Training and recovery glossary

The numbers your watch, ring and training apps show you, explained in plain English. What each one means, why it matters for your next training decision, and where wearables get it wrong.

Wearables give endurance, HYROX and hybrid athletes more data than ever, but the same word can mean different things on different devices. Each entry below has a short definition and a full page covering how the metric is measured, its limits, related terms and the research behind it.

Recovery and HRV

Overnight signals your wearable uses to judge how recovered you are.

Heart rate variability (HRV)

The small, constantly changing gaps between heartbeats. Measured overnight and compared with your own baseline, it's one of the most useful recovery signals a wearable gives you.

Overreaching and overtraining

Overreaching is a short-term drop in performance from heavy training. If recovery is planned it can lead to improvement, but if load stays too high for too long it can slide into a longer, more serious slump.

Recovery score

A single number that rolls overnight data such as HRV, resting heart rate and sleep into one daily read on how ready you are to train. Each brand builds theirs differently.

Respiratory rate

How many breaths you take per minute, usually measured during sleep. It's normally very steady night to night, so a clear rise above your usual range stands out.

Resting heart rate (RHR)

How fast your heart beats when you're fully at rest. A reading that sits above your usual range for a few days can point to fatigue, illness, heat or poor sleep.

RMSSD

A way of calculating HRV from the differences between consecutive heartbeats. It mainly reflects parasympathetic activity and is what WHOOP, Oura and many HRV apps report.

SDNN

An HRV measure based on the overall spread of the gaps between normal heartbeats. It's the HRV metric Apple Health stores from Apple Watch.

Heart and fitness

Fitness markers and the heart rate numbers that set your training intensity.

Cardiac drift

The slow rise in heart rate during a long, steady session even though your pace or power hasn't changed. Heat and dehydration make it bigger.

Heart rate zones

Bands of heart rate, usually five, that divide effort from very easy to maximal. They help you keep easy days easy and hard days on target.

Lactate threshold (LT)

The effort at which lactate starts building up in your blood faster than your body clears it. Threshold pace, power or heart rate is a strong guide to endurance performance.

Maximum heart rate (HRmax)

The highest heart rate you can reach in an all-out effort. Age formulas give only a rough guess, so a tested value makes heart rate zones far more reliable.

VO2 max

The most oxygen your body can take in and use during all-out exercise, in millilitres per kilogram per minute. Watches estimate it from heart rate and pace rather than measuring it.

Zone 2

Steady, comfortable aerobic training at an effort you could hold for a long time while still talking. Its exact boundaries depend on which zone system you use.

Training load and racing

How training stress is counted, modelled and managed, from build weeks to race day.

Acute:chronic workload ratio (ACWR)

Your recent (usually 1-week) training load divided by your longer-term (usually 4-week) load. It's popular for spotting spikes, but its link to injury risk is disputed.

Compromised running

Running straight after a non-running effort, such as a sled push, lunges or wall balls, when your legs and breathing are already taxed. In HYROX, seven of the eight 1 km runs are compromised.

CTL and ATL

Two rolling averages of daily training load. CTL, shown as Fitness in intervals.icu, covers about six weeks. ATL, shown as Fatigue, covers about one week.

Form (TSB)

Fitness minus fatigue (CTL minus ATL). Negative means recent training is heavier than you're used to. Rising towards positive usually means you're freshening up.

Tapering

Cutting training back in the days or weeks before a key race so fatigue clears while fitness holds. Most of the evidence favours cutting volume, not intensity.

Training load

A number for how much training stress you've done: how long and how hard, combined. Watching how it changes week to week matters more than any single session.

Training monotony and strain

Monotony measures how little your daily training load varies across a week. Strain combines that with total load. High values mean hard days with no real easy days between them.

TRIMP

A heart-rate-based training load score that multiplies session time by how hard your heart was working, with extra weight on higher intensities.

Sleep

What sleep trackers report, and which parts of sleep matter most for training.

Deep sleep

The deepest stage of non-REM sleep, known clinically as N3 or slow-wave sleep. Wearables estimate it from movement and heart rate, and it's the stage they most often get wrong.

REM sleep

The sleep stage with rapid eye movements, irregular breathing and brain activity that looks a lot like being awake. Most dreaming happens here.

Sleep consistency

How similar your sleep and wake times are from night to night. Regular timing supports your body clock, and Apple's sleep score gives it 30 of its 100 points.

Sleep debt

The running shortfall between the sleep you need and the sleep you actually got. It builds up over several short nights, often faster than you notice.

Blood tests

Common blood markers for endurance athletes. Always read results with your doctor.

Ferritin

A blood test that reflects how much iron your body has stored. Low ferritin is common in endurance athletes, but inflammation and hard training can push the result up.

Haemoglobin (Hb)

The oxygen-carrying protein in red blood cells, measured in a full blood count. Endurance training can lower its concentration slightly by expanding plasma volume, which isn't the same as anaemia.

Vitamin D

A vitamin your skin makes in sunlight, important for bone health. The blood test measures 25-hydroxy-vitamin D, and most Australians get enough from sun exposure.

This glossary is general information to help you understand your training data. It isn't medical advice, diagnosis or treatment. For injury, illness, medication, blood test results or any health concern, speak to a qualified health professional.

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