than basal metabolic rate (BMR) and accounts for the largest portion of total daily energy expenditure.

The cell membrane that surrounds a muscle fiber (muscle cell).

The basic functional unit of striated muscle, defined as the segment between two Z-disks. It contains organized thick (myosin) and thin (actin) filaments whose sliding interaction during contraction shortens the sarcomere, producing muscle movement.

The cytoplasm of a muscle fiber (muscle cell). It is the gel-like substance that fills the space between the sarcolemma (muscle cell membrane) and the myofibrils (contractile structures).

A specialized form of smooth endoplasmic reticulum found in muscle fibers, responsible for storing, releasing, and reabsorbing calcium ions (Ca²⁺) during the process of muscle contraction and relaxation.

Specialized stem cells located between the sarcolemma and the basal lamina of skeletal muscle fibers. They play a key role in muscle growth, repair, and regeneration by proliferating and differentiating into new muscle fibers or fusing with existing ones following injury or stress.

A group of three paired muscles (anterior, middle, and posterior) located in the lateral neck. During forced inhalation, they assist in elevating the first and second ribs, helping to expand the thoracic cavity and facilitate airflow into the lungs.

Heart valves located at the bases of the large arteries leaving the ventricles—the aortic valve and the pulmonary valve. They prevent backflow of blood into the ventricles after ventricular contraction.

The part of the peripheral nervous system responsible for transmitting sensory information from receptors in the body (e.g., skin, muscles, joints, and organs) to the central nervous system (CNS). It enables the perception of stimuli such as touch, temperature, pain, and body position (proprioception).

A specialized cluster of pacemaker cells located in the right atrium of the heart that initiates electrical impulses, setting the rhythm for the heartbeat. The SA node generates action potentials that spread through the atria, causing atrial contraction and establishing the heart’s intrinsic rate, typically 60–100 beats per minute in adults.

A heart rhythm originating from the sinoatrial (SA) node with a rate of less than 60 beats per minute in adults. It is characterized by normal P waves preceding each QRS complex and a regular rhythm. Sinus bradycardia can be normal in well-conditioned athletes or occur due to medications, increased vagal tone, or certain medical conditions.

A heart rhythm originating from the sinoatrial (SA) node with a rate greater than 100 beats per minute in adults. It maintains normal P wave morphology and a regular rhythm, with each P wave preceding a QRS complex. Sinus tachycardia often occurs as a physiological response to exercise, stress, or fever, but can also indicate underlying conditions such as anemia, hyperthyroidism, or hypovolemia.

A fundamental rule of motor unit recruitment stating that motor units are activated in order of increasing size of their motor neurons. Smaller motor units, which typically contain slow-twitch (fatigue-resistant) fibers, are recruited first for low-force activities. As force demands increase, progressively larger motor units with fast-twitch fibers are recruited. This ensures efficient, smooth, and energy-conserving muscle activation.

A type of striated muscle tissue attached to bones via tendons, responsible for voluntary movements of the body.

A model explaining muscle contraction, stating that actin (thin) and myosin (thick) filaments slide past each other within the sarcomere without changing length. This sliding shortens the sarcomere, producing contraction, and is powered by ATP-driven cross-bridge cycling.

A type of non-striated, involuntary muscle tissue found in the walls of hollow organs such as the intestines, blood vessels, bladder, and uterus.

The division of the peripheral nervous system responsible for voluntary control of skeletal muscles and the transmission of sensory information to the central nervous system (CNS). It includes motor neurons that activate muscles and sensory neurons that convey information such as touch, pain, and proprioception.

A peptide hormone that functions primarily as an inhibitory regulator of hormone secretion. It is produced in several tissues, including the hypothalamus, pancreas (delta cells), and gastrointestinal tract.

The process by which multiple postsynaptic potentials (EPSPs or IPSPs) from different presynaptic neurons combine at the same time on a postsynaptic neuron. When these simultaneous inputs occur at different synapses on the dendrites or cell body, their combined effect can bring the membrane potential to threshold, triggering an action potential.

A common pulmonary function test that measures the volume and speed of air a person can inhale and exhale. It is used to assess lung function and diagnose respiratory conditions such as asthma, chronic obstructive pulmonary disease (COPD), and restrictive lung disease.

The functional tissue of the lungs that has a soft, porous, and elastic structure, primarily composed of alveoli, bronchioles, and associated capillaries.

The field of business dealing with sports and recreation. Sports management involves any combination of skills that correspond with planning, organizing, directing, controlling, budgeting, leading, or evaluating of any organization or business within the sports field.

A specialty that uses psychological knowledge and skills to address optimal performance and well-being of athletes, developmental and social aspects of sports participation, and systemic issues associated with sports settings and organizations.

A sub-discipline of sociology which focuses on sports as social phenomena.

A type of epithelial cell characterized by their thin, flat shape and single-layer arrangement.

The flat section of the ECG tracing between the end of the QRS complex and the beginning of the T wave. It represents the period when the ventricles are depolarized and in the early phase of repolarization. Abnormalities in the ST segment can indicate myocardial ischemia or infarction.

A condition in which the key variables of a system remain constant over time, even though energy or matter may be continuously entering and leaving the system. It is a dynamic equilibrium, not a static one.

The condition during submaximal, constant-load exercise where oxygen uptake (VO₂) plateaus and remains stable, indicating that aerobic energy supply meets the energy demand of the activity.

A prominent neck muscle that extends from the sternum and clavicle to the mastoid process of the skull. In addition to its role in head rotation and flexion, it assists in elevating the sternum during forced inhalation, thereby increasing the volume of the thoracic cavity.

A class of hormones derived from cholesterol that are lipid-soluble and capable of passing through cell membranes to bind with intracellular receptors. They regulate a wide range of physiological processes including metabolism, immune response, salt and water balance, and reproductive functions.

A class of lipids characterized by a core structure of four fused carbon rings (three six-membered rings and one five-membered ring). Unlike triglycerides, steroids are not composed of fatty acids and glycerol, but they are still classified as lipids because they are hydrophobic and insoluble in water.

A group of proteins that are produced by cells in response to stressful conditions, such as: Heat (e.g., fever or high environmental temperatures); Oxidative stress; Toxins or heavy metals; Inflammation; Exercise; Infection or injury.

The amount of blood ejected by a ventricle during a single contraction, typically measured in milliliters. Stroke volume is calculated as the difference between end-diastolic volume (EDV) and end-systolic volume (ESV) and is a key determinant of cardiac output.

A substrate is the specific reactant that an enzyme acts upon during a chemical reaction.

A disaccharide of glucose and fructose.

The additive effect of multiple muscle twitches occurring in rapid succession, leading to a greater overall force of contraction. It occurs because the muscle does not fully relax between stimuli, allowing calcium levels to remain elevated.

A branch of the autonomic nervous system that prepares the body for “fight or flight” responses during stress or emergency situations. It increases heart rate, dilates airways, mobilizes energy stores, and redirects blood flow to skeletal muscles, enhancing the body’s ability to respond to perceived threats.

The phase of the cardiac cycle during which the heart muscle contracts, causing the ventricles (and to a lesser extent the atria) to eject blood into the arteries.

The deflection on an electrocardiogram (ECG) that represents ventricular repolarization, the process by which the ventricles recover electrically after contraction. It follows the QRS complex and precedes the next cardiac cycle.

The process by which multiple postsynaptic potentials (EPSPs or IPSPs) from a single presynaptic neuron accumulate over time at the same synapse. If these signals occur in rapid succession, their combined effect can bring the postsynaptic membrane to threshold, triggering an action potential.

Enlarged regions of the sarcoplasmic reticulum located adjacent to the transverse tubules in skeletal and cardiac muscle fibers. They serve as major calcium storage sites and release calcium ions rapidly during excitation-contraction coupling, enabling efficient muscle contraction.

The male gonads responsible for producing sperm and secreting male sex hormones, primarily testosterone. Located in the scrotum, the testes consist of seminiferous tubules (where spermatogenesis occurs) and interstitial (Leydig) cells that produce androgens. They play a central role in male reproductive function and secondary sexual characteristics.

The primary male sex hormone, classified as an androgen. It is produced mainly by the testes and in smaller amounts by the adrenal cortex. Testosterone plays a key role in the development of male reproductive tissues, secondary sexual characteristics (such as increased muscle mass and body hair), and the regulation of libido and sperm production.

A sustained and continuous contraction of a muscle resulting from rapid, repeated stimulation without sufficient relaxation between stimuli. It occurs when the frequency of nerve impulses is high enough that individual muscle twitches fuse into a smooth, prolonged contraction.

(Also called the ATP–PCr system) is the body’s fastest energy system, providing immediate energy for short-duration, high-intensity activities (e.g., sprinting, heavy lifting).

A butterfly-shaped endocrine gland located in the front of the neck, just below the larynx. It produces the hormones thyroxine (T₄) and triiodothyronine (T₃), which regulate metabolism, growth, and development. The gland also secretes calcitonin, which helps regulate calcium levels in the blood.
Thyroid hormone production is controlled by thyroid-stimulating hormone (TSH) from the anterior pituitary, forming part of the hypothalamic-pituitary-thyroid axis.

A glycoprotein hormone produced by the anterior pituitary gland that stimulates the thyroid gland to produce and release thyroxine (T₄) and triiodothyronine (T₃). These thyroid hormones regulate metabolism, growth, and development.

One of the two primary hormones produced by the thyroid gland, the other being T₃ (triiodothyronine). T₄ is the predominant form of thyroid hormone circulating in the blood, but it is less biologically active than T₃. Most T₄ is converted into T₃ in peripheral tissues such as the liver and kidneys.

The amount of air inhaled or exhaled during a normal, resting breath. It represents the baseline volume of air exchanged in the lungs without conscious effort and is typically around 500 milliliters in a healthy adult.

A giant elastic protein that spans from the Z-disk to the M-line within a sarcomere, anchoring thick (myosin) filaments and providing structural stability and elasticity.

The total amount of energy an individual expends in a 24-hour period, encompassing all physiological and physical activities. TEE is composed of three main components: basal or resting metabolic rate (BMR/RMR), the thermic effect of food (TEF), and energy expended during physical activity. It represents the overall energy requirement for maintaining body weight and supporting daily functions.

The maximum volume of air the lungs can hold after a full, deep inhalation. It includes all the air in the lungs, comprising tidal volume, inspiratory reserve volume, expiratory reserve volume, and residual volume.

The deepest of the abdominal muscles, wrapping horizontally around the abdomen. It plays a key role in forced exhalation by compressing the abdominal cavity, which increases intra-abdominal pressure and helps elevate the diaphragm.

Invaginations of the sarcolemma that penetrate into the interior of skeletal and cardiac muscle fibers, forming a network of membranous channels.

Also known as the citric acid cycle or Krebs cycle, is a central metabolic pathway that takes place in the mitochondrial matrix of eukaryotic cells. It oxidizes acetyl-CoA (derived from carbohydrates, fats, and proteins) into carbon dioxide (CO₂) while generating high-energy electron carriers and a small amount of ATP.

The most common type of fat (lipid) found in the body and in food. They are composed of one glycerol molecule bonded to three fatty acids through ester bonds. Triglycerides serve as a major form of long-term energy storage, provide insulation, and protect organs.

One of the two primary hormones produced by the thyroid gland, the other being T₄ (thyroxine). Although T₃ is produced in smaller quantities than T₄, it is the more biologically active form and is responsible for regulating metabolism, growth, development, and body temperature.

A regulatory protein that runs along the length of actin filaments in muscle fibers, blocking myosin-binding sites on actin in a relaxed state.

A regulatory protein complex attached to tropomyosin on actin filaments. It controls muscle contraction by binding calcium ions, which triggers a shift in tropomyosin to expose myosin-binding sites on actin.

A single, brief contraction and relaxation cycle of a muscle fiber in response to one action potential. It consists of three phases: latent period, contraction phase, and relaxation phase.

Motor units composed of large motor neurons and fast-twitch glycolytic (Type IIx or IIb) muscle fibers. These fibers contract very rapidly and generate high force but fatigue quickly because they rely primarily on anaerobic metabolism. Type FF units are recruited last, during short-duration, high-intensity activities like sprinting or heavy lifting.

Motor units composed of medium-sized motor neurons and fast-twitch oxidative-glycolytic (Type IIa) muscle fibers. These fibers contract quickly and produce moderate force, with greater fatigue resistance than fast fatigable units due to their mixed oxidative and glycolytic metabolism. Type FR units are recruited for activities requiring both speed and endurance, such as running or cycling at moderate intensity.

(Also called slow-twitch fibers) are skeletal muscle fibers specialized for endurance and continuous, low-intensity activity. They rely primarily on aerobic metabolism for ATP production and are highly resistant to fatigue.

Specialized epithelial cells found in the alveoli of the lungs. Their primary function is to produce and secrete pulmonary surfactant, a lipoprotein substance that reduces surface tension within the alveoli. This surfactant prevents alveolar collapse during exhalation and helps maintain lung compliance, making breathing more efficient. In addition to surfactant production, these cells also play a role in alveolar repair and regeneration.

Skeletal muscle fibers specialized for rapid, powerful contractions. They rely primarily on anaerobic metabolism, fatigue quickly, and are suited for short bursts of high-intensity activity. Subtypes include Type IIa (fast oxidative-glycolytic) and Type IIx (fast glycolytic).

Motor units composed of a small motor neuron and slow-twitch (Type I) muscle fibers. These fibers contract slowly, generate low force, and are highly resistant to fatigue due to their rich supply of mitochondria, capillaries, and oxidative enzymes. Type S motor units are primarily recruited for sustained, low-intensity activities such as posture maintenance and endurance exercise, following the size principle.

A small, rounded deflection on an electrocardiogram (ECG) that follows the T wave and precedes the next P wave. It is thought to represent repolarization of the Purkinje fibers or papillary muscles. Prominent U waves may indicate conditions such as hypokalemia, bradycardia, or certain drug effects.

Blood vessels that carry blood toward the heart, typically deoxygenated (except in pulmonary and umbilical veins). Veins have thinner walls and larger lumens than arteries, and they contain valves that prevent backflow of blood, aiding its return to the heart under low pressure.

The ratio between the amount of air reaching the alveoli (ventilation) and the amount of blood reaching the alveoli via the capillaries (perfusion).

The exercise intensity at which ventilation (breathing rate and volume) increases disproportionately to oxygen uptake (VO₂). This occurs because carbon dioxide (CO₂) production rises sharply due to increased buffering of hydrogen ions (H⁺), signaling a greater reliance on anaerobic metabolism.

Small blood vessels that collect blood from capillary beds and transport it to veins. Venules have thin walls composed of endothelium and a small amount of smooth muscle, allowing them to facilitate the exchange of some substances and serve as a transition point between the microcirculation and the larger venous system.

The inner layer of the pleural membrane that directly covers the surface of each lung. It closely adheres to the lung tissue, following its contours and fissures.

The maximum amount of air a person can exhale after a full, deep inhalation. It represents the total volume of air that can be voluntarily moved in and out of the lungs and includes the inspiratory reserve volume (IRV), tidal volume (TV), and expiratory reserve volume (ERV).

The gradual increase in oxygen uptake (VO₂) during prolonged, steady-state exercise at a constant submaximal workload, typically observed after 10–15 minutes of exercise, even though the external workload remains unchanged.

Refers to the product of force applied to an object and the distance over which the force is applied, typically expressed in joules (J).

The total amount of physical effort or stress placed on the body during a bout of exercise.

A dense protein structure that defines the boundary of a sarcomere in striated muscle.