Training terms

Threshold pace is the running pace at around the hardest controlled effort you can hold for a long time. It is the running counterpart of cycling’s FTP, and it anchors your pace zones and the load calculation for your runs.

FTP (Functional Threshold Power) is a practical estimate of the highest cycling power you can hold for a long time, fairly steadily, around your threshold. It is the anchor for your power zones and for the load calculation of your rides.

LTHR (Lactate Threshold Heart Rate) is your heart rate around the lactate threshold: the effort you can just about keep up in a controlled way for a long time. For many athletes it is a more reliable anchor for heart-rate zones than formulas based on your maximum heart rate.

Heart-rate zones split the range between rest and maximum effort into bands, each with its own training purpose: from recovery and easy endurance to threshold work and VO2max intervals. Well-set zones are the difference between training on purpose and guessing.

Your aerobic threshold (LT1) is the top of your genuinely easy endurance work: up to this effort your heart rate stays flat during steady work and you can keep talking in full sentences. It sits below your threshold heart rate (LT2/LTHR) — they are two different turn points.

TSS (Training Stress Score) sums up the total load of a session in a single number, based on duration and intensity relative to your threshold. That makes sessions and training weeks comparable — the foundation under terms like fitness (CTL) and form (TSB).

TSB (Training Stress Balance), often just called "form", is the difference between your slowly built-up training load (fitness, CTL) and your recent load (fatigue, ATL). The number sums up how fresh or tired the load model thinks you are.

CTL (Chronic Training Load), called "fitness" in many apps, is a slow-moving measure of the training load you have built up over the past weeks. The number rises with consistent training and gradually fades when you do less.

ACWR (Acute:Chronic Workload Ratio) is the ratio between your recent training load and the base you built up in the weeks before. The ratio makes it visible at a glance how big a jump in load you’re making.

HRV (Heart Rate Variability) is the variation in the time between consecutive heartbeats. A healthy, rested nervous system keeps the heart from beating quite metronome-tight; that small variation is a window on your recovery, and so it is widely used to dose training load.

Critical speed (CS) is the running speed on the boundary between hard and very hard effort, estimated from your performances over several distances or durations. The swimming variant is called Critical Swim Speed (CSS); in cycling the same quantity, expressed in power, is called Critical Power (CP).

Sweat loss is the amount of fluid you lose through sweat during an effort, usually expressed in millilitres per hour (your sweat rate). It varies strongly from person to person and condition to condition, and it determines how much you sensibly drink during longer or hot sessions.

VO2max is the maximum amount of oxygen your body can take up and use per minute during hard effort, usually expressed per kilogram of body weight. It is one of the best-known measures of aerobic capacity: the ceiling within which all your endurance performances take place.

RPE (Rate of Perceived Exertion) is a simple scale for how hard an effort feels to you, usually from 1 to 10. It makes training by feel concrete and works without sensors — a full signal alongside pace, power and heart rate.

Heat acclimatisation is the degree to which your body is adapted to exercise in the heat. Someone who is used to it performs better and more safely on hot days than someone who goes in cold.

The menstrual cycle brings hormonal swings that, for some athletes, carry through noticeably into energy, recovery, body temperature and how training feels. How strongly varies enormously from person to person — and that is exactly why general per-phase rules often work poorly.

Progressive overload is the basic principle behind progress: to get stronger, faster or fitter, the stimulus has to grow gradually heavier over time. If the load stays the same, your body gets used to it and the adaptation stops.

Grade-adjusted pace (GAP) is your running pace corrected for the elevation along the way: the pace you would have run on flat ground at the same effort. That way a hard hilly run counts fairly, even when the raw pace says little.

Carbohydrates per hour is the measure of your fuelling during exercise: how much fast energy you take in along the way, in grams per hour. On longer or harder sessions it helps decide whether you stay upright or run empty.

Periodisation is splitting your training into phases, each with its own purpose, so your fitness builds up on purpose and your form coincides with your most important race. It is the framework that makes separate sessions work towards something together.

The Intensity Factor (IF) expresses in a single number how hard a session was on average relative to your own threshold. An IF of 1.0 means, roughly: the whole session at threshold level. It is the intensity side of your load, apart from how long you were out.

Your one-rep max (1RM) is the heaviest weight you can handle for exactly one repetition. It is the common measure of maximum strength — and you can estimate it reliably without ever doing a true max test.

Your resting heart rate (RHR) is the number of times your heart beats per minute at complete rest, often measured overnight or just after waking. It is a simple, free window on your recovery and overall condition.

The recovery window is the period after a hard or long session when replacing energy, protein and fluid supports your recovery most. How important that window is depends above all on when your next hard effort follows.

Your anaerobic reserve is the finite amount of extra work you can produce above your endurance limit before you have to ease off. That endurance limit is called Critical Power (cycling) or Critical Speed (running); the reserve above it is called W′ (cycling, pronounced "W-prime", measured in kilojoules) or D′ (running, measured as distance).

Your power curve is a graph of your highest average cycling power for every duration — from a sprint of a few seconds to holding an hour. At a glance it shows where you’re strong and where there’s room to grow.

Aerobic efficiency (AE) is the ratio between what you produce (speed or power) and how hard your heart beats for it, measured in a comparable easy endurance effort. Put simply: how much pace do you get per heartbeat?

Cardiac drift is the gradual rise in your heart rate while your pace or power stays the same, during a longer effort. You’re not going harder, and yet your heart beats faster as the session goes on.

W/kg is your power divided by your body weight: 240 watts at 75 kilos is 3.2 W/kg. It is the measure that decides how fast you get up a hill — climbing means lifting your own weight, after all.

Normalized power (NP) is an estimate of the steady power that would have stressed your body just as much as the variable ride you actually did. On a ride full of surges, hills and traffic lights, NP comes out higher than your average watts.

Sweet spot is the intensity just below your threshold — hard enough to push your threshold up, easy enough to stay there for quite a while. It is popular because you get relatively much training stimulus for relatively little recovery cost.

Sleep debt is the shortfall that stacks up when you sleep less than your body needs, night after night. One short night you barely notice; a week of an hour too little each time adds up to something your training definitely does feel.

Readiness is an umbrella term for scores that try to sum up in one number how well prepared you are for something. The word covers two very different things: readiness for today (your daily form) and readiness for your goal months from now.

Critical Swim Speed (CSS) is the swim pace you can hold in a long, controlled effort, usually expressed in seconds per 100 metres. For swimming it is what threshold pace is for running and FTP for cycling: the anchor under your zones and your load calculation.

Your muscles all run on the same fuel, but your body makes it in three ways: lightning fast and very briefly, quickly and in limited supply, or slowly and virtually without limit. Which route dominates depends on how hard and how long you go — and that determines what a session actually trains.