Glossary

1 Repetition Maximum (1RM)Physiology | unit:kg

The maximum weight that can be lifted for a single complete repetition of an exercise. It is the gold standard for quantifying exercise load in strength training. Estimated via multi-rep prediction formulas (e.g., Epley) when direct testing is impractical.

2,3-Bisphosphoglycerate (2,3-BPG)Physiology

A small molecule produced in red blood cells that reduces hemoglobin's affinity for oxygen, promoting oxygen release to working muscles. Elevated during intense exercise and altitude exposure, 2,3-BPG acts as a fine-tuning regulator of oxygen delivery.

AcclimatizationPhysiology

The process of physiological adaptation to a changed environment — heat, humidity, or altitude — that develops over days to weeks. Heat acclimatization (10–14 days) increases plasma volume, lowers resting heart rate, and triggers earlier sweating. Altitude acclimatization increases red blood cell production via EPO.

Achilles TendonBiomechanics

The strongest tendon in the human body, connecting the calf muscles to the calcaneus (heel bone) and acting as the primary spring in running's stretch-shortening cycle. It absorbs forces several times body weight with each stride. Achilles tendinopathy is the most common overuse injury in endurance athletes.

Adenosine Triphosphate (ATP)Physiology

The universal energy currency of the cell, required for muscle contraction, ion pumping, and all biosynthetic reactions. Cellular ATP stores are tiny and deplete within seconds; the body continuously regenerates ATP via the phosphocreatine system, glycolysis, and oxidative phosphorylation.

Aerobic MetabolismPhysiology

Energy production using oxygen to oxidize glucose and fatty acids in the mitochondria, yielding 36–38 ATP per glucose molecule. The dominant pathway during prolonged exercise, aerobic metabolism accumulates fatigue slowly and is virtually unlimited in substrate.

Anaerobic MetabolismPhysiology

Energy production in the absence of adequate oxygen, generating ATP via glycolysis at the cost of producing lactate and hydrogen ions. Only 2 ATP are produced per glucose molecule, but the rate is rapid, supporting high-intensity efforts of seconds to a few minutes.

Athlete's HeartPhysiology

Cardiac hypertrophy induced by prolonged endurance training, characterized by an enlarged left ventricular chamber and thickened heart muscle. Unlike pathological hypertrophy, athlete's heart results in a larger, stronger pump that delivers more blood per beat.

Barometric PressurePhysiology | unit:hPa or mmHg

The pressure exerted by the weight of the atmosphere above a given point. At altitude, barometric pressure drops, reducing the partial pressure of oxygen in inspired air and impairing hemoglobin saturation. This is the fundamental physiological challenge of high-altitude exercise.

Beta-OxidationPhysiology

The metabolic process by which free fatty acids are progressively cleaved in the mitochondria to produce acetyl-CoA for the Krebs cycle. Fat oxidation yields far more ATP per molecule than glucose but requires more oxygen and proceeds more slowly, making it the primary fuel source at low-to-moderate intensities.

BradycardiaPhysiology | unit:bpm

A resting heart rate below 60 bpm; in elite endurance athletes it often falls below 40 bpm. Athletic bradycardia results from a high stroke volume enabling the heart to maintain cardiac output with fewer beats. It is a hallmark of aerobic fitness, not a pathology.

Brown Adipose Tissue (BAT)Physiology

A specialized fat tissue rich in mitochondria that generates heat by uncoupling oxidative phosphorylation from ATP synthesis. Abundant in infants, retained in small amounts around the neck and shoulders of adults. Regular cold exposure may activate BAT thermogenesis.

Buffering CapacityPhysiology

The ability of blood and intramuscular systems to neutralize hydrogen ions and resist acidosis during high-intensity exercise. The primary blood buffer is bicarbonate (HCO3-); muscle carnosine is the dominant intramuscular buffer. Training and beta-alanine supplementation increase buffering capacity.

CadenceBiomechanics | unit:steps/min or rpm

The number of steps per minute in running, or pedal revolutions per minute in cycling. Elite runners typically cadence at 170–180 steps/min, minimizing ground contact time and braking forces. Cyclists generally optimize efficiency at 80–100 rpm.

Calcium Release ImpairmentPhysiology

A reduction in the release of calcium ions from the sarcoplasmic reticulum into the muscle cell during repeated or prolonged contractions, impairing cross-bridge cycling and force production. This is a key molecular mechanism of peripheral fatigue; adequate sleep and cooling accelerate recovery of calcium channel function.

Capillary DensityPhysiology | unit:capillaries/mm²

The number of capillaries per unit area of muscle tissue. Endurance training increases capillary density, improving oxygen and nutrient delivery to muscle fibers and enhancing lactate clearance. Higher capillary density directly boosts aerobic capacity.

Carbon Dioxide Production (VCO2)Physiology | unit:L/min

The rate of CO2 produced by metabolic processes during exercise. Measured alongside VO2 to calculate the respiratory exchange ratio, it reflects substrate utilization. During heavy anaerobic work, excess CO2 is produced as bicarbonate buffers hydrogen ions, causing VCO2 to rise disproportionately.

Cardiac OutputPhysiology | unit:L/min

The total volume of blood pumped by the heart per minute, equal to heart rate multiplied by stroke volume. Resting cardiac output is approximately 5 L/min; elite endurance athletes can reach 40 L/min during maximal exercise.

Central FatiguePhysiology

Fatigue originating in the brain and central nervous system rather than the muscles themselves, characterized by reduced motor cortex output and weakened drive to muscles. Contributing factors include altered serotonin/dopamine balance, elevated brain temperature, and ammonia accumulation. Central and peripheral fatigue interact continuously during prolonged exercise.

Central Governor ModelPsychology

Tim Noakes's model proposing that the brain subconsciously regulates exercise intensity to protect vital organs from damage, creating a 'physiological reserve' that is never fully depleted. Fatigue is thus a protective brain-generated emotion rather than a peripheral muscle failure. The model is supported by end-sprint phenomena and controversial in mechanistic detail.

Critical Power (CP)Training Science | unit:W

The theoretical maximum power or speed that can be sustained indefinitely by the aerobic energy system. In practice, CP can be held for 30–60 minutes. Exercise above CP depletes the finite anaerobic work capacity (W') reservoir; once exhausted, exercise must stop or intensity must fall below CP.

DetrainingTraining Science

The partial or complete reversal of physiological adaptations when training ceases or is substantially reduced. VO2max declines measurably within 2–3 weeks; muscle mass diminishes more slowly. 'Muscle memory' facilitates faster retraining compared to first-time training.

Diastolic PressurePhysiology | unit:mmHg

The arterial blood pressure during the heart's relaxation phase between beats. Regular aerobic exercise lowers resting diastolic pressure, reducing cardiovascular disease risk. In a reading like 120/80 mmHg, the lower number represents diastolic pressure.

Dissociation (Attentional Focus)Psychology

A cognitive strategy in which an athlete deliberately shifts attention away from fatigue and discomfort toward external stimuli such as scenery or music. Common among recreational endurance athletes, dissociation can temporarily reduce perceived exertion; elite athletes more often use associative focus (monitoring body signals) for optimized pacing.

Ejection FractionPhysiology | unit:%

The percentage of blood in the left ventricle that is pumped out with each contraction. Normal values are 55–70%; in athletes this should be interpreted alongside absolute stroke volume. It is a key measure of cardiac pumping efficiency.

EPOC (Excess Post-Exercise Oxygen Consumption)Physiology

The elevated metabolic rate persisting after exercise ceases, reflecting restoration of oxygen stores, lactate clearance, temperature normalization, and hormonal recovery. High-intensity and resistance training produce greater EPOC than moderate steady-state work, extending the caloric cost of a session.

Erythropoietin (EPO)Physiology

A glycoprotein hormone produced by the kidneys that stimulates red blood cell production in bone marrow. EPO rises naturally at altitude; synthetic EPO is a banned performance-enhancing drug that artificially elevates hematocrit and oxygen-carrying capacity.

Exercise Economy TestTraining Science

A laboratory test measuring steady-state oxygen consumption at multiple fixed exercise intensities (speeds or power outputs) to quantify metabolic efficiency. Typically performed on a treadmill or cycle ergometer with 4–6 minutes at each stage. Results are independent of VO2max and provide strong performance predictions.

Fat AdaptationNutrition

The process of enhancing skeletal muscle fat oxidation capacity through low-carbohydrate diet and/or long slow-distance training, sparing glycogen during prolonged exercise. While fat adaptation improves lipid utilization, high-intensity performance still depends on carbohydrate availability.

Free Fatty Acids (FFA)Physiology

Lipid molecules released from adipose tissue into the bloodstream, serving as a major fuel during low-to-moderate-intensity endurance exercise. Beta-oxidation in the mitochondria yields substantially more ATP per molecule than glucose. Adipose fat stores are practically unlimited compared to glycogen.

GlucagonPhysiology

A pancreatic hormone released when blood glucose falls, stimulating hepatic glycogenolysis and gluconeogenesis to maintain blood glucose levels. During prolonged exercise, glucagon rises as insulin falls, ensuring a continuous glucose supply to working muscles.

GlucoseNutrition

The primary simple sugar and the most readily available fuel for cells. Ingested carbohydrates are digested to glucose and stored as glycogen in muscles and liver. During exercise, glucose from both glycogen breakdown and dietary intake serves as a key energy substrate.

GlycogenNutrition | unit:mmol/kg wet weight

The polymer form of glucose stored in muscles and the liver. During exercise, glycogen is broken down to glucose for energy. Depletion of muscle glycogen during events longer than 90 minutes — 'bonking' or 'hitting the wall' — is a primary limiting factor in endurance performance.

Glycogen Loading (Carbohydrate Loading)Nutrition

A pre-competition nutritional strategy of several days of high carbohydrate intake combined with reduced training volume to maximize muscle and liver glycogen stores. Effective for events exceeding 90 minutes, it delays glycogen depletion and the associated performance decline.

GlycolysisPhysiology

The cytoplasmic pathway that breaks glucose down to pyruvate, producing 2 ATP net. Glycolysis operates with or without oxygen; in the presence of oxygen, pyruvate enters the Krebs cycle; under oxygen-limited conditions it is converted to lactate.

Ground Contact Time (GCT)Biomechanics | unit:ms

The duration that the foot remains in contact with the ground during each running step. Elite runners maintain GCT of 160–200 ms; longer contact times reduce the efficiency of the stretch-shortening cycle and accelerate fatigue. Rising GCT during a race is an early fatigue indicator.

Heart RatePhysiology | unit:bpm

The number of times the heart beats per minute. It increases proportionally with exercise intensity to meet rising oxygen demand. Maximum heart rate declines with age; resting heart rate decreases with training and serves as a fitness indicator.

Heart Rate Variability (HRV)Recovery | unit:ms

The beat-to-beat variation in the time intervals between consecutive heartbeats, measured in milliseconds. High HRV indicates good recovery and parasympathetic dominance; low HRV signals fatigue, illness, or overreaching. Morning HRV monitoring is increasingly used to individualize training load.

Heat StrokePhysiology

A medical emergency in which thermoregulatory failure allows core temperature to exceed 40°C, causing central nervous system dysfunction, confusion, and potential multi-organ damage. Immediate cooling is the only effective treatment; event medical teams and cooling tents are mandatory.

HematocritPhysiology | unit:%

The proportion of blood volume occupied by red blood cells. Altitude training and EPO stimulation increase hematocrit, enhancing oxygen transport. Anti-doping regulations set the legal upper limit at 50% for males and 47% for females.

HematopoiesisPhysiology

The production of all blood cells — red cells, white cells, and platelets — in the bone marrow. Endurance training and altitude exposure stimulate EPO-driven hematopoiesis, increasing red blood cell count and oxygen transport capacity.

Hemoglobin Saturation (SpO2)Physiology | unit:%

The percentage of hemoglobin molecules in the blood that are bound to oxygen. At sea level in healthy individuals, SpO2 is 97–99%. It falls at altitude or with pulmonary disease and is easily measured with a pulse oximeter.

Henneman's Size PrinciplePhysiology

The principle that motor units are recruited in order from smallest (Type I, fatigue-resistant) to largest (Type IIx, fast-fatiguing) as exercise intensity increases. This orderly recruitment optimizes efficiency and fatigue resistance across a wide range of loads.

HydrationNutrition

The maintenance of optimal fluid balance for health and exercise performance. Body mass losses of 2% from sweat substantially impair aerobic performance; losses of 5–8% create serious medical risk. Adequate fluid and electrolyte intake before, during, and after exercise supports thermoregulation and muscle function.

HyperthermiaPhysiology | unit:°C

A core body temperature significantly above 37°C, commonly reaching 39–41°C during exercise in the heat. The central nervous system uses hyperthermia as a fatigue signal, reducing exercise intensity. Core temperatures above 40°C pose serious health risk.

HyponatremiaPhysiology | unit:mmol/L Na+

Below-normal serum sodium concentration, caused during endurance events by excessive hypotonic fluid intake without sodium replacement. A potentially life-threatening condition most common in ultra-distance events, presenting with nausea, headache, and in severe cases, seizure or coma.

HypothermiaPhysiology | unit:°C

A core body temperature below 35°C, risking cardiac arrhythmia, impaired cognition, and loss of motor coordination. A hazard in open-water swimming, winter ultramarathons, and high-mountain races. Early symptoms include shivering and reduced performance; advanced stages involve cardiac arrest.

HypoxemiaPhysiology | unit:SaO2 % or PaO2 mmHg

A below-normal oxygen level in arterial blood. It can result from altitude, pulmonary disease, or extreme exercise intensity. Some elite athletes experience exercise-induced arterial hypoxemia (EIAH) during maximal efforts when ventilatory capacity is outpaced by oxygen demand.

HypoxiaPhysiology

Insufficient oxygen delivery to tissues. Altitude hypoxia triggers EPO secretion, increased ventilation, and over time greater capillarization. Controlled hypoxic exposure via altitude camps and hypoxic tents is used deliberately to stimulate erythropoietic adaptations.

IGF-1 (Insulin-like Growth Factor-1)Physiology

A hormone produced mainly in the liver and locally in muscles in response to growth hormone, promoting muscle cell growth, satellite cell activation, and protein synthesis. Exercise raises IGF-1 levels, accelerating post-training repair and hypertrophy. IGF-1 declines with age, contributing to sarcopenia.

InsulinPhysiology

A pancreatic hormone that promotes glucose uptake by cells and glycogen synthesis after carbohydrate ingestion. Insulin levels fall during exercise as muscles can take up glucose insulin-independently. Regular exercise increases insulin sensitivity, reducing type 2 diabetes risk.

Interval TrainingTraining Science

A training method alternating bouts of high-intensity work with periods of active or passive recovery. Interval training allows a greater total volume of high-intensity work than continuous training at the same intensity. It effectively develops both aerobic and anaerobic capacity.

Krebs Cycle (Citric Acid Cycle)Physiology

A series of chemical reactions in the mitochondrial matrix that oxidize acetyl-CoA to carbon dioxide, producing electron carriers (NADH, FADH2) used in oxidative phosphorylation. Carbohydrates, fats, and proteins all converge on this central metabolic pathway.

LactatePhysiology | unit:mmol/L

A molecule produced from pyruvate when glycolysis accelerates beyond the aerobic system's capacity. Long mischaracterized as a waste product, lactate is a valuable fuel shuttled between cells and metabolized aerobically. Elevated blood lactate indicates the onset of metabolic acidosis.

Lactate Shuttle HypothesisPhysiology

George Brooks's hypothesis proposing that lactate is an energy intermediate shuttled between producing cells and consuming cells (heart, brain, oxidative muscle fibers) rather than a metabolic dead end. This view transformed understanding of high-intensity exercise and lactate physiology.

Lactate Threshold (LT)Physiology | unit:%VO2max or pace/power

The exercise intensity at which blood lactate begins to accumulate above resting levels. LT1 marks the onset of lactate accumulation; LT2 (or MLSS) marks the point where the buffering system is overwhelmed. Both thresholds shift to higher intensities with endurance training.

Lactate Threshold TestTraining Science | unit:mmol/L

An incremental exercise test with blood lactate measurements at each stage (typically from earlobe or fingertip) to construct a lactate-intensity curve. LT1 and LT2 are identified from inflection points in this curve, enabling precise personalized training zone prescription.

Live High – Train Low (LHTL)Training Science

A strategy in which athletes sleep and rest at altitude (typically 2,000–2,500 m) to elevate EPO and red blood cell production while performing high-quality workouts at sea level. LHTL preserves both the hematological benefits of altitude and the mechanical quality of low-altitude training.

Maximal Aerobic Power (MAP)Training Science | unit:W

The highest power output at which VO2max is achieved during an incremental test on a cycle ergometer. MAP reflects both aerobic capacity and mechanical efficiency. Athletes with identical VO2max but different mechanical efficiency will differ in MAP.

Maximal Lactate Steady State (MLSS)Training Science | unit:mmol/L or pace/power

The highest exercise intensity at which blood lactate concentration remains stable over time, reflecting perfect balance between lactate production and clearance. MLSS is the most physiologically meaningful training threshold but requires multiple sessions to determine. The 4 mmol/L lactate level is a common clinical approximation.

Mechanical EfficiencyBiomechanics | unit:%

The fraction of metabolic energy input converted into useful mechanical work. The human body converts only 20–25% of fuel energy into mechanical work during exercise; the remainder is released as heat. Training, SSC utilization, and optimal biomechanics improve this ratio.

Minute VentilationPhysiology | unit:L/min

The total volume of air inhaled and exhaled per minute, calculated as tidal volume multiplied by respiratory rate. At rest it is 6–10 L/min; during maximal exercise it can exceed 150–200 L/min. Higher ventilation increases oxygen delivery and carbon dioxide removal.

MitochondriaPhysiology

The organelles in which aerobic metabolism occurs. Endurance training stimulates mitochondrial biogenesis, increasing both mitochondrial number and oxidative enzyme activity per unit of muscle. This adaptation is the principal cellular mechanism underlying improved aerobic capacity.

Mitochondrial BiogenesisPhysiology

The creation of new mitochondria from existing ones, triggered by endurance training through the master regulator PGC-1alpha. Increased mitochondrial volume density directly enhances aerobic capacity, fat oxidation, and lactate clearance. Both volume training and HIIT activate PGC-1alpha via distinct signaling pathways.

MotivationPsychology

The internal and external drives that initiate and sustain goal-directed behavior. Intrinsic motivation (love of the activity, mastery seeking) predicts long-term adherence better than extrinsic motivation (rewards, approval). Under extreme fatigue, psychological motivation can override perceived physiological limits.

Motor UnitPhysiology

A single motor neuron and all the muscle fibers it innervates. Small motor units control fine movements; large motor units generate powerful contractions. Endurance training improves motor unit coordination and fatigue resistance.

Muscle BiopsyTraining Science

A minimally invasive procedure that extracts a small sample of muscle tissue (typically from the vastus lateralis) for analysis of fiber type distribution, mitochondrial density, glycogen content, and enzyme activity. Muscle biopsy is the definitive tool for studying cellular adaptations to training.

Muscular AcidosisPhysiology

The fall in intramuscular pH caused by hydrogen ion accumulation from rapid glycolysis and ATP hydrolysis during high-intensity exercise. When pH falls below approximately 6.9, enzyme activity and cross-bridge cycling are impaired, reducing force production. Muscular acidosis is a central mechanism of peripheral fatigue.

Myofibrillar HypertrophyPhysiology

Muscle growth driven by an increase in the number and size of contractile units (myofibrils) within muscle fibers. Resistance training produces this type of hypertrophy, increasing both muscle size and strength. Endurance training, by contrast, primarily increases mitochondrial density without significant fiber enlargement.

MyoglobinPhysiology

An iron-containing oxygen-binding protein in muscle cells that stores and facilitates oxygen diffusion within the fiber. Abundant in Type I (slow-twitch) fibers, giving them their red color. Myoglobin serves as a short-term oxygen reservoir and increases with endurance training.

Myosin Heavy Chain (MHC)Physiology

The primary contractile protein that determines muscle fiber shortening velocity and force production. The three main isoforms — MHC-I (slow), MHC-IIa (intermediate), and MHC-IIx (fast) — correspond to slow-twitch, intermediate, and fast-twitch fiber types. Training can shift MHC isoform expression toward more oxidative profiles.

Neural Motor ControlPsychology

The central nervous system's ability to coordinate motor commands to produce efficient, well-timed muscle activation patterns. Repeated training automates motor programs, reducing cognitive load and metabolic cost per movement. Elite athletes execute complex movements with less neural effort than novices.

Neural PlasticityPsychology

The ability of the nervous system to reorganize structurally and functionally in response to training, experience, or injury. Motor cortex training develops more efficient neural programs, reducing cognitive effort for learned movements. In elite athletes, complex movement patterns become near-automatic through neural plasticity.

Neuromuscular JunctionPhysiology

The specialized synapse between a motor neuron's terminal and a muscle fiber, where acetylcholine transmits the electrical signal triggering contraction. Training increases the reliability and speed of neuromuscular transmission; fatigue can transiently impair synaptic function. The efficiency of neuromuscular junctions determines how quickly muscles respond to motor commands.

Overload PrincipleTraining Science

The foundational training principle stating that physiological adaptation occurs only when exercise stress exceeds the body's current capacity. Insufficient overload produces no adaptation; excessive overload causes injury or OTS. The optimal stimulus lies in a narrow zone between these extremes.

Overtraining Syndrome (OTS)Recovery

A clinical condition arising from sustained excessive training load without adequate recovery, characterized by persistent performance decline, chronic fatigue, mood disturbances, and impaired immune function. Full recovery may require weeks to months of rest.

Oxidative PhosphorylationPhysiology

The final stage of aerobic metabolism occurring in the mitochondrial inner membrane, where electrons from NADH and FADH2 are passed along the electron transport chain to oxygen, driving ATP synthase to produce the majority of cellular ATP (32–34 per glucose).

Partial Pressure of Oxygen (PaO2)Physiology | unit:mmHg

The pressure exerted by oxygen molecules in a gas mixture or in blood. At altitude, reduced barometric pressure lowers PaO2, decreasing hemoglobin oxygen loading in the lungs. This mechanism underlies the performance impairment observed at high elevation.

Pennation AngleBiomechanics

The angle at which muscle fibers are oriented relative to the axis of force production. A greater pennation angle allows more fibers to pack into a given cross-sectional area, increasing force output but reducing shortening velocity. Optimal pennation angle depends on whether power or speed is the priority.

PeriodizationTraining Science

The systematic organization of training into hierarchical cycles — macrocycle, mesocycle, and microcycle — that progressively manipulate load and recovery to peak performance for key competitions. Linear, undulating, and block periodization are common models.

Peripheral FatiguePhysiology

Fatigue localized within the muscle itself, resulting from calcium release impairment, inorganic phosphate accumulation, acidosis, and substrate depletion. It is quantified as a decline in muscle force in response to electrical stimulation. Peripheral and central fatigue develop simultaneously and mutually amplify each other.

PGC-1alphaPhysiology

The master transcriptional co-activator orchestrating mitochondrial biogenesis, fatty acid oxidation, and angiogenesis in response to endurance exercise. Activated by AMPK, CaMK, and SIRT1 signaling, PGC-1alpha is the central switch linking exercise stimulus to cellular aerobic adaptation.

Phosphocreatine (PCr) SystemTraining Science

The fastest energy system, regenerating ATP by transferring a phosphate group from stored phosphocreatine to ADP. It requires no chemical reaction chain, making it instantaneous. PCr stores are exhausted within 5–10 seconds, limiting its use to explosive efforts like sprinting and weightlifting.

Plasma Volume ExpansionPhysiology

An early training adaptation in which the fluid component of blood increases, diluting hematocrit temporarily but increasing stroke volume, thermoregulatory capacity, and lactate buffering capacity. Plasma volume expansion is one of the fastest cardiovascular adaptations, occurring within the first days of training.

Plyometric TrainingTraining Science

Training that exploits the stretch-shortening cycle through explosive jumping, depth drops, and bounding to develop tendon stiffness, neural activation, and muscle contraction speed. Plyometrics improve running economy and power in endurance athletes without adding substantial muscle mass.

Power (Watts)Training Science | unit:W or W/kg

The rate of mechanical work output, calculated as force multiplied by velocity. Measured directly via cycling power meters, power is unaffected by wind, gradient, or fatigue pacing errors, making it the most objective measure of exercise output. Power-to-weight ratio (W/kg) enables fair comparison between athletes.

Psychobiological ModelPsychology

Samuele Marcora's model proposing that exercise is terminated when perceived effort becomes intolerable, not when muscles fail. As long as motivation is sufficient, athletes can continue beyond apparent physical limits. The model validates mental training and psychological strategies as genuine performance enhancers.

Pulmonary DiffusionPhysiology

The passive exchange of oxygen from alveoli into pulmonary capillaries and carbon dioxide in the reverse direction, driven by partial pressure gradients. Elite endurance training increases diffusion capacity, accelerating oxygen loading of the blood during heavy exercise.

PyruvatePhysiology

The end product of glycolysis and the metabolic crossroads between aerobic and anaerobic pathways. When oxygen is available, pyruvate enters the mitochondria as acetyl-CoA for the Krebs cycle; under oxygen shortage it is reduced to lactate.

Rating of Perceived Exertion (RPE)Psychology | unit:6-20 or 0-10

A subjective scale quantifying how hard an exercise bout feels. Gunnar Borg's classic scale runs from 6 (rest) to 20 (maximal effort); the modified CR10 scale spans 0–10. RPE is highly reliable and correlates strongly with heart rate, making it a valid intensity monitoring tool.

Respiratory Exchange Ratio (RER)Physiology | unit:VCO2/VO2

The ratio of carbon dioxide produced to oxygen consumed (VCO2/VO2). An RER near 0.70 indicates pure fat oxidation; near 1.00 indicates pure carbohydrate oxidation. Values above 1.10 signal significant anaerobic metabolism.

Running EconomyBiomechanics | unit:mL/kg/km

The oxygen cost of running at a given speed; lower oxygen consumption at a set pace indicates better economy. Running economy is influenced by stride mechanics, tendon stiffness, footwear, and years of training. Two athletes with identical VO2max can differ substantially in race performance due to differences in running economy.

SarcomerePhysiology

The smallest functional unit of a muscle fiber, formed by overlapping actin and myosin filaments. When a contraction signal arrives, myosin heads pull actin filaments inward, shortening the sarcomere. Thousands of sarcomeres in series form a single muscle fiber.

Satellite CellPhysiology

Muscle stem cells located on the surface of muscle fibers that activate after exercise-induced damage or overload. Satellite cells proliferate and fuse with existing fibers or form new fibers, underpinning muscle repair and hypertrophy.

Self-TalkPsychology

Internal or spoken verbal cues an athlete uses during exercise to manage effort, correct technique, or sustain motivation. Short motivational phrases (e.g., 'keep going', 'relax') have been shown in controlled studies to reduce perceived exertion and extend time-to-exhaustion. Both motivational and instructional self-talk are effective.

Skeletal Muscle PumpPhysiology

The mechanism by which rhythmic muscle contractions during exercise compress veins, propelling blood toward the heart and augmenting venous return. This pump prevents venous pooling during upright exercise and supports the elevated cardiac output required by working muscles.

Sleep and RecoveryRecovery

Sleep is the primary window for training-induced adaptations, when growth hormone and IGF-1 peak and protein synthesis accelerates. Endurance athletes require 8–9 hours per night, rising to 9–10 hours during heavy training blocks. Chronic sleep restriction impairs performance, immunity, and cognition.

Stretch-Shortening Cycle (SSC)Biomechanics

A mechanical cycle in which a muscle-tendon unit is rapidly stretched (eccentric phase) immediately before it shortens (concentric phase), recovering elastic energy stored in tendons. The SSC underlies running and jumping efficiency and is the basis of plyometric training.

Stride LengthBiomechanics | unit:meters

The horizontal distance covered in a single running stride. Running velocity equals stride length multiplied by cadence. Overstriding (landing the foot far ahead of the center of mass) increases braking forces and injury risk.

Stroke VolumePhysiology | unit:mL/beat

The volume of blood ejected from the left ventricle with each heartbeat. Endurance training increases stroke volume through cardiac muscle strengthening and plasma volume expansion, allowing the heart to pump more blood per beat at rest and during exercise.

SupercompensationTraining Science

The biological adaptation cycle in which the body, after recovering from a training stress, temporarily achieves a fitness level slightly above its previous baseline. Timing the next training session within this elevated window accumulates progressive gains; too early or too late reduces them.

Sweating RatePhysiology | unit:L/hr

The volume of sweat produced per unit time as a thermoregulatory response. In hot, humid conditions, sweat rate can exceed 1–2 L/hour. Unreplaced sweat loss leads to dehydration, impairing performance and thermoregulation. Heat acclimatization increases sweat rate and lowers the temperature threshold for sweating onset.

Systolic PressurePhysiology | unit:mmHg

The arterial blood pressure at the moment of cardiac contraction. It rises during exercise and decreases at rest with regular aerobic training. The first (higher) number in a blood pressure reading represents systolic pressure.

TendonBiomechanics

A dense connective tissue composed mainly of type I collagen that transmits muscle force to bone. Tendons store and release elastic energy in the stretch-shortening cycle, acting as biological springs. Training increases both tendon strength and stiffness, improving running economy.

Tendon StiffnessBiomechanics | unit:N/m

The resistance of a tendon to elongation under load; a stiffer tendon deforms less for a given force. Endurance training stiffens the Achilles and patellar tendons, enhancing elastic energy storage and return in the stretch-shortening cycle. Extreme stiffness, however, may increase injury risk.

ThermoregulationPhysiology

The set of physiological mechanisms that maintain core body temperature within a narrow range (36.5–37.5°C). During exercise, heat produced by working muscles is dissipated via sweating, radiation, and increased skin blood flow. In hot and humid conditions these mechanisms are challenged, and performance declines.

Training LoadTraining Science

A quantification of the total physiological and psychological stress imposed by training, combining intensity, volume, and frequency. Appropriate training load leads to supercompensation; excessive load without sufficient recovery results in overtraining syndrome.

Type I Fiber (Slow-Twitch)Physiology

Fatigue-resistant, aerobically powered muscle fibers with high mitochondrial and myoglobin content, giving them a red appearance. They are activated first during low-intensity exercise and can sustain aerobic work for hours. Marathon runners' muscles are predominantly Type I.

Type IIa Fiber (Fast Oxidative-Glycolytic)Physiology

An intermediate fiber type that is faster and more powerful than Type I but more fatigue-resistant than Type IIx. Activated during moderate-to-high-intensity exercise, these fibers can shift toward a more oxidative profile with endurance training.

Type IIx Fiber (Fast Glycolytic)Physiology

The fastest and most powerful muscle fiber type, relying primarily on glycolysis. Low mitochondrial density makes them fatigue rapidly, limiting their contribution to seconds of maximal effort. Dominant in sprinters and power athletes.

Venous ReturnPhysiology

The volume of blood returning from the peripheral circulation to the heart per unit time. During exercise, the skeletal muscle pump, respiratory pump, and venoconstriction enhance venous return, directly increasing stroke volume and cardiac output.

Ventilatory Threshold (VT1/VT2)Physiology

Two inflection points in ventilation rate during incremental exercise. VT1 marks the onset of lactate accumulation; VT2 (equivalent to LT2) reflects the limit of bicarbonate buffering. These thresholds can be detected non-invasively via respiratory gas analysis.

Vertical OscillationBiomechanics | unit:cm

The up-and-down displacement of the center of mass during running. Elite runners typically show 6–8 cm of vertical oscillation; excess vertical movement wastes energy that could propel horizontal motion. Lower oscillation generally indicates superior running economy.

VO2maxPhysiology | unit:mL/kg/min

The maximum rate at which the body can consume oxygen during incremental exercise. Expressed in mL/kg/min, it is the gold standard of aerobic fitness. Genetics set the ceiling; endurance training can raise it 15–25%.

VO2max TestTraining Science | unit:mL/kg/min

An incremental exercise test to exhaustion on a treadmill or cycle ergometer with breath-by-breath respiratory gas analysis. VO2 plateaus at maximal aerobic capacity. It is the reference standard for aerobic fitness in endurance sport and research.

W-prime (W')Training Science | unit:J

The finite anaerobic work capacity available above Critical Power, measured in joules. Every second spent above CP depletes W'; every second below CP replenishes it at a known rate. Cyclists use W' models to simulate race attacks, breakaways, and pacing strategy.