Reference

Sports science glossary

Recovery research has a vocabulary problem. These are the terms you meet in the studies and in your own app, defined in plain language, with links to the articles where we work through them properly. Read any of these numbers against your own baseline rather than against anyone else’s.

A runner mid-stride during a road race, panned in black and white.

A

Acute load ATL
Roughly what you have trained in the last seven days, added up. On its own it says little. It matters when you hold it next to your chronic load, which is what the acute:chronic workload ratio does.
Acute:chronic workload ratio ACWR
Your last week of training divided by your last month’s average. Around 1.0 means this week looks like a normal week for you. Well above that means you are asking for more than your body has recently adapted to, which research links to higher injury risk. It reads best as a prompt to look closer, not a verdict.
Aerobic base
The endurance foundation built from steady, comfortable training rather than hard efforts. It develops over months, and it is what lets you tolerate harder work later without the hard work tearing you down.
Autonomic nervous system
The part of your nervous system running what you do not consciously control: heart rate, breathing, digestion. Its two branches, sympathetic (activating) and parasympathetic (recovering), shift balance through the day. Heart rate variability is one window onto where that balance currently sits.

B

Baseline
Your own recent normal for a measure, rather than a population average. Most recovery metrics only mean something against a baseline: an HRV of 45 ms is unremarkable for one person and clearly low for another.
Bone mineral density
How much mineral is packed into your bone tissue, which is a measure of how strong the bone is. It tends to peak in your late twenties and decline gradually after, faster around menopause. Loading the skeleton through resistance and impact training is the main trainable influence on it.

C

Cardiorespiratory fitness
How well your heart, lungs, and blood vessels deliver oxygen to working muscle. VO2 max is the usual way of putting a number on it.
Chronic load CTL
Your training averaged over roughly the last four weeks: the base your body has actually adapted to. Built up gradually, a high chronic load appears to protect against injury rather than cause it, which is why “just train less” is the wrong reading of a high workload ratio.
Circadian rhythm
The roughly 24-hour cycle setting when you feel alert, when you sleep, and when body temperature and hormones rise and fall. Light is its strongest signal, which is why sleep timing moves when your light exposure does.
Concurrent training
Training strength and endurance in the same program. The two do interfere with each other, though the effect is modest and shows up mainly in lower-body strength and power.

D

Deep sleep slow-wave sleep, N3
The deepest stage of non-REM sleep, concentrated in the first half of the night. Growth hormone release and the clearing of metabolic waste are associated with it. It is also the stage your body protects most when a night gets cut short, which is why deep sleep can hold up while total sleep falls.
Deload
A planned lighter week, taken before fatigue forces one. Volume, intensity, or both come down so the adaptation from the preceding weeks has room to surface.
Detraining
The loss of adaptations when training stops or drops sharply. Endurance markers fade within a couple of weeks, while strength and muscle hold on considerably longer.

E

Eumenorrheic
Having a regular, naturally occurring menstrual cycle, without hormonal contraception. Research on cycle and recovery usually specifies eumenorrheic participants, because the picture looks different on hormonal contraception.
Exponentially weighted moving average EWMA
An average giving recent days more weight than older ones. Applied to training load it tracks how your body actually experiences a week better than a flat average does, where a session three weeks ago counts the same as yesterday’s.

F

Follicular phase
The first half of a menstrual cycle, from the start of your period to ovulation. Estrogen rises through it, and HRV is often relatively higher than in the second half. One study of international footballers found muscle and tendon injuries clustered in the days just before ovulation.
FRIEND
The reference dataset behind a VO2 max percentile: thousands of lab-measured treadmill tests, sorted into brackets by age and sex. The letters stand for the Fitness Registry and the Importance of Exercise National Database, and it is the most widely used American reference for cardiorespiratory fitness. When something tells you where your VO2 max sits for your age and sex, this table is usually what it is reading from. It starts at age twenty, so if you are eighteen or nineteen there is no bracket of your own and the twenties column is the practical comparison.

G

Glycogen
The form your body stores carbohydrate in, mostly in muscle and liver. It is the main fuel for hard efforts, and running low on it is a large part of why long sessions get harder near the end.

H

Heart rate reserve HRR
The gap between your resting heart rate and your maximum. Effort is often read as a share of that gap rather than as a raw heart rate, so the same pace can mean something quite different for two people with different resting rates.
Heart rate variability HRV
The variation in the time between consecutive heartbeats, usually reported in milliseconds. More variation generally reflects a nervous system in a recovered state; less reflects one dealing with something, whether that is hard training, short sleep, illness, or stress. The absolute number varies enormously between people, so it only reads meaningfully against your own baseline.
Heart rate zones
Bands of intensity defined as shares of your maximum heart rate or your heart rate reserve. Most schemes use five, running from very easy to near-maximal.
Hypnogram
The stepped chart of your sleep stages across a night, drawn in the order they happened. It shows the shape of a night rather than just its total.

I

Interference effect
The specific finding behind concurrent training: endurance work blunts some strength and power adaptation, more so for the lower body and when the two sessions sit close together. It is real, and smaller than its reputation.

L

Lactate threshold
The intensity at which lactate starts accumulating in your blood faster than you clear it. In practice it marks the edge of the effort you can hold for a long time.
Luteal phase
The second half of a menstrual cycle, after ovulation. Progesterone rises, which lifts resting body temperature and resting heart rate slightly and often lowers HRV. A recovery number that dips here is usually the phase rather than a problem.

M

Maximum heart rate
The highest rate your heart reaches at all-out effort. Age-based formulas estimate it, but individual variation around them is wide enough that a measured figure is worth considerably more than a calculated one.
Metabolic equivalent MET
A unit of effort where 1 MET is roughly your energy use sitting still. A 7 MET activity costs about seven times that. Public activity guidelines usually express intensity this way.

N

Non-REM sleep
The three stages of sleep outside REM: light sleep (N1 and N2) and deep sleep (N3). Most of a normal night is spent here.

O

Orthosomnia
A documented pattern where pursuing a perfect sleep score makes sleep worse. It is the clearest argument for reading a sleep metric as a weekly trend rather than a nightly grade.
Overreaching
Training beyond what you are currently recovering from. Planned and kept short, it is a normal part of a training block and performance rebounds afterwards. Extended too long, it stops rebounding.
Overtraining syndrome
A long-lasting drop in performance, with fatigue that rest does not quickly resolve, measured in months rather than days. It is rare. Most of what gets called overtraining is overreaching.

P

Perimenopause
The years of hormonal transition before menopause, often beginning in the forties. Cycles become irregular, sleep is more often disturbed, and HRV and resting heart rate commonly shift. These changes are expected, and a recovery app that reads them as failures is reading them wrong.
Periodization
Structuring training over time into phases with different emphases, instead of repeating the same week indefinitely. The point is to make the hard work land when you are able to absorb it.
Photoplethysmography PPG
The optical method a wrist wearable uses to read your pulse. Light goes into the skin and the amount reflected back changes as blood volume rises and falls with each beat. It is how a watch derives heart rate and HRV without a chest strap, and it is also why motion degrades the reading.
Progressive overload
Gradually increasing the demand you place on your body so it keeps adapting. The gradual part carries as much weight as the increase.

R

Rate of perceived exertion RPE
How hard an effort felt, rated on a simple scale, usually 1 to 10. It needs no hardware, and it tracks objective measures of training stress closely enough that coaches use it on its own to monitor athletes.
Readiness
A composite score estimating how prepared you are for hard work today, usually built from HRV, resting heart rate, sleep, and recent training load, each compared against your own recent normal. Apps weight those ingredients differently, so a readiness number from one app does not mean the same thing as the same number from another.
Recovery
The process of returning to, and past, your previous capacity after training stress. It is not only rest: sleep, food, and the size of the preceding load all shape how long it takes.
REM sleep
The stage where most vivid dreaming happens, the eyes move, and the body is largely still. It concentrates in the second half of the night, which is why a night cut short at the end loses REM out of proportion to the hours lost.
Resting heart rate RHR
Your heart rate at complete rest, usually captured during sleep or first thing in the morning. It drifts down as fitness improves, and rises with illness, alcohol, heat, and accumulated fatigue.
RMSSD
The specific HRV calculation most recovery apps use: the root mean square of successive differences between heartbeats. It reflects parasympathetic activity and stays reasonably stable over short recordings, which is what makes it suitable for overnight measurement.

S

Sarcopenia
The age-related loss of muscle mass and strength. It starts earlier than most people expect, from around your thirties, and resistance training is the most effective counter to it.
Session RPE
Training load for a single session, worked out as how hard it felt multiplied by how long it lasted. Sixty minutes at RPE 7 gives 420 units. It is the simplest usable measure of load, and it needs nothing but a watch face and an honest answer.
Sleep debt
The running shortfall between the sleep you need and the sleep you got, accumulated across recent nights. Short nights add to it and a long night pays some of it back, though not hour for hour.
Sleep efficiency
The share of your time in bed that you actually spent asleep. Around 85% and above is typical for a healthy adult.
Sleep latency
How long it takes you to fall asleep after lights out. Very short latency is not automatically a good sign, since falling asleep almost instantly can indicate you are running short on sleep.
Sleep quality index SQI
A single score for one night, built from how much of your time in bed you spent asleep, how much of that sleep was deep and REM, and how the night sits against your own recent ones. It exists because no one number describes a night on its own: eight broken hours and eight solid ones deserve different scores. Every app builds its own version and weights the ingredients differently, so a sleep score from one does not mean the same thing as the same score from another.
Sleep regularity index SRI
A measure of how closely one day’s sleep and wake times match the next, given as a percentage. It asks a simple question for every minute of the day: were you in the same state, asleep or awake, at this minute yesterday? Large population studies have found how steady your timing is to be at least as informative as how long you sleep, which is why a consistent bedtime is worth as much attention as the hours themselves.
Sleep stages
The states a night cycles through: light sleep, deep sleep, and REM, in cycles of roughly 90 minutes. Deep sleep dominates the early cycles and REM the later ones, so where a night gets shortened changes what you lose.
Smallest worthwhile change Hopkins’ buffer
The minimum shift in a measure that counts as real rather than noise, often set at about half your typical day-to-day variation. The half-a-swing rule comes from the sports scientist Will Hopkins, whose work on reliability in sports measurement is where most recovery apps get it from. It is the reason a well-built score ignores a small dip and reacts to a larger one.
Supercompensation
The rebound above your previous capacity after a training stress, once you have recovered from it. Training again at the right point on that curve is the basic logic behind hard-easy scheduling.

T

Taper
A planned reduction in training volume before a competition, holding intensity roughly steady. Done well it lets accumulated fatigue clear while the fitness underneath it stays put.
Training impulse TRIMP
A single number for the cost of one workout, combining how long you trained with how hard your heart was working, and weighting the high-intensity minutes disproportionately. It is why an hour of intervals can score three times an hour of walking. The formula comes from the physiologist Eric Banister, and the curve it uses to weight intensity is slightly different for women and men.
Two-process model of sleep
The standard explanation for when you feel sleepy: pressure that builds the longer you have been awake, working against your circadian rhythm, which pushes alertness up and down on a roughly 24-hour cycle. It is why deep sleep concentrates in the first cycles of a night, when that pressure is at its highest, and why the same seven hours feel different taken early and taken late. Described by Alexander Borbély in 1982, it is still the reason a sleep score weights a protected early stretch above the same hours broken up later.

V

VO2 max
The maximum rate at which you can take in and use oxygen during hard exercise, in milliliters per kilogram per minute. It is the standard measure of aerobic capacity and one of the stronger predictors of long-term health. Wearable figures are estimates carrying real error, so your trend tells you more than the exact number does.

W

Wake after sleep onset WASO
The total time spent awake between first falling asleep and finally getting up. It is what separates eight fragmented hours from eight solid ones.
Welch’s t-test
A way of asking whether the gap between two averages is bigger than the noise around them, for groups that may differ in size and in how much they vary. That last part is what separates it from the plain t-test, and it is the safer assumption when comparing, say, your mornings after a late dinner against your mornings without one. What it returns is best read as a range the real difference is consistent with, rather than a single number carrying more precision than the data holds.

Z

Z-score
How far a reading sits from your own average, counted in units of your own typical variation. A Z-score of 0 matches your average; −2 sits two typical swings below it. It is the arithmetic that lets a score compare you to yourself instead of to everyone else.
Zone 2
The second of the five heart rate zones: steady, conversational effort you could hold for a long time. It has become shorthand for the easy aerobic work that makes up most of a well-built endurance program.

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