Glossary

The endurance training 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.

Training Concepts & Physiology

The core ideas that explain how endurance fitness is built, measured and improved.

Aerobic Endurance

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.

Aerobic ThresholdAeT

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.

Base Training

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.

Bonking

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.

Carbohydrate Intake

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.

Fartlek

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.

Fat Oxidation

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.

Glycogen

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.

Hydration

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.

Interval Training

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.

Overtraining & Overreaching

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.

Polarized Training

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.

Rate of Perceived ExertionRPE

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.

Repetition TrainingReps

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.

Running Economy

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.

Supercompensation

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.

Sweat Rate

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.

Taper

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.

Tempo

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.

VO₂maxVO₂max

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.

Heart Rate

Heart-rate metrics and models used to set and monitor training intensity.

Heart Rate VariabilityHRV

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.

MAF MethodMAF

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.

Power

Power-based metrics for cycling and running that quantify mechanical output.

Power-to-Weight RatioW/kg

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.

Training Stress ScoreTSS

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.

W PrimeW′

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

Pace, speed and form metrics that describe how fast and how efficiently you move.

Cadence

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.

Maximal Aerobic SpeedMAS

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.

Negative Split

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.

Pace Zones

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.

Positive Split

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.

Velocity at VO₂maxvVO₂max

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.

Swimming

Swim-specific measures of speed, efficiency and threshold in the water.

Bilateral Breathing

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.

Critical Swim SpeedCSS

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.

Open Water Swimming

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.

Stroke LengthDPS

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.

Stroke RateSR

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.

Swim Pace per 100

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.

SWOLF

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.

Racing & Events

Race formats, pacing tactics and the language of competition and event day.

5K

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.

Brick Workout

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.

Did Not FinishDNF

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.

Half Marathon

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.

Pacing

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.

Personal RecordPR / PB

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.

Race Time Prediction

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.

TransitionT1/T2

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.

Triathlon

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.

UltramarathonUltra

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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