Glossary
Every term an endurance athlete runs into, defined in plain English and grounded in the science, across running, cycling, swimming and triathlon. Where a term has a calculator, you can jump straight to it.
The core ideas that explain how endurance fitness is built, measured and improved.
The capacity to sustain prolonged exercise primarily using oxygen-based energy production. It is developed mainly through high volumes of easy and moderate training that strengthen the heart, grow capillaries and increase mitochondria.
The lower intensity at which blood lactate first rises slightly above resting levels, typically around the top of easy Zone 2. Most long, low-intensity endurance training is done at or just below this point to build a large aerobic base with low fatigue.
Another name for the higher of the two metabolic thresholds, roughly the point at which lactate production clearly outpaces clearance. In practice it overlaps closely with lactate threshold and functional threshold pace or power.
An extended block of mostly low-intensity, high-volume work early in a training cycle that builds aerobic endurance, durability and efficiency. It lays the foundation that allows higher-intensity work later to be absorbed and turned into race fitness.
The sudden, severe fatigue and drop in performance that occurs when glycogen stores run low and blood glucose falls during long efforts. It is largely preventable with adequate carbohydrate fuelling before and during exercise.
The grams of carbohydrate consumed per hour during prolonged exercise to maintain blood glucose and spare glycogen. Endurance athletes commonly target around 30 to 90 grams per hour depending on event length and gut tolerance.
A Swedish term meaning speed play: a continuous session that mixes faster surges with easier running in an unstructured, feel-based way. It blends aerobic and faster work and is a flexible, lower-pressure way to add intensity.
The use of fat as fuel during exercise, which dominates at low intensities and spares limited glycogen stores. Building a strong aerobic base and training at easy intensities can increase the rate at which the body burns fat.
The stored form of carbohydrate held in muscle and liver, and the dominant fuel for moderate to high intensity endurance exercise. Limited stores mean glycogen depletion is a common cause of bonking in long events, which fuelling strategies aim to delay.
The management of body fluid and electrolyte balance before, during and after exercise to support performance and safety. Needs vary widely with body size, intensity, heat and individual sweat rate.
Structured repeats of hard effort separated by recovery, used to spend meaningful time at high intensities that cannot be held continuously. Intervals are the main tool for raising VO₂max and threshold.
The exercise intensity at which lactate begins to accumulate in the blood faster than the body can clear it, marking the upper edge of comfortably sustainable effort. Training near this point is one of the most effective ways to raise sustainable race pace.
Overreaching is a short-term performance dip from heavy training that resolves with a few days of rest, while overtraining is a deeper, longer-lasting decline tied to inadequate recovery. Warning signs include stalled performance, elevated resting heart rate, poor sleep and persistent fatigue.
The planned organisation of training into phases and cycles that progressively shift volume and intensity toward a goal event. It manages the balance between stress and recovery so fitness peaks at the right time.
A distribution of training in which roughly 80 percent of sessions are easy and around 20 percent are hard, with little time spent in the moderate middle. Research in well-trained endurance athletes links this approach to strong improvements while limiting accumulated fatigue.
A subjective rating of how hard an effort feels, commonly on a 1 to 10 or 6 to 20 scale. It lets athletes gauge intensity without devices and is valuable when heart rate or power data are unreliable, such as in heat or at altitude.
Short, fast repeats run or ridden at speeds well above threshold with full or near-full recovery, targeting speed, power and neuromuscular economy rather than the aerobic system. Reps are typically brief and not meant to accumulate fatigue.
How much oxygen or energy you use to run at a given submaximal pace, with lower consumption meaning better economy. Two runners with the same VO₂max can perform very differently depending on their economy.
The rebound in which the body, after recovering from a training stress, rebuilds to a slightly higher level of fitness than before. Timing the next hard session to land on this upswing is the basic logic behind progressive training.
The amount of fluid lost through sweat per hour of exercise, usually measured in litres per hour by weighing yourself before and after a session. Knowing your sweat rate guides how much to drink to avoid both dehydration and overdrinking.
A planned reduction in training volume in the days or weeks before a key event, designed to shed fatigue while preserving fitness. A good taper usually cuts volume substantially while keeping some intensity to stay sharp.
A moderately hard, controlled effort sitting between easy aerobic work and threshold, often described as comfortably hard. Sustained tempo runs or rides improve fatigue resistance and the ability to hold a strong pace for long durations.
Training performed at or near the lactate or functional threshold, the highest intensity sustainable for roughly 30 to 60 minutes. Repeated threshold sessions raise the pace or power you can hold before fatigue accelerates.
The maximum rate at which your body can take in, transport and use oxygen during all-out exercise, usually expressed in millilitres of oxygen per kilogram of body weight per minute (ml/kg/min). It sets the ceiling on your aerobic power and is a strong predictor of endurance potential across running, cycling, rowing and skiing.
A low-intensity training band sitting just below the aerobic threshold, where you can hold a conversation and fat is a major fuel. It is the cornerstone of aerobic base building and typically makes up the bulk of an endurance athlete's weekly hours.
Heart-rate metrics and models used to set and monitor training intensity.
The difference between your maximum and resting heart rates, representing the working range available for exercise. It is the basis of the Karvonen method, which sets zones as percentages of this reserve rather than of maximum heart rate alone.
The variation in time between consecutive heartbeats, which reflects the balance of the nervous system and overall recovery state. Higher values generally indicate readiness to train, while a persistent drop can flag accumulated fatigue or stress.
Bands of heart-rate intensity, commonly five or seven, that group training into easy, moderate and hard efforts. They translate physiology into practical targets so each session hits its intended purpose.
A way of calculating heart-rate zones that applies target percentages to the heart rate reserve and then adds the resting heart rate back in. By accounting for resting heart rate, it personalises zones better than percent-of-maximum methods, especially for very fit or unfit individuals.
The heart rate corresponding to your lactate threshold, often estimated from your average heart rate in a hard sustained time trial. Many coaches anchor heart-rate zones to LTHR because it is more individual and stable than maximum heart rate.
The Maximum Aerobic Function approach by Phil Maffetone, which caps most training at a heart rate of 180 minus your age in years, adjusted for health and experience. It is designed to keep training aerobic and build fat-burning endurance with minimal stress.
The highest number of beats per minute your heart can reach during all-out effort, largely genetic and declining slowly with age. It anchors many heart-rate zone models, though it is best confirmed by a hard field test rather than an age formula.
Your heart rate when fully at rest, ideally measured first thing in the morning before getting up. It tends to fall as aerobic fitness improves and a sustained rise can signal fatigue, illness or overtraining.
Power-based metrics for cycling and running that quantify mechanical output.
The highest power output sustainable in a metabolic steady state, derived mathematically from a set of maximal efforts of different durations. It is closely related to FTP but rests on a physiological model that also yields the finite W′ above it.
The highest power in watts a cyclist can sustain in a quasi-steady state for roughly an hour, commonly estimated as 95 percent of a 20-minute test. It is the standard reference for setting cycling power zones and tracking fitness over time.
An adjusted average power that weights surges and variability to better reflect the true physiological cost of a ride than a simple mean. It is the basis for intensity and load metrics on variable-terrain efforts.
Bands of power output, typically expressed as percentages of FTP or critical power, that structure cycling and running intensity. Because power responds instantly and is unaffected by fatigue or terrain, zones based on it are precise for prescribing efforts.
Sustainable power divided by body mass, expressed in watts per kilogram, which determines climbing and acceleration performance in cycling. It is a more meaningful comparison between riders than raw watts, especially when gravity is involved.
An estimate of the mechanical power a runner produces, measured by foot pods or watches and expressed in watts. It aims to give runners a real-time, terrain-aware intensity metric similar to what cyclists get from a power meter.
A single number that summarises the total training stress of a session by combining its intensity relative to threshold with its duration. An hour at threshold is defined as 100, giving a consistent scale to compare and plan workouts.
The fixed amount of work, measured in kilojoules, that can be performed above critical power before exhaustion, often pictured as a battery that drains during hard efforts and recharges below critical power. Managing W′ is key to pacing repeated surges and breakaways.
Pace, speed and form metrics that describe how fast and how efficiently you move.
The rhythm of movement, measured as steps per minute in running or pedal revolutions per minute in cycling. Adjusting cadence can influence efficiency, impact forces and muscular versus cardiovascular load.
The lowest running speed at which VO₂max is reached, used as a reference to prescribe interval paces and distances. Working at fractions of MAS gives a simple, individualised way to target the aerobic power system.
A pacing strategy in which the second half of a race is completed faster than the first. It is widely regarded as one of the most efficient ways to race distance events because it limits early overexertion.
Target pace ranges, often derived from a recent race or threshold test, that define easy, marathon, threshold, interval and repetition efforts. They let runners control intensity directly by speed, which is precise on flat, stable terrain.
A race finished with the second half slower than the first, usually the result of starting too fast. While sometimes deliberate on hilly courses, an unintended positive split often signals poor pacing or fuelling.
A single fitness number from Jack Daniels that represents a runner's effective VO₂max as inferred from a recent race result. It is used to prescribe consistent training paces and to predict performances at other distances.
The running velocity that elicits maximal oxygen uptake, closely related to maximal aerobic speed and combining VO₂max with running economy into one speed. Intervals around this velocity are a classic way to develop aerobic power.
Swim-specific measures of speed, efficiency and threshold in the water.
Breathing alternately to both sides while swimming freestyle, typically every third stroke. It promotes a balanced, symmetrical stroke and is useful in open water for sighting and adapting to conditions on either side.
The fastest pace a swimmer can sustain for a long distance without fatiguing, estimated from the times of a short and a longer time trial such as 200 and 400 metres. It functions as the swimming equivalent of threshold and anchors swim training paces.
Swimming in lakes, rivers or the sea rather than a pool, where there are no walls, lane lines or fixed black line to follow. It demands skills like sighting, drafting and navigating currents and waves that pool swimming does not.
The distance travelled with each stroke, also called distance per stroke, which combines with stroke rate to set swimming speed. Improving stroke length through better technique often yields faster swimming at the same effort.
The number of arm strokes a swimmer takes per minute, the swimming counterpart to running cadence. Balancing stroke rate against the distance covered per stroke is central to efficient, sustainable swimming.
The time taken to swim 100 metres or yards, the standard unit for expressing and comparing swimming speed. Training sets and goal paces are usually written relative to a swimmer's per-100 pace.
A swimming efficiency score that adds the seconds taken to swim a pool length to the number of strokes used to cover it. A lower SWOLF means more distance per stroke for the time, indicating more efficient swimming.
Race formats, pacing tactics and the language of competition and event day.
A ten-kilometre race held mostly just below to around lactate threshold, demanding strong sustained aerobic fitness. It bridges the gap between the speed-oriented 5K and longer endurance distances.
A road or track race of five kilometres, a popular entry-point distance run largely at or above lactate threshold. It rewards a blend of aerobic power and speed and is a common benchmark for fitness.
A training session in triathlon that combines two disciplines back to back, most often a bike ride immediately followed by a run. It teaches the body to handle the awkward transition and the heavy-legged feeling of running off the bike.
A result recorded when an athlete starts a race but does not complete it, whether from injury, illness, mechanical failure or missing a cut-off. It is a normal part of long and demanding events and is distinct from not starting at all.
A road race of 21.1 kilometres (13.1 miles), run close to but generally just below lactate threshold for most athletes. It demands a high aerobic capacity and disciplined pacing and fuelling.
A road race of 42.195 kilometres (26.2 miles), run at a sustainable aerobic intensity where endurance, durability and fuelling are decisive. Avoiding glycogen depletion and pacing the early miles conservatively are central to a strong finish.
The strategy of distributing effort across a race so that energy lasts to the finish without slowing or stopping early. Good pacing matches intensity to fitness, distance, terrain and conditions.
An athlete's fastest ever time at a given distance or event, called a personal record in the United States and a personal best elsewhere. It is the most common individual benchmark of progress.
Estimating a likely finish time at one distance from a known performance at another, using formulas that model how speed fades as distance grows. It guides realistic goal setting and race pacing.
The time taken to cover a defined segment of a race, such as each kilometre, mile or lap. Comparing splits across a race reveals pacing patterns like negative or positive splitting.
The timed changeover between disciplines in a triathlon, from swim to bike (T1) and from bike to run (T2). Smooth, practised transitions can save meaningful time and are sometimes called the fourth discipline.
A multisport race combining swimming, cycling and running in immediate succession, raced over distances from sprint up to the long-course Ironman. Performance depends on fitness across all three disciplines plus efficient transitions and pacing.
Any running race longer than the standard marathon distance, frequently on trails and over distances such as 50 kilometres, 100 kilometres or 100 miles. Success hinges on aerobic endurance, fuelling, hiking efficiency and mental resilience rather than raw speed.
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