Endurance training glossary

Acute training load

ATL · fatigue

A 7-day exponentially weighted average of daily training stress, read as fatigue. It rises and falls quickly, which is the point: tiredness accumulates and dissipates far faster than fitness does.

Acute-to-chronic workload ratio

ACWR

This week's training load divided by the chronic baseline you have built. The commonly cited sweet spot is 0.8 to 1.3, with values above 1.5 associated with elevated injury risk. Below 0.8 is not safety — it is detraining. It is a population-level association, not a personal prediction.

Apple Health

HealthKit

Apple's on-device health data store on iPhone, into which watches, straps and apps write sleep, heart rate, workouts and more. Its practical significance is that an app reading from it works with whatever hardware you already own, rather than requiring a specific brand.

Base phase

The block prioritising aerobic capacity through high volume at low intensity. It is the least interesting phase and the one that determines the ceiling of everything after it.

Baseline

Your own rolling recent distribution for a metric, against which today is compared. Using a personal baseline instead of a population norm is what makes HRV and resting heart rate usable at all, since between-person variation in both dwarfs the day-to-day variation that carries the signal.

Brick session

A triathlon workout combining two disciplines back to back, almost always a ride followed immediately by a run. It trains the specific discomfort of running on legs that have just cycled, which no separate session reproduces.

Build phase

The block layering event-specific intensity onto an established aerobic base. Volume typically plateaus or drops slightly as intensity rises.

Carbohydrate periodization

Matching carbohydrate intake to the demand of each day rather than eating the same way daily. High-load days need carbohydrate; easy days need less. The evidence for deliberately training in a depleted state is much weaker than the evidence for simply fuelling hard sessions properly.

Cardiac lag

The delay between a change in effort and the corresponding change in heart rate, typically 30–120 seconds. It is why heart rate is a poor control signal for short intervals, where power or pace responds immediately.

Chronic training load

CTL · fitness

A 42-day exponentially weighted average of daily training stress, read as fitness. It moves slowly by design, because fitness is built over weeks. It is not a measure of how fast you are — it is a measure of how much training you have absorbed recently.

Critical power

CP

The asymptote of the power-duration curve — the highest power sustainable without continuously drawing on finite anaerobic reserves. Physiologically better founded than functional threshold power, and derived from several maximal efforts of different durations rather than from a single test.

Critical speed

CS

The running and swimming equivalent of critical power — the pace asymptote of the speed-duration curve. Its finite companion, D′, is the distance available above critical speed.

Decoupling

aerobic decoupling · cardiac drift

The drift of heart rate relative to output over a long steady effort, expressed as a percentage between the first and second halves. Under about 5% suggests good aerobic durability at that intensity; a large drift suggests the effort was above sustainable, or that heat or dehydration intervened.

Deload week

recovery week

A planned low-load week inside a training block, commonly every fourth week in a 3:1 cadence. It is scheduled in advance rather than taken when you finally break, which is the entire distinction between a deload and a collapse.

Deterministic

Producing the same output for the same input, every time. Applied to a training metric it means the score can be checked, reproduced and argued with — the opposite of a figure generated by a language model, which may differ between runs on identical data.

Detraining

The loss of adaptation when training load drops for long enough. Cardiovascular fitness begins measurably declining within about two weeks of inactivity, though it returns faster than it was built the first time.

Durability

The ability to hold power or pace late in a long session, after significant work has already been done. Increasingly recognised as a distinct trainable quality: fresh threshold power says little about what you can do in the fourth hour, which is what actually decides long races.

Energy availability

EA

Energy left for basic physiological function after exercise is paid for, expressed per kilogram of fat-free mass. Chronically low energy availability — under roughly 30 kcal/kg FFM/day — is the mechanism behind most of what gets loosely called overtraining, and it affects athletes of every gender.

Exponentially weighted moving average

EWMA

An average giving recent values more weight than older ones, decaying smoothly rather than dropping observations at a cutoff. Both fitness and fatigue are computed this way, which is why a single missed day nudges them rather than breaking them.

Functional threshold power

FTP

The highest average power a cyclist can hold for approximately an hour, in watts. It anchors cycling training zones and is the reference intensity for training stress scoring. Commonly estimated as about 95% of a 20-minute maximal effort.

Glycogen

The body's stored form of carbohydrate, in muscle and liver, holding roughly 90–120 minutes of hard endurance work. Running out is what "bonking" or "hitting the wall" describes.

Grade-adjusted pace

GAP

Running pace normalised for gradient, estimating the equivalent flat pace for the same effort. It exists so a hilly run can be compared with a flat one and so uphill work is not misread as an easy day.

Heart-rate variability

HRV

The variation in time between consecutive heartbeats, used as an indirect readout of autonomic nervous system state. Higher generally indicates a more recovered, parasympathetically dominant state. There is no good absolute value: it varies enormously between individuals, so only your own trend and your deviation from your own baseline carry information.

Heart-rate zones

Intensity bands defined as fractions of a reference heart rate. Zones derived from measured threshold are usable; zones derived from "220 minus age" are a population regression with an error band of roughly ±10–12 beats, wide enough to put an individual in the wrong zone entirely.

Interval

A defined segment of a structured session with its own target intensity and duration, separated by recovery. Structure exists so the hard parts are hard enough and the easy parts are actually easy.

Lactate threshold

LT2 · anaerobic threshold

The highest intensity at which lactate production and clearance stay in balance, sustainable for roughly an hour in trained athletes. It predicts endurance performance better than maximal oxygen uptake does, and unlike VO2max it responds well to training.

Monotony

Foster monotony

How similar every day of a training week is to every other day, computed as the week's mean daily load divided by its standard deviation. High monotony — above about 1.5 — means you have no real hard days and no real rest days, which is more dangerous than a week with genuine peaks.

Overreaching

A deliberate short-term overload that leaves you temporarily worse, in order to be better after recovery. Functional overreaching resolves in days and produces adaptation; non-functional overreaching takes weeks and produces nothing. The difference is only visible afterwards, which is why load is capped in advance.

Overtraining syndrome

A clinical state of long-term performance decline with hormonal, immune and mood disturbance, taking months to resolve. It is genuinely rare and quite different from being tired. Most of what athletes call overtraining is chronic underfuelling or non-functional overreaching.

Peak phase

The short sharpening block before a taper: race-pace work, trimmed volume, full recovery between hard sessions.

Performance management chart

PMC

The chart plotting fitness, fatigue and form together over time. It is the standard way endurance software visualises a training block, and its value is in the shape rather than in any single day: a healthy block shows fitness climbing while form oscillates, not a straight line anywhere.

Periodization

Structuring training into phases with different goals rather than repeating one week indefinitely. The classic sequence moves from general aerobic volume through event-specific intensity to sharpening and a taper.

Polarized training

80/20

An intensity distribution putting roughly 80% of training time at low intensity and the remainder genuinely hard, with little in between. The common failure it prevents is the "moderate trap": sessions run hard enough to accumulate fatigue but too easy to drive adaptation.

Progressive overload

The principle that adaptation requires load that gradually exceeds what you are used to. The word doing the work is "gradually": overload without a cap is just injury with extra steps.

Ramp rate

How fast planned training load grows week over week, as a percentage. Conservative practice keeps it under about 8%, which is roughly 3–5 points of fitness a week — fast enough to progress, slow enough for tendons and bone to keep up with the cardiovascular system.

Readiness

training readiness · recovery score

A composite daily score estimating how much training load you can absorb today, blended from recovery signals and current fatigue. Its usefulness depends entirely on whether the app shows you the drivers behind it — a readiness number without a reason cannot be checked against how you actually feel.

Relative energy deficiency in sport

RED-S

The syndrome of impaired function arising from prolonged low energy availability, affecting bone density, endocrine function, immunity, mood and performance. It replaced the older female-athlete-triad framing precisely because the problem is not sex-specific.

Resting heart rate

RHR

Your heart rate at complete rest, usually measured during sleep. A rise of several beats above your own baseline is a general stress signal — training load, illness, alcohol, heat or poor sleep can all produce it, and the measure cannot distinguish between them.

RMSSD

The root mean square of successive differences between heartbeats — the specific HRV statistic most wearables report, because it is the most robust to short recordings and reflects parasympathetic activity. When an app says "your HRV", it usually means overnight RMSSD.

Running economy

The oxygen cost of running at a given submaximal speed. Among athletes with similar maximal oxygen uptake it is often the largest remaining differentiator, and it improves with years of volume, strength work and technique rather than with any single session.

Sleep debt

Accumulated shortfall against sleep need over recent nights. It matters more than any single night: one bad night before a race is far less costly than a fortnight of six-hour nights before it.

Sleep need

The amount of sleep an individual actually requires, estimated from their own history rather than assumed to be eight hours. Scoring against personal need is what stops a habitually short sleeper from being permanently marked as under-recovered.

Strain

Foster strain

Weekly training load multiplied by monotony. It captures the fact that high load is survivable but high load with no variation is what tends to precede illness and injury. Not to be confused with the exertion score some wearables call "strain".

Taper

The planned load reduction before a race, shedding fatigue while preserving fitness. Evidence supports cutting volume substantially — often 40–60% over one to three weeks — while keeping intensity, since it is volume that carries the fatigue.

Time constant

The period governing how fast an exponentially weighted average responds. Fitness conventionally uses 42 days and fatigue 7, and those two numbers are most of what separates the two curves.

Training stress balance

TSB · form · freshness

Chronic training load minus acute training load — fitness minus fatigue, read as form. Strongly negative means you are buried under recent work; strongly positive means you are fresh, and if it stays there for weeks it means you are detraining. Race form is usually a small positive number, not the largest one you can reach.

Training stress score

TSS

A single number describing how much a session took out of you, combining intensity with duration. It is scaled so that roughly one hour at threshold effort equals 100. Its purpose is to make a swim, a ride and a run comparable in one currency so they can be added up.

VO2max

maximal oxygen uptake

The maximum rate at which the body can consume oxygen during exercise, in millilitres per kilogram per minute. It sets a ceiling on endurance performance but predicts it poorly among trained athletes, most of whom are separated by threshold and efficiency rather than by ceiling. Wearable estimates are correlations, not measurements.

W′

W prime · anaerobic work capacity

The finite amount of work available above critical power, in kilojoules. Think of it as a battery: it depletes when you exceed critical power and recharges below it. It explains why two riders with the same threshold can have completely different repeatability on climbs.

Zone 2

Low-intensity aerobic work, conversational, below the first ventilatory threshold. It is where most endurance volume should sit, and its benefits — mitochondrial density, fat oxidation, capillarisation — accrue from time spent there rather than from effort.