Health and Fitness Glossary: 139 Gym, Nutrition and Science Terms Explained
Plain-English definitions of 139 gym, workout, nutrition, sleep and research terms, from progressive overload and VO2 max to meta-analysis, each linked to the article that explains it.
A
- Absolute risk (absolute risk difference)
The actual chance of an outcome in a group over a set time, such as deaths per 100,000 people a year. It keeps relative figures in proportion: heavy smoking adds over 2,000 excess deaths per 100,000 a year, against about 190 for sitting the most.
- Adaptive thermogenesis (metabolic adaptation)
A drop in calorie burn during weight loss that is bigger than the smaller body alone explains, one reason progress slows. Six years after extreme, rapid loss, 14 TV weight-loss contestants still burned about 500 fewer calories a day at rest than predicted.
- Ankle dorsiflexion (ankle mobility)
The ankle motion that brings your shin forward over a planted foot, or the top of the foot toward the shin. You need it to squat, walk downhill and use stairs; limited dorsiflexion is linked to shallower squats, knees caving inward and Achilles problems.
- Anti-extension (anti-extension core exercise)
Core exercise in which the job is to stop the lower back from arching while gravity or moving limbs try to extend it, as in the plank and dead bug. Related categories are anti-rotation and anti-lateral flexion, as in the side plank.
- Autophagy
Cells’ system for breaking down and recycling worn-out parts, whose mechanisms earned a 2016 Nobel Prize. Fasting ramps it up in yeast and mice and hard exercise raises its markers in human muscle, but popular fasting timelines for it were never measured in people.
B
- BDNF (brain-derived neurotrophic factor)
Brain-derived neurotrophic factor, a protein that supports the growth and survival of brain cells. In a meta-analysis of 29 studies, a single exercise session moderately raised BDNF, more so in regular exercisers. It is one plausible reason exercise helps thinking and mood.
- Berberine
A compound from barberry and goldenseal that lowers blood sugar, partly by activating AMPK, as metformin does. In a meta-analysis of 37 trials in type 2 diabetes it cut HbA1c by about 0.6 points. It is not a GLP-1 drug and interacts with many medicines.
- Bioelectrical impedance (BIA) (bioimpedance)
The method smart scales use: a tiny, safe current passes through the body, and its resistance, with your age, sex and height, is turned into a body fat estimate. Readings repeat well but can be several points off, so they suit tracking trends.
- Blood flow restriction (BFR) training
Training with light weights while a cuff around the upper arm or thigh partly restricts blood flowing out of the muscle. In adults over 60 it built muscle comparably to heavy lifting, though heavy loads still produced larger gains in maximal strength.
- BMI (body mass index)
Weight in kilograms divided by height in meters squared. Adult categories are under 18.5 underweight, 18.5–24.9 healthy, 25–29.9 overweight and 30 or more obesity, with lower cut-offs advised for some ethnic groups. It cannot tell muscle from fat.
- BMR (basal metabolic rate; resting metabolic rate (RMR))
Basal metabolic rate: the calories your body burns lying still, running your organs and keeping you warm. It is usually the largest part of daily burn, and lean mass predicts it best. The Mifflin-St Jeor equation is the most reliable common estimate.
- Body fat percentage (body fat %)
The share of your body weight that is fat. Healthy ranges are higher for women than for men and rise a little with age. Every method, from DEXA scans to smart scales and tape measures, only estimates it, so trends matter more than one reading.
- Body recomposition (recomp)
Gaining muscle and losing fat over the same period, often while scale weight barely changes. It works best for beginners, people returning after a break and people with more body fat, and relies on resistance training, high protein and a modest calorie deficit.
- Bone mineral density (BMD) (bone density)
How much mineral is packed into your bones, usually measured with a DXA scan and reported as a T-score. Low values mean osteopenia or osteoporosis and a higher fracture risk. Bone adapts to load, so resistance and impact training help preserve it.
C
- C-reactive protein (CRP) (CRP; hs-CRP)
A protein the liver releases during inflammation. A high-sensitivity CRP test gauges low-grade inflammation: for heart risk, under 1 mg/L is lower risk, 1–3 average and over 3 higher. A recent cold or injury can raise it temporarily, so doctors may retest.
- Caffeine half-life
The time your body takes to clear half a dose of caffeine. It runs to several hours and varies widely between people, so afternoon coffee can still disturb sleep: in one trial, a large dose taken six hours before bed still measurably cut total sleep.
- Calorie deficit (energy deficit)
Eating fewer calories than your body burns, so it draws on stored energy such as body fat. The classic rule that a 500-calorie daily deficit loses 1 lb a week overpromises over time, because your daily burn falls as you lose weight.
- Cardiorespiratory fitness (aerobic fitness, CRF)
How well your heart, lungs, blood and muscles work together to supply oxygen during sustained exercise, usually measured as VO2 max or in METs. In a meta-analysis of healthy adults, each 1-MET higher fitness level was linked to 13% lower all-cause mortality.
- CBT-I (cognitive behavioral therapy for insomnia)
A structured, multi-week treatment for chronic insomnia built on sleep restriction, stimulus control and challenging anxious thoughts about sleep. The American College of Physicians recommends it as the first treatment, ahead of sleeping pills, and its benefits tend to last after treatment ends.
- Chronic low-grade inflammation (systemic inflammation)
A quiet, body-wide immune activation that can last years, unlike the brief acute inflammation that heals a cut. It is linked to heart disease, type 2 diabetes and other chronic illness; inactivity, stress and poor diet help drive it, and losing excess weight lowers it.
- Chronotype (morning lark; night owl)
Your natural tendency to feel alert and perform best earlier or later in the day, the biology behind being a lark or an owl. It is substantially biological; in trained athletes, time since waking predicted peak performance better than clock time did.
- Circadian rhythm (body clock)
Your body’s roughly 24-hour internal timing system: a master clock in the brain plus peripheral clocks in organs such as the liver and pancreas. Light is its main signal and meal timing tunes the organ clocks. It shapes sleepiness, hormones and insulin sensitivity.
- Cohort study (prospective cohort study)
An observational study that measures a group’s habits or health markers at the start, then follows them for years to see who develops disease or dies. Large cohorts link a higher resting heart rate to higher mortality, for example, but show association, not causation.
- Cold water immersion (ice bath, cold plunge)
Sitting in cold water, often after exercise, for recovery or well-being. Small, low-quality trials suggest it eases muscle soreness, but used right after strength training it may blunt muscle growth, though not strength gains, so timing matters when size is the goal.
- Compound vs isolation exercise (multi-joint vs single-joint exercise)
A compound exercise moves several joints and works several muscles at once, such as a row, squat or press. An isolation exercise moves one joint to target one muscle, such as a curl or lateral raise. Compounds cover more muscle; isolations give more direct work.
- Concentric contraction (concentric phase, positive)
The part of a rep where the muscle shortens to move the weight, such as curling a dumbbell up. It is the phase most people think of as the lift itself; the controlled lowering that follows is the eccentric phase.
- Confidence interval (95% CI)
The range within which a study’s true effect plausibly lies, usually given at 95% confidence. A narrow interval means a precise estimate; a very wide one, like a blue-light glasses trial spanning real benefit, real harm and no effect, means the study settles little.
- Conflict of interest (COI; competing interests)
A financial or professional tie that could bias research, such as funding from a company selling the product studied. It does not make a finding wrong, but it is a reason to weigh it against independent work; our articles flag such ties.
- Confounding (confounder; residual confounding)
When a third factor drives both an exposure and an outcome, making them look linked. People who sit most also tend to smoke more or earn less, for example. Studies adjust for known confounders, but unmeasured ones leave residual confounding that cannot be removed.
- Cortisol (stress hormone)
The body’s main stress hormone. It follows a daily rhythm, highest shortly after waking and lowest at night, and rises during exercise, stress or a skipped meal before falling back. Problems come from levels that stay high, for example with ongoing stress or poor sleep.
- Creatine loading phase (creatine loading)
Taking about 20 g of creatine a day, as four 5 g servings, for 5–7 days to fill muscle stores within about a week, then 3–5 g daily. Skipping it reaches roughly the same stores in 3–4 weeks, with less early water weight.
- Creatine monohydrate (creatine)
A compound your body makes and also gets from meat and seafood, stored mostly in muscle to fuel short, hard efforts. Monohydrate is the most studied supplement form; pricier forms have not been shown to work better. Typical doses are 3–5 g a day.
D
- Deep sleep (slow-wave sleep; N3)
The deepest stage of non-REM sleep, when you are hardest to wake. It is concentrated early in the night, growth hormone release peaks during it, and it plays a bigger role than REM in consolidating facts and events. It declines with age.
- Degenerative meniscus tear (degenerative meniscal tear)
A wear-related tear in the knee’s cartilage cushion that develops gradually, usually after 35, and is common even in knees without pain. Randomized trials found arthroscopic partial meniscectomy no better than exercise therapy, or sham surgery, for this type of tear.
- Delayed gratification (marshmallow test)
Choosing a larger later reward over a smaller immediate one, as in the marshmallow test. A 2018 replication found the link between waiting time and later achievement was half the original size, and smaller still after accounting for family background. Waiting draws on learnable strategies.
- Deload (deload week)
A planned, temporary reduction in training volume, intensity or both, typically for about a week, scheduled to let fatigue that has built up in muscles, joints and the nervous system clear. Some training continues; it is not a full week off.
- DEXA scan (DXA, dual-energy X-ray absorptiometry)
A low-dose X-ray scan that splits the body into fat, lean tissue and bone. It is a reference method for body composition research and the standard test for bone density, but cost and radiation make it a poor tool for frequent tracking.
- Dietary fiber (soluble fiber; insoluble fiber)
Carbohydrates your digestive enzymes cannot break down. Viscous soluble fiber helps lower LDL cholesterol and blunts blood sugar rises; insoluble fiber adds bulk to stool. WHO-commissioned reviews found the greatest risk reductions at 25–29 g a day, more than most US adults eat.
- DOMS (delayed onset muscle soreness) (muscle soreness after exercise)
The tenderness and stiffness that builds in the days after hard or unfamiliar exercise, especially lowering-heavy eccentric work. It reflects inflammation from muscle stress, not lactic acid, eases with time and light movement, and is a poor measure of how much muscle you are building.
- Dynamic stretching
Stretching by moving joints through their range in a controlled way, such as leg swings, walking lunges or arm circles. It raises muscle temperature and rehearses movement patterns, preparing the body for exercise without the temporary strength loss of long static holds.
E
- Eccentric contraction (eccentric phase, negative)
The part of a rep where the muscle lengthens under load, such as lowering a dumbbell in a curl. Muscles can produce more force while lengthening than shortening, and eccentric work drives most of the muscle damage and soreness after a new or hard session.
- Effect size (standardized mean difference; Cohen’s d; Hedges’ g)
A number for how big a difference or relationship is, not just whether one exists. On the common standardized scale, roughly 0.2 is small, 0.5 moderate and 0.8 large. A result can be statistically significant and still have a trivially small effect size.
- Ego depletion (willpower depletion)
The theory that self-control draws on a limited reserve that runs down with use, leaving less willpower for the next task. Large preregistered replications across 23 labs and later 36 labs found the effect close to zero, undercutting the fuel-tank view of willpower.
- Electrolytes (sodium; potassium; magnesium)
Minerals that carry an electrical charge in body fluids. Sodium, potassium and magnesium matter most for hydration, controlling where water goes and letting nerves and muscles work. Plain water covers most days; replacement matters with heavy sweating, long endurance sessions, vomiting or diarrhea.
- EMG (electromyography)
A method that records the electrical activity of muscles to show how strongly they are switched on during an exercise. High activation is a clue, not proof of growth: in one trial the hip thrust out-activated the squat, yet both built similar glute muscle.
- EPOC (afterburn effect) (excess post-exercise oxygen consumption, afterburn)
The extra energy your body uses while it recovers after exercise. It rises steeply with intensity, but even after long or very hard sessions it adds only about 6–15% to the energy cost of the workout itself: a real but small bonus.
- Exercise snacks (exercise snacking)
Short, deliberate bursts of hard movement, typically one to two minutes, spread through the day, such as climbing stairs fast. Trials show they can raise cardio fitness in inactive adults. They differ from NEAT, which is incidental, low-intensity movement.
F
- Flow state (flow; being in the zone)
A state of total absorption in a task, where effort feels effortless and time seems to vanish, named by psychologist Mihaly Csikszentmihalyi. It tends to need clear goals, immediate feedback and a challenge closely matched to your skill. Its brain mechanisms are still debated.
- Foam rolling (self-myofascial release)
Applying sustained pressure to muscles with a foam roller. It does not physically break up fascia; its effects appear to be mainly through the nervous system. It gives a short-term gain in range of motion and modestly reduces soreness without lowering strength.
- Forward head posture (text neck)
A posture in which the head drifts forward of the shoulders, often while looking down at a phone. The viral 60-pound neck load comes from a 2014 computer model, not measurements in people, and research linking posture itself to neck pain is mixed.
G
- GLP-1 drugs (GLP-1 receptor agonists; semaglutide (Ozempic, Wegovy); tirzepatide (Mounjaro, Zepbound))
Diabetes and weight-loss medicines that mimic GLP-1, a gut hormone, slowing digestion and sharply reducing appetite. Body scans from the drug trials show a sizable share of the weight lost is lean mass rather than fat, which higher protein intake and resistance training help limit.
- GRADE (Grading of Recommendations Assessment, Development and Evaluation)
A widely used system for rating how confident we can be that a research finding is real, from high to very low certainty. Evidence is rated down for risk of bias, inconsistency, indirectness, imprecision and publication bias. Our Evidence Index uses a simplified version.
- Grip strength (handgrip strength)
How hard you can squeeze with your hand, a simple proxy for overall strength. In the PURE study of almost 140,000 adults in 17 countries, each 5 kg lower grip strength was linked to a 16% higher risk of death from any cause.
- Grit
Psychologist Angela Duckworth’s term for passion and perseverance toward long-term goals, measured with a short self-report scale. A 2017 meta-analysis of 66,807 people found grit only moderately linked to performance and strongly overlapping with conscientiousness, with perseverance doing most of the work.
- Growth mindset
The belief that abilities can be developed through effort and strategy rather than being fixed. Carol Dweck’s idea is real but modest: in 2018 meta-analyses, mindset was only weakly linked to academic achievement, and teaching it had a tiny average effect, larger for at-risk students.
- Gut microbiome (gut microbiota; gut bacteria)
The community of bacteria, fungi, viruses and other microbes living mostly in your large intestine. They ferment fiber into short-chain fatty acids that feed your colon’s lining and interact with your immune system. Greater diversity is linked to health, mostly by association, not proven cause.
H
- Habit stacking (habit anchoring)
Attaching a new habit to one you already do daily, such as stretching right after your morning coffee, so the existing routine becomes the cue. In a small study, people told to floss after brushing built stronger flossing habits than those told to floss before.
- HbA1c (A1c; glycated hemoglobin)
A blood test reflecting your average blood sugar over roughly the past three months, reported as the percentage of hemoglobin with glucose attached. Doctors use it to diagnose and monitor diabetes, and trials of diet, walking or supplements often report it as an outcome.
- Heart rate variability (HRV)
The small variation in time between one heartbeat and the next, set by the autonomic nervous system. It rises when the rest-and-digest side dominates and tends to fall with age, stress, poor sleep, illness and alcohol, so your own trend matters more than any chart.
- HIIT (high-intensity interval training) (interval training, sprint interval training)
Short bouts of hard, vigorous effort separated by easy recovery or rest; sprint interval training is the all-out version. For fat loss, trials find results close to steady-state cardio, usually reached in less training time.
- Hip hinge (hinge pattern)
A movement pattern in which you push your hips back with a flat back and only a slight knee bend, then drive them forward to stand tall. It is the basis of the Romanian deadlift and kettlebell swing and mainly trains the hamstrings and glutes.
- Hypertrophy (muscle hypertrophy, muscle growth)
An increase in the physical size of muscle tissue. It is a separate outcome from strength: a beginner’s early strength gains come mostly from the nervous system, and new muscle usually becomes measurable on scans between about three and ten weeks of consistent training.
I
- Insulin resistance (low insulin sensitivity)
When muscle, liver and other cells respond weakly to insulin, the hormone that signals them to take up blood sugar, so levels run higher. Its opposite, insulin sensitivity, is naturally higher earlier in the day, and a few nights of short sleep can lower it.
- Interference effect (concurrent training interference)
The idea that doing endurance training alongside strength training blunts gains in muscle and strength. A 2022 meta-analysis of 43 studies found no meaningful loss of muscle growth or maximal strength; explosive strength was the exception, mainly when both were done in one session.
- Intermittent fasting (IF; time-restricted eating; 16:8)
An eating pattern defined by when you eat rather than what, most often time-restricted eating within an 8- or 10-hour daily window. With calories and protein matched, trials find fat loss similar to ordinary dieting; its main advantage is that some people find it easier.
- Interval walking training (IWT) (Japanese interval walking)
A walking method developed at Shinshu University in Japan: three minutes fast alternated with three minutes slow, five or more times, on four or more days a week. In a five-month trial it beat steady walking on fitness, leg strength and systolic blood pressure.
- Isometric exercise (isometric contraction, static hold)
Exercise in which a muscle works hard while the joint stays still, as in a plank or wall sit. A 2023 meta-analysis of 270 trials ranked it first for lowering resting blood pressure, though on far fewer trials than aerobic exercise.
K
- Ketogenic diet (keto; ketosis)
A very-low-carbohydrate, high-fat eating pattern that pushes the body to run largely on fat and produce ketones, a state called ketosis. It can bring short-term weight loss and better blood sugar markers, but long-term outcome data are thin; its strongest medical use is drug-resistant epilepsy.
L
- L-theanine (theanine)
An amino acid found in tea leaves, especially green tea. Alone it does little for test performance, but in small trials about 100–200 mg taken with caffeine improved attention switching and focus more than caffeine alone. The research base is still small.
- Lactate threshold (first lactate threshold, ventilatory threshold)
The intensity at which lactate first starts to build up in the blood and breathing picks up noticeably; below it, effort can be sustained for a long time. In a study of 100 adults it fell between 60% and 90% of maximum heart rate.
- Lean body mass (lean mass, fat-free mass)
Everything in your body that is not fat: muscle, bone, organs and water. It is the main driver of your resting calorie burn, and keeping it while losing weight depends largely on strength training and eating enough protein.
- Leucine threshold
The idea that a meal must supply enough leucine, an essential amino acid, to fully switch on muscle building. Older muscle seems to need more: in one small study, 2.7 g of leucine raised muscle protein synthesis in older adults; 1.7 g did not.
- Long muscle length training (stretched-position training, lengthened partials)
Training a muscle in the part of its range where it is most stretched, such as the bottom of a curl or calf raise. Several trials found more growth than training only the shortened range, though a 2025 meta-analysis found similar growth across muscle regions.
M
- Macronutrients (macros)
The three nutrients that supply most of your calories: protein, carbohydrate and fat. Setting macros means splitting a calorie target among them; our calorie calculator sets protein first by body weight, puts fat near 30% of calories and lets carbohydrate fill the rest.
- Magnesium citrate
Magnesium bound to citric acid, about 16% magnesium by weight. In human studies it is absorbed better than magnesium oxide. It is also sold as a saline laxative: magnesium the gut does not absorb draws water into the bowel, which can loosen stools.
- Magnesium glycinate (magnesium bisglycinate)
Magnesium bound to the amino acid glycine, only about 14% magnesium by weight, so compare the label’s elemental magnesium figure. It looks less likely to loosen stools than other forms, but its reputation for superior absorption rests on thin human evidence.
- Maximum heart rate (max heart rate, HRmax)
The highest heart rate you can reach in an all-out effort. Formulas estimate it from age, such as Tanaka’s 208 minus 0.7 × age, but a person’s true maximum often differs from the prediction by 10 beats or more, so a measured value is better.
- Melatonin
A hormone the pineal gland releases in the evening as darkness falls, signaling night to your body clock rather than sedating you. In trials, benefits for falling asleep rise with dose only to about 4 mg a day; even 0.3 mg has shortened sleep onset.
- MET (metabolic equivalent) (metabolic equivalent of task, MET-minutes)
A unit of exercise intensity: 1 MET is the energy you use sitting at rest, so a 4-MET activity costs four times as much. The World Health Organization counts 3 to under 6 METs as moderate activity and 6 or more as vigorous.
- Meta-analysis
A study that statistically pools results from many earlier studies into one estimate, usually as the last step of a systematic review. It is only as good as the studies it combines, so reviewers also rate their risk of bias and check for publication bias.
- Mobility vs flexibility (joint mobility, flexibility)
Mobility is the ability to move a joint freely and with control through its full range, as in squatting deep or reaching overhead. Flexibility is how far a muscle can be passively stretched. Good mobility needs flexibility plus the strength and control to use it.
- Moderate vs vigorous activity (moderate-intensity activity, vigorous-intensity activity)
The two intensity bands used in physical activity guidelines. Moderate activity, like brisk walking, lets you talk but not sing; vigorous activity, like running, costs roughly twice the energy per minute. The WHO advises 150–300 moderate or 75–150 vigorous minutes a week.
- Muscle memory
The faster regain of muscle size and strength after a break compared with building it the first time. Animal studies suggest muscle fibers keep the extra nuclei gained in training even after shrinking, and human studies find molecular traces of past training.
- Muscle protein synthesis (MPS)
The process of building new muscle protein from amino acids. Strength training and protein-rich meals both raise it, with the amino acid leucine as a key on switch. Muscle grows when synthesis outpaces breakdown over weeks and months, not after any single meal.
N
- NEAT (non-exercise activity thermogenesis)
The calories burned by all movement that is not sleeping, eating or deliberate exercise, such as walking around, chores, standing and fidgeting. It varies widely between people, usually exceeds exercise as a share of daily burn and tends to fall during dieting.
- Neural adaptation (neural adaptations to strength training)
Improvements in how the nervous system activates and coordinates muscles. They explain most of the strength a beginner gains in the first few weeks of training, before muscle growth becomes measurable, and they level off relatively quickly.
- Neuroplasticity (brain plasticity)
The brain’s capacity to reorganize itself in response to practice and experience, at any age. It is well established, but it does not mean brain-training games make you smarter overall: their gains mostly stay with the trained task rather than transferring to everyday thinking.
- NMN (nicotinamide mononucleotide)
A precursor the body converts into NAD+, a coenzyme cells need for energy production and repair. In small, short human trials oral NMN raises blood NAD+ markers, and one 10-week trial improved muscle insulin sensitivity in prediabetic women, but benefits for human aging remain unproven.
- Non-specific low back pain (nonspecific lower back pain)
Lower back pain with no identifiable specific cause, which describes nearly all cases; serious causes such as fracture, infection or cancer are rare. Gentle activity helps more than bed rest, and exercise reduces chronic back pain by an amount patients notice.
O
- Observational study (epidemiological study)
Research that tracks what people already do, such as how much they sit or sleep, and their later health, without assigning anything. It can reveal links across huge populations but cannot prove cause, because of confounding, reverse causation and errors in self-reported habits.
- Omega-3 fatty acids (EPA; DHA; fish oil)
Polyunsaturated fats, chiefly EPA and DHA from fatty fish and algae, that are built into brain cell membranes. The American Heart Association advises at least two servings of fatty fish a week. On supplements, compare combined EPA plus DHA per serving, not total fish oil.
- One-rep max (1RM) (one-repetition maximum)
The heaviest weight you can lift for one full repetition with good form. It is the standard measure of maximal strength, and training loads are often written as a percentage of it, such as 70% of 1RM.
P
- Placebo (placebo effect)
A dummy treatment, such as a look-alike pill with no active ingredient, given to a comparison group so a real treatment can be judged fairly. People often improve on placebo too: in one leg-cramp trial, cramps fell by over a third on placebo alone.
- Power vs strength (muscular power, maximal strength)
Strength is the most force a muscle can produce, usually measured as a one-rep max. Power is force multiplied by velocity, or how fast you can produce force. It declines earlier and faster than strength with age and closely tracks function and fall risk.
- Preregistration (trial registration)
Publicly recording a study’s hypotheses, outcomes and analysis plan before collecting data, so results cannot be quietly reshaped afterward. It guards against cherry-picking: large preregistered replications found the famous ego-depletion effect close to zero.
- Progressive overload
Gradually increasing the demand on a muscle over weeks and months, by adding weight, reps, sets, range of motion or control, so it keeps being pushed slightly past what it has already adapted to. It is the principle behind every program that builds muscle.
- Proprioception (body awareness, joint position sense)
Your body’s sense of where its joints and limbs are, from sensors in the feet, ankles and joints. With the inner ear and vision, it underpins balance. All three weaken with age and inactivity, but balance and strength training improve stability and cut fall risk.
- Publication bias (file-drawer effect)
The tendency for studies with positive, tidy results to be published more readily than those finding nothing, which can make the visible evidence look stronger than it is. Reviewers check for it with tools such as funnel plots, and GRADE rates evidence down for it.
R
- Randomized controlled trial (RCT)
A study that assigns people by chance to a treatment or a comparison group, such as a placebo, then compares outcomes. Because randomization balances other differences between the groups, it can show cause and effect in a way observational studies cannot.
- Rate of force development (RFD)
How quickly muscle tension climbs from zero toward its maximum at the start of an effort. It matters when there is no time to build full strength, such as recovering from a stumble or standing up quickly from a low chair.
- Relative risk (risk ratio; RR)
How much more or less likely an outcome is in one group than another, as a ratio. Large cohorts found a relative risk of death of 1.09, or 9% higher, per extra 10 resting heartbeats a minute. Alone, it says little about one person’s chances.
- REM sleep (rapid eye movement sleep)
The dreaming stage of sleep, when brain activity resembles wakefulness while the body is temporarily paralyzed. REM periods lengthen toward morning, so cutting sleep short at the end of the night removes a disproportionate share. It is tied especially to emotional processing and skill-based memory.
- Repeated bout effect
The protective adaptation that leaves a muscle less disrupted and less sore the next time it does the same kind of exercise. It is why soreness fades as a program becomes familiar, even while the muscle keeps growing.
- Replication (replication crisis)
Repeating a study, ideally with more people and independent researchers, to see whether the result holds. Several famous psychology findings shrank or vanished when retested, the so-called replication crisis; the marshmallow test’s link to later achievement was half the size in a larger 2018 study.
- Reps in reserve (RIR)
How many more good reps you could have completed when you ended a set. Stopping at 2 RIR means two clean reps were left. RIR maps onto the RPE scale, and modern training research uses it to describe how close sets come to failure.
- Resting heart rate (RHR)
How many times your heart beats per minute while you are awake, calm and at rest. The American Heart Association gives 60 to 100 bpm as the normal adult range; endurance athletes often sit lower, and large cohorts link higher resting rates to higher mortality.
- Reverse causation (reverse causality)
When the supposed effect is actually the cause. Illness can make people sleep longer or move less before it kills them, so long sleep or inactivity can look deadly when it is partly a symptom. Measuring a habit only once cannot separate the two.
- RPE (rating of perceived exertion)
A scale for how hard an effort feels. Strength training often uses a 1 to 10 version where 10 is failure, 9 means about one rep left and 8 about two. Cardio research often uses other versions, such as the 6 to 20 Borg scale.
S
- Sarcopenia (age-related muscle loss)
The progressive, age-related loss of muscle mass, strength and function. It is linked to falls, loss of independence and higher mortality in older adults. Resistance training is the most effective intervention research has found, and it works at any age.
- Sarcopenic obesity (skinny fat (informal))
Low muscle mass combined with high body fat, sometimes in a person whose weight or BMI looks normal. The combination carries more metabolic risk than either alone, and a meta-analysis of 23 studies linked it to about 21% higher all-cause mortality.
- Sedentary behavior (sitting time)
Any waking activity at very low energy cost while sitting, reclining or lying down. It is distinct from physical inactivity, which means not meeting activity guidelines, so a person can be highly sedentary and highly active at the same time.
- Sitting-rising test (SRT) (sit-and-rise test)
A floor test scored from 0 to 10: sit down cross-legged and stand back up, losing points for each hand, knee or other support used. In 2,002 adults aged 51 to 80, each point higher was linked to 21% better survival.
- Sleep debt (sleep deficit; catch-up sleep)
The shortfall that builds when you sleep less than you need, night after night. A short weekend lie-in may offset part of it, but regularly needing two or more extra hours signals a large debt, and big weekend schedule shifts disrupt your circadian rhythm.
- Sleep efficiency
The percentage of your time in bed actually spent asleep, a gauge of sleep quality rather than quantity. When healthy adults were deliberately exposed to a cold virus, those with lower sleep efficiency over the previous two weeks were more likely to get sick.
- Sleep inertia
The groggy, slowed thinking that can follow waking, especially when an alarm pulls you out of deep sleep. It is why medium-length naps, long enough to reach deep sleep but too short to finish a cycle, tend to leave people groggier than short ones.
- Sleep regularity (consistent sleep schedule)
How consistent your sleep and wake times are from day to day. In a 2024 study of about 61,000 UK Biobank adults wearing activity trackers, regularity predicted risk of death better than sleep duration did, which is why a steady wake time matters.
- Spot reduction
The idea that exercising one body part burns the fat covering it, such as crunches for belly fat. Most research finds fat loss happens body-wide; one small 2023 trial found slightly more trunk fat loss, so it is less settled than often claimed.
- Static stretching
Holding a muscle in a stretched position without moving for a sustained time. It improves long-term flexibility, but long holds right before strength or power work can temporarily reduce force output, which is why many warm-ups use dynamic stretching instead.
- Statistical significance (p-value)
A judgment that a result would be unlikely if there were truly no effect, usually based on a p-value falling below a preset cutoff. It says nothing about size or importance: growth mindset interventions show a statistically real effect that is practically tiny.
- Steady-state cardio (MICT, moderate-intensity continuous training, LISS)
Cardio done as one continuous bout at a steady easy or moderate pace, such as brisk walking or relaxed cycling. Researchers call it moderate-intensity continuous training; its easiest form is often called LISS and overlaps heavily with Zone 2.
- Stretch tolerance
How much stretch sensation you can handle before it feels uncomfortable. Research suggests a meaningful share of the flexibility gained over weeks of stretching comes from higher stretch tolerance, a change in the nervous system, not only from muscles becoming longer.
- Systematic review
A review that searches for every relevant study on a question using a method set in advance, screens them against fixed criteria and appraises their quality, so conclusions rest on the whole evidence base, not handpicked studies. It often ends with a meta-analysis.
T
- Talk test
A simple check of exercise intensity: if you can speak comfortably in full sentences, you are likely below your ventilatory or lactate threshold, in Zone 2 or easier. If you can manage only a few words at a time, you are above it.
- TDEE (total daily energy expenditure; maintenance calories)
Total daily energy expenditure: all the calories you burn in a day. It combines resting metabolism, the largest share for most people, with the thermic effect of food, exercise and NEAT, your everyday non-exercise movement. Eating at your TDEE keeps weight stable.
- Testosterone
The main male sex hormone, also made in women, involved in muscle, bone and sex drive. In healthy men, average levels hold steady after 40; big drops usually travel with weight gain, illness or heavy drinking, and losing excess fat is the best-supported natural fix.
- Testosterone replacement therapy (TRT)
Prescription testosterone that restores normal blood levels in men with diagnosed hypogonadism, confirmed by symptoms plus repeated low fasting morning tests. Trials show real but modest gains in sexual function, bone density and lean mass, little for energy, and it suppresses natural production and fertility.
- Thermic effect of food (TEF; diet-induced thermogenesis)
The energy your body spends digesting, absorbing and storing food, about 5–15% of daily energy expenditure on a mixed diet. Protein costs the most to process, then carbohydrate, then fat, so a higher-protein diet raises it slightly, though not enough to transform your daily total.
- Time under tension (TUT) (rep tempo)
The total time a muscle works during a set, mostly set by rep speed. A meta-analysis found reps lasting from about half a second to 8 seconds built similar muscle when sets were hard; very slow reps of over 10 seconds did worse.
- Tolerable upper intake level (UL; upper limit)
The highest daily intake of a nutrient unlikely to harm almost all adults. For magnesium from supplements and medicines, the US limit is 350 mg a day, set because diarrhea becomes likely above it, not because of organ damage. Food magnesium does not count.
- Training frequency
How often a muscle or movement is trained, usually counted as sessions per muscle per week. It interacts with weekly volume: full-body or upper and lower routines, for example, spread the same hard sets across two or more sessions a week.
- Training to failure (momentary muscular failure, muscle failure)
Ending a set only when you cannot complete another full rep despite maximal effort. Technical failure, when form starts to break down, comes slightly earlier. Meta-analyses find sets stopped close to failure build muscle about as well, with less fatigue.
- Training volume (weekly sets per muscle)
How much training a muscle gets, usually counted as hard sets per muscle per week. More weekly sets generally produce more growth, but with diminishing returns, and studies differ in how they count indirect sets, such as triceps work during presses.
U
- Ultra-processed foods (UPF; NOVA group 4)
Industrial food formulations, typically containing additives and ingredients rarely used in home kitchens, as defined by the NOVA system. In a tightly controlled NIH trial, people ate about 500 more calories a day on an ultra-processed diet, though scientists still debate the classification itself.
- Unilateral exercise (single-leg exercise, single-arm exercise)
An exercise that trains one arm or leg at a time, such as a Bulgarian split squat, step-up or one-arm row. It adds a balance challenge, lets light loads work hard and makes it easier to spot and even out a weaker side.
V
- Visceral fat (deep belly fat, intra-abdominal fat)
Fat stored deep in the abdomen around organs such as the liver and intestines, unlike subcutaneous fat just under the skin. It is more metabolically active and is linked to type 2 diabetes, heart disease and fatty liver, independent of total body weight.
- Vitamin D (vitamin D3; 25-hydroxyvitamin D)
A fat-soluble vitamin your skin makes in sunlight, measured in blood as 25-hydroxyvitamin D. Correcting real deficiency matters, but large trials in people with mostly adequate levels, such as VITAL and D-Health, found extra vitamin D did not cut cancer, heart events, fractures or deaths.
- VO2 max (maximal oxygen uptake, VO2 peak)
The most oxygen your body can take in, carry and use per minute during all-out exercise, written in mL/kg/min. It is the main measure of cardiorespiratory fitness, falls with age, and low values are strongly linked to higher death rates.
W
- Waist-to-height ratio (WHtR)
Your waist measurement divided by your height, in the same units. Under 0.5 is healthy, 0.5 to 0.59 means increased risk and 0.6 or more high risk. Because it reflects belly fat, it flags heart and diabetes risk better than BMI.
Z
- Zone 2 (Zone 2 cardio, Zone 2 training)
An easy-to-moderate cardio intensity at which breathing deepens but you can still talk in full sentences. We estimate it as 60–70% of maximum heart rate; physiologists define it as just below the first lactate or ventilatory threshold, which varies from person to person.
No terms match. Try a shorter word, or clear the topic filter.
Plain-English definitions written to match our evidence-based articles. Each term links to the full explanation and its sources. Educational only, not medical advice. Last updated October 2026.
The Wonder Drop health and fitness glossary explains 139 terms you meet in training, nutrition, sleep and health research in plain English. Each definition matches our evidence-based articles and links to the full explanation, from VO2 max and progressive overload to protein synthesis, insulin resistance and what a randomized controlled trial actually is.
Topics at a glance
| Topic | Terms | For example |
|---|---|---|
| Training | 24 | Progressive overload, Reps in reserve (RIR), Hypertrophy |
| Heart & Fitness | 18 | VO2 max, Zone 2, Heart rate variability (HRV) |
| Body Composition | 13 | Bioelectrical impedance (BIA), BMI (body mass index), Body fat percentage |
| Recovery & Mobility | 14 | Ankle dorsiflexion, Cold water immersion, Degenerative meniscus tear |
| Nutrition | 13 | BMR, Calorie deficit, Dietary fiber |
| Supplements | 10 | Berberine, Creatine loading phase, Creatine monohydrate |
| Sleep | 11 | Caffeine half-life, CBT-I, Chronotype |
| Hormones & Metabolism | 10 | Adaptive thermogenesis, Autophagy, C-reactive protein (CRP) |
| Brain & Mind | 8 | BDNF, Delayed gratification, Ego depletion |
| Research Terms | 18 | Randomized controlled trial, Meta-analysis, Confounding |
How to use the glossary
- Search for any word in a term or its definition.
- Filter by topic to browse one area, such as Training, Sleep or Research Terms.
- Jump to a letter with the A to Z bar.
- Link to a term. Every definition has its own # link, so you can share or cite a single term.
Why we include research terms
Health headlines often hide the most important detail: what kind of study the claim comes from. A small observational study, a randomized controlled trial and a meta-analysis of many trials carry very different weight. Knowing what terms like relative risk, confounding and effect size mean is the quickest way to judge a claim for yourself. Our Evidence Index applies the same thinking to 120 popular health claims, grading each one from A to F.
Keep exploring
Put the terms to work with our free tools: the exercise library for step-by-step form guides, the workout plan generator for a weekly program, and the VO2 max and fitness age calculator to see where your fitness stands. The glossary is educational and is not medical advice; see our medical disclaimer.
This article is for informational purposes only and is not medical advice. See our Medical Disclaimer before changing your exercise, diet, or supplement routine.
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Frequently asked questions
What does RIR mean in the gym?
RIR stands for reps in reserve: how many more good reps you could have done when you stopped a set. Ending sets with about 1 to 3 reps in reserve is a common way to train hard without going to complete failure every time.
What is progressive overload?
Progressive overload means gradually asking more of your muscles over time, by adding weight, reps or sets, so they keep adapting. It is the core principle behind getting stronger and building muscle.
What is the difference between a randomized controlled trial and an observational study?
In a randomized controlled trial, researchers randomly assign people to a treatment or a comparison group, which lets them test cause and effect. An observational study only watches what people already do, so it can show links but cannot rule out other explanations.
Can I link to a single term?
Yes. Every definition has its own link, shown as a # next to the term. Use it to share or cite one definition directly.