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Fitness & longevity glossary

Plain-English definitions of the training, nutrition and longevity terms used across the site. Underlined terms open a full explainer — what it means, why it matters, and the calculators that put it to work.

309Terms · A to Z

All terms

309 terms

A25 terms

Absolute vs relative riskRelative risk reduction, Absolute risk reduction: "Halves your risk" is meaningless until you know what the risk was to start with.
ActinThin filament: The thin filament that myosin pulls against to shorten a muscle.
Adiponectin: A hormone released by fat cells that improves insulin sensitivity and reduces inflammation.
Adjusted calciumCorrected calcium: Blood calcium recalculated for your albumin level, so a low protein does not look like low calcium.
AdrenalineEpinephrine: The fight-or-flight hormone that raises heart rate and mobilises fuel.
Aerobic capacity: The body's overall ability to take in, transport and use oxygen to produce energy.
Aerobic metabolismOxidative metabolism: Energy production that uses oxygen: slow to start, but efficient and effectively unlimited.
Albumin: The most abundant protein in blood, made by the liver.
Alkaline phosphataseALP: An enzyme from the liver and bones, read as part of a liver panel.
All-cause mortality: The risk of dying from any cause, a common yardstick in health research.
ALTAlanine aminotransferase, SGPT: A liver enzyme that leaks into the blood when liver cells are stressed.
AMPKAMP-activated protein kinase: The cell's low-energy sensor, switched on by exercise and fasting.
Anabolic hormone: A hormone that promotes tissue building: testosterone, insulin, growth hormone and IGF-1.
Anabolism: The metabolic building of larger molecules and tissue, which costs energy.
Anaerobic metabolism: Energy production without oxygen: fast, limited in capacity, and costly to sustain.
Anaerobic thresholdVentilatory threshold: The intensity at which energy production shifts sharply towards anaerobic pathways.
Antioxidant: A compound that neutralises free radicals by safely donating an electron.
Apnoea–hypopnoea index: A count of how many times an hour breathing stops or becomes shallow during sleep, used to grade sleep-disordered breathing.
ApoBApolipoprotein B: A marker of the number of atherogenic particles in your blood.
ApoptosisProgrammed cell death: The orderly, controlled dismantling of a cell that is damaged or no longer needed.
ASTAspartate aminotransferase, SGOT: An enzyme found in liver, heart and muscle, so a raised value is not automatically the liver.
Atherosclerosis: The build-up of plaque in artery walls that narrows and stiffens them.
ATPAdenosine triphosphate: The cell's immediate energy currency: the molecule every contraction actually spends.
Autonomic nervous systemANS: The branch of the nervous system controlling involuntary functions like heart rate and digestion.
Autophagy: The cell's recycling process that clears and reuses damaged components.

B14 terms

BasophilsBasophil count: The rarest white cells, involved in allergic and inflammatory reactions.
BicarbonateHCO3, Total CO2: The blood's main buffer against acid, and a clue to breathing and kidney function.
BilirubinTotal bilirubin: The yellow pigment from recycled red blood cells, processed by the liver.
Bioavailability: The proportion of a nutrient or compound that is actually absorbed and usable.
Biological ageBio age: An estimate of how old your body seems, versus your calendar age.
Blood pressure: The force blood exerts against artery walls, reported as systolic over diastolic.
BMIBody mass index: Weight divided by height squared: a crude population screening measure, not a diagnosis.
BMRBasal Metabolic Rate: The calories your body burns at complete rest just to stay alive.
Body composition: The proportions of fat, muscle, bone and water that make up your body mass.
Body recompositionRecomp: Losing fat and gaining muscle at the same time.
Body-fat percentageBody fat %: The share of your body weight that is fat rather than lean tissue.
Bone densityBone mineral density, BMD: The mineral content packed into bone: the standard marker of fracture risk.
Broad-spectrum SPFSunscreen, SPF: Sun protection against both UVA and UVB: the best-evidenced skin habit.
BulkingGaining phase, Muscle-gain phase: Deliberately eating in a surplus for a period to prioritise muscle gain.

C30 terms

CadenceStride rate, Step rate: How many steps you take, or pedal strokes you turn, per minute.
CalciumSerum calcium, Ca: Blood calcium, kept in a narrow range by hormones regardless of what bone holds.
CalorieKilocalorie, kcal: A unit of food energy: in everyday use, a kilocalorie.
Calorie deficitCaloric deficit, Energy deficit: Taking in less energy than you expend: the condition under which fat loss happens.
Calorie surplusCaloric surplus, Energy surplus: Taking in more energy than you expend: the condition that allows weight and muscle gain.
Capillary density: How richly a tissue is supplied with the smallest blood vessels.
Carb cycling: Alternating higher- and lower-carbohydrate days, usually matched to training load.
CarbohydrateCarbs: The body's primary fast fuel, stored in muscle and liver as glycogen.
Cardiac muscleMyocardium: The specialised involuntary muscle of the heart, contracting continuously for a lifetime.
Cardiac output: The volume of blood the heart pumps each minute: heart rate multiplied by stroke volume.
Cartilage: Smooth, flexible connective tissue that cushions joints and reduces friction.
Catabolic hormone: A hormone that promotes tissue breakdown and fuel mobilisation: chiefly cortisol.
Catabolism: The metabolic breakdown of larger molecules and tissue, releasing energy.
Central nervous systemCNS: The brain and spinal cord: where movement is planned and commanded.
ChlorideCl: The salt partner to sodium, used mainly to check the blood's acid balance.
Cholesterol / HDL ratioTotal cholesterol to HDL ratio: Total cholesterol divided by HDL, a single figure used in risk scoring.
Chronic low-grade inflammation: A persistent, mild elevation of inflammatory signals in the blood that tends to rise with age.
Circadian rhythmBody clock: The body's roughly 24-hour internal clock, governing sleep, hormones and metabolism.
Collagen: The main structural protein in skin, tendon and bone, and a popular supplement.
Complete proteinHigh-quality protein: A protein source containing all nine essential amino acids in useful amounts.
Compound exerciseMulti-joint exercise: An exercise that moves multiple joints and trains several muscles at once.
ConcentricConcentric phase, Lifting phase: The phase of a lift where the muscle shortens under load, the 'lifting' part.
Confounding: When a third factor explains the link you thought you'd found between two things.
CopperSerum copper: A trace mineral needed for iron transport and connective tissue, balanced against zinc.
CortisolStress hormone: A hormone central to the stress response, energy and recovery.
Creatine kinaseCK, Creatine phosphokinase, CPK: A muscle enzyme that rises when muscle is damaged, including by hard training.
Creatine phosphatePhosphocreatine, PCr: A rapid phosphate store that regenerates ATP during the first seconds of hard effort.
Creatinine: A muscle waste product used to gauge how well the kidneys filter.
CuttingFat-loss phase: Eating in a deficit for a period to lose fat while holding on to as much muscle as possible.
Cytokine: A signalling protein of the immune system that coordinates inflammation and repair.

D7 terms

Deload: A planned easier period to shed fatigue and allow recovery.
DHEADehydroepiandrosterone: An adrenal hormone that serves as a precursor to testosterone and oestrogen.
DHEA-SDHEA sulphate, Dehydroepiandrosterone sulphate: An adrenal hormone that acts as raw material for sex hormones.
Dietary fat: The most energy-dense macronutrient, essential for hormones and vitamin absorption.
Direct bilirubinConjugated bilirubin: The portion of bilirubin the liver has already processed, used to locate a problem.
DOMSDelayed-Onset Muscle Soreness: The muscle soreness that appears a day or two after unfamiliar or hard exercise.
Drop setStrip set: Continuing a set at a lighter load immediately after reaching failure.

E9 terms

EccentricEccentric phase, Negative: The phase of a lift where the muscle lengthens under load, the 'lowering' part.
eGFREstimated glomerular filtration rate: An estimate of how fast your kidneys are filtering blood, calculated rather than measured.
Energy balance: The relationship between calories consumed and calories expended.
Enzyme: A protein that speeds up a specific biochemical reaction without being consumed by it.
EosinophilsEosinophil count: White cells associated with allergy and parasitic infection.
EPOCExcess Post-exercise Oxygen Consumption, Afterburn: The extra calories burned recovering after hard exercise.
ESRErythrocyte sedimentation rate: How fast red cells settle in a tube, an old but useful inflammation marker.
Essential amino acidsEAAs: The nine amino acids the body can't make and must get from food.
Executive function: The brain's control system for focus, planning, self-control and switching between tasks.

F16 terms

Fartlek: Swedish for 'speed play': unstructured mixing of faster and slower running by feel.
Fascia: Sheets of connective tissue that wrap, separate and connect muscles and organs.
Fasting glucoseFasting blood glucose, FBG: Your blood-sugar level after an overnight fast: a first-line metabolic marker.
Fasting insulinHOMA-IR: How much insulin your body needs at rest to keep blood sugar in check.
Fermented foods: Foods carrying live microbial cultures, linked to gut diversity.
Ferritin: A measure of the body's stored iron.
FFMIFat-free mass index: A muscularity index: fat-free mass adjusted for height, the lean-mass answer to BMI.
FibreFiber, Dietary fibre: Carbohydrate the body cannot digest, feeding gut bacteria and supporting satiety.
FibrinogenFactor I: The clotting protein that also rises with inflammation.
FolateVitamin B9, Folic acid: A B-vitamin that partners with B12 for healthy blood and nerves.
Follicle stimulating hormoneFSH: A pituitary hormone that drives egg development and sperm production.
Free androgen indexFAI: A calculated estimate of how much of your testosterone is available to tissues.
Free radicalReactive oxygen species, ROS: A highly reactive molecule with an unpaired electron that can damage cell components.
Free T3Free triiodothyronine, FT3: The unbound portion of the more active thyroid hormone.
Free T4Free thyroxine, FT4: The unbound, active portion of the main thyroid hormone.
Free testosterone: The small unbound fraction available to tissues: often the number that matters.

G16 terms

Gamma GTGGT, Gamma-glutamyl transferase: A liver enzyme that is sensitive to alcohol and to bile flow problems.
Gastric emptying: How fast food leaves the stomach: slowed markedly by GLP-1 medicines.
Ghrelin: The hunger hormone, released by the stomach to stimulate appetite.
GlobulinSerum globulin, Total globulin: The non-albumin blood proteins, including antibodies and transport proteins.
GLP-1Glucagon-like peptide-1: A gut hormone released when you eat that signals fullness to the brain.
GLP-1 receptor agonistsGLP-1 agonists, Incretin medicines: Prescription medicines that mimic the GLP-1 satiety signal for far longer.
Glucagon: Insulin's counterpart: the hormone that raises blood glucose between meals.
Gluconeogenesis: The body making glucose from non-carbohydrate sources such as amino acids and glycerol.
GLUT4: The glucose transporter in muscle and fat that insulin, and exercise, bring to the surface.
Glycaemic indexGI: A ranking of how quickly a carbohydrate food raises blood sugar.
Glycaemic loadGlycemic load, GL: Glycaemic index adjusted for how much carbohydrate a realistic portion actually contains.
Glycogen: The body's stored form of carbohydrate, held in muscle and liver.
Glycolysis: The breakdown of glucose for energy, which can run with or without oxygen.
Grip strength: How hard you can squeeze, a surprisingly strong marker of overall health.
Growth hormoneGH, hGH, Somatotropin: A pituitary hormone supporting growth, repair and fat metabolism, released mainly during deep sleep.
Gut microbiomeGut microbiota, Gut flora: The community of gut microbes that diet and fibre help shape.

H17 terms

HaematocritHematocrit, Packed cell volume: The proportion of blood volume made up of red cells.
HaemoglobinHemoglobin, Hb: The iron-containing protein in red blood cells that carries oxygen.
HbA1cGlycated haemoglobin, Haemoglobin A1c: A marker of your average blood sugar over roughly the past three months.
HDL cholesterolHDL-C, 'Good' cholesterol: Cholesterol on the particles that help clear it from the arteries.
Health-related quality of life (HRQoL): A measure of how a health condition and its treatment affect a person's physical, mental and social wellbeing.
Healthspan: The years of life spent in good health, free of chronic disease.
Heart rate reserveHRR, Karvonen method: The gap between your resting and maximum heart rate, used to set training zones.
HIITHigh-Intensity Interval Training: Alternating short, hard efforts with recovery periods.
Hippocampal subfields: The distinct anatomical subregions of the hippocampus, such as CA1, the subiculum and the CA2, CA3 and dentate gyrus area, which can be measured separately on high-resolution brain scans.
Homeostasis: The body's maintenance of stable internal conditions despite external change.
Homocysteine: An amino acid tied to B-vitamin status and cardiovascular risk.
Hormone: A chemical messenger released into the blood to act on distant tissues.
HRVHeart Rate Variability: The variation in time between heartbeats, used as a readiness and recovery signal.
hs-CRPHigh-sensitivity C-reactive protein: A sensitive blood marker of low-grade inflammation in the body.
Hyaluronic acidHA, Sodium hyaluronate: A humectant that holds water at the skin's surface: hydration, not rejuvenation.
Hyperplasia: An increase in the number of muscle cells, as distinct from enlarging existing ones.
Hypertrophy: The growth of muscle fibres in size, the goal of most physique training.

I12 terms

IGF-1Insulin-like growth factor 1, Somatomedin C: A growth factor that mediates many of growth hormone's effects on tissue.
Inflammation: The immune response to injury or stress: protective when acute, damaging when chronic.
Insulin: The hormone that lowers blood glucose and directs nutrients into storage.
Insulin resistance: A state in which cells respond poorly to insulin, so more is needed to do the same job.
Insulin sensitivity: How responsive your cells are to insulin, central to metabolic health.
Insulin signallingInsulin signaling: The chain of events by which insulin tells a cell to take up glucose.
Intermittent fastingIF, Time-restricted eating: Cycling between defined eating and fasting windows rather than eating across the whole day.
Interval training: Alternating bouts of harder work with recovery periods, at planned intensities and durations.
Intestinal permeabilityLeaky gut: How much passes through the gut lining: real biology, oversold as a diagnosis.
IronSerum iron: The iron travelling in your blood right now, which is not the same as your iron stores.
Isolation exerciseSingle-joint exercise: An exercise that moves a single joint to target one muscle.
IsometricIsometric hold, Static hold: A contraction where the muscle produces force without changing length.

K3 terms

Ketogenic dietKeto diet: A very-low-carbohydrate, high-fat diet that shifts the body into ketosis.
Ketosis: A metabolic state in which the body runs largely on fat-derived ketone bodies.
Krebs cycleCitric acid cycle, TCA cycle: The mitochondrial cycle that extracts energy carriers from fuel for aerobic ATP production.

L16 terms

LactateLactic acid: A product of fast glucose breakdown: a usable fuel, not a waste product.
Lactate threshold: The intensity above which lactate accumulates faster than the body can clear it.
LDHLactate dehydrogenase: An enzyme present in almost every tissue, so it flags damage without saying where.
LDL cholesterolLDL-C, 'Bad' cholesterol: Cholesterol carried on the particles most linked to clogged arteries.
Lean body massLBM, Fat-free mass: Everything in your body that isn't fat, chiefly muscle, bone and water.
Lean mass preservation: Keeping muscle while losing fat: the part rapid weight loss threatens.
Left ventricular hypertrophyLVH: Thickening of the heart's main pumping chamber, most often from long-term high blood pressure.
Leptin: The satiety hormone released by fat tissue to signal energy stores to the brain.
Leucine: The key branched-chain amino acid that triggers muscle protein synthesis.
Ligament: Connective tissue joining bone to bone and stabilising joints.
Lipid panelLipid profile, Cholesterol test: A blood test reporting cholesterol fractions and triglycerides together.
LISSLow-intensity steady-state: Low-intensity steady-state cardio: easy, continuous, conversational work.
Lp(a)Lipoprotein(a): A largely genetic, independent risk factor for cardiovascular disease.
Luteinising hormoneLH: The pituitary hormone that triggers ovulation and testosterone production.
Lymphocyte countAbsolute lymphocytes: The white cells behind targeted immunity and immune memory.
Lymphocyte percentageLymphocyte %: The share of your white blood cells that are lymphocytes.

M28 terms

MacronutrientMacros: The three energy-providing nutrients: protein, carbohydrate and fat.
MagnesiumSerum magnesium, Mg: A mineral needed by hundreds of enzymes, of which blood holds only a fraction.
Maintenance calories: The daily intake at which your weight stays stable: your expenditure, expressed as food.
Maximum heart rateMHR, HRmax: The highest rate your heart can beat during all-out effort.
MCHMean corpuscular haemoglobin, Mean cell haemoglobin: The average amount of haemoglobin inside each red blood cell.
MCHCMean corpuscular haemoglobin concentration: How concentrated the haemoglobin is inside red cells, rather than how much there is.
Mean corpuscular volumeMCV: The average size of your red blood cells.
Mean platelet volumeMPV: The average size of your platelets, which hints at how new they are.
Mechanical tension: The force a muscle produces under load, the primary driver of muscle growth.
Melanotan IIMelanotan, tanning injection: An unlicensed injectable 'tanning' peptide with documented safety concerns.
Melatonin: The hormone that signals darkness and sets the timing of sleep.
Meta-analysisSystematic review: A study of studies: pooling results statistically to get a more precise answer than any single trial gives.
Metabolic adaptationAdaptive thermogenesis: The body's tendency to burn fewer calories as you diet.
Metabolic flexibility: The ability to switch efficiently between burning fat and burning carbohydrate.
Metabolic stress: The build-up of metabolites during hard training: the source of the burn and the pump.
Metabolic syndrome: A cluster of risk factors that together raise cardiometabolic risk.
MicronutrientVitamins and minerals: Vitamins and minerals the body needs in small amounts for health.
Mind-body therapies: Practices that pair mental focus with movement, breath or relaxation, such as yoga, mindfulness, Qigong and dance therapy.
Mind-muscle connectionInternal focus of attention: Deliberately focusing attention on the muscle you are trying to work during a lift.
MitochondriaMitochondrion: The organelles that produce most of the cell's ATP using oxygen.
MonocytesMonocyte count: White cells that clear debris and become tissue macrophages.
Motor unit: A single motor neuron together with every muscle fibre it controls.
mTORMechanistic target of rapamycin: A cellular pathway that drives growth, including muscle building.
Muscle damageExercise-induced muscle damage: Microscopic disruption of muscle tissue from training, which the body then repairs.
Muscle protein breakdownMPB, Muscle proteolysis: The continuous breakdown of muscle protein, the counterweight to synthesis.
Muscle protein synthesisMPS: The process of building new muscle protein, stimulated by training and protein intake.
Muscular endurance: The ability of a muscle to keep producing force over repeated or sustained contractions.
MyosinThick filament: The thick filament whose heads pull on actin to generate muscle force.

N12 terms

NAD+Nicotinamide adenine dinucleotide: A coenzyme central to energy metabolism and DNA repair, which declines with age.
NEATNon-Exercise Activity Thermogenesis: Calories burned through everyday movement outside of formal exercise.
Net carbs: Total carbohydrate minus fibre, and sometimes minus sugar alcohols.
Net protein balance: Muscle protein synthesis minus breakdown: positive over time means muscle is being gained.
Neuromuscular junctionMotor end plate: The synapse where a nerve meets a muscle fibre and triggers it to contract.
Neurotransmitter: A chemical messenger carrying signals across the gap between nerve cells.
NeutrophilsNeutrophil count, ANC: The most numerous white cells, and the first responders to bacterial infection.
NiacinamideNicotinamide, Vitamin B3: A vitamin-B3 form used in skincare for barrier support and pigmentation.
Non-HDL cholesterol: Everything except HDL: all the cholesterol carried by particles that can lodge in artery walls.
NoradrenalineNorepinephrine: A stress hormone and neurotransmitter driving arousal, focus and vascular tone.
Nutrient density: How many useful nutrients a food provides per calorie it costs.
Nutrient timingMeal timing: Arranging when you eat around training, rather than only what and how much.

O8 terms

Observational studyCohort study, Epidemiological study: A study that watches what people already do: powerful for spotting patterns, unable to prove cause on its own.
OestrogenEstrogen, Oestradiol, Estradiol: The principal female sex hormone, also important for bone, brain and vascular health in everyone.
One-rep max1RM: The most weight you can lift for a single rep of an exercise.
Osteoblast: The cell that builds new bone by laying down and mineralising matrix.
Osteoclast: The cell that breaks bone down, releasing its mineral: the other half of remodelling.
Overtraining syndromeOTS, Unexplained underperformance syndrome: A prolonged performance decline caused by training load outpacing recovery.
Oxidative phosphorylationElectron transport chain: The oxygen-using process in mitochondria that produces most of the body's ATP.
Oxidative stress: An imbalance between free radical production and the body's ability to neutralise it.

P20 terms

Parasympathetic nervous system: The rest-and-digest branch that restores the body after exertion.
Pelvic floor muscle training: Structured, repeated voluntary contraction of the muscles that support the bladder, bowel and pelvic organs, used to improve their strength and control.
PerimenopauseMenopause transition: The years of fluctuating hormones before periods stop for good.
Periodisation: Structuring training into phases to manage fatigue and peak performance.
PhosphateSerum phosphate, Inorganic phosphate, PO4: A mineral partnered with calcium in bone and central to the body's energy currency.
PhotoageingPhotoaging: Skin ageing driven by UV exposure rather than time alone.
Pittsburgh Sleep Quality Index (PSQI): A widely used questionnaire that scores self-reported sleep quality over the past month, where higher scores mean worse sleep.
PlateletsThrombocytes, PLT: Cell fragments that plug damaged blood vessels and start clotting.
Postural feedback: The idea that the position the body is held in feeds back on emotional state and behaviour, rather than simply reflecting them.
PotassiumK: The main electrolyte inside cells, tightly controlled because the heart depends on it.
PowerMuscular power: Force produced quickly: work divided by time, the combination of strength and speed.
Prebiotics: Food components your gut microbes use, to your benefit.
Probiotics: Live bacteria whose benefits are specific to the strain and dose.
Progesterone: The hormone released after ovulation that prepares the womb lining.
Progressive overload: Gradually increasing the demand on a muscle over time so it keeps adapting.
ProlactinPRL: The pituitary hormone behind milk production, which also suppresses sex hormones when high.
ProprioceptionPosition sense, Kinaesthesia: The body's internal sense of where its parts are and how they are moving.
Protein: The macronutrient made of amino acids, used to build and repair tissue.
Protein leverageProtein leverage hypothesis: The idea that appetite is driven by protein, so you keep eating until protein needs are met.
PSAProstate specific antigen: A protein made by the prostate, used in prostate screening.

R18 terms

Radial pressure wave therapy: A clinic device that delivers rapid mechanical pressure pulses through the skin, commonly used for stubborn tendon pain.
Randomised controlled trialRCT: A study that assigns people to treatment or control at random: the design that can show cause rather than association.
Range of motionROM: How far a joint or lift travels through its full available movement.
Rate of force developmentRFD: How quickly you can generate force: the slope of the force curve, not its peak.
Red blood cell countRBC, Erythrocyte count: How many oxygen-carrying cells are in a given volume of blood.
Red cell distribution widthRDW: A measure of how varied your red blood cells are in size.
RefeedDiet break, Refeed day: A planned higher-calorie day, usually extra carbohydrate, inserted into a diet.
Referred pain: Pain felt in one part of the body that actually originates from another.
REM sleepRapid eye movement sleep, Paradoxical sleep: The dreaming stage of sleep, important for memory consolidation and emotional regulation.
RepetitionRep, Reps: One complete movement of an exercise, from start position back to start position.
Rest-pause: Taking short pauses inside a set to squeeze out extra reps beyond failure.
Resting heart rateRHR: Your heart rate at complete rest, a simple fitness and recovery marker.
RetinoidsRetinol, Tretinoin: Vitamin-A derivatives: the best-evidenced topical active for skin ageing.
Reverse dieting: Raising calories gradually after a diet, aiming to limit fat regain on the way back up.
RIRReps in Reserve: How many more reps you could have done, a way to gauge proximity to failure.
RMRResting metabolic rate: The energy your body uses at rest: slightly higher, and easier to measure, than BMR.
RPERate of Perceived Exertion: A 1 to 10 scale for how hard a set felt, used to regulate training intensity.
Running economy: How much oxygen, and so energy, you use to run at a given pace.

S23 terms

Sarcomere: The basic contractile unit of muscle, where actin and myosin slide past each other.
Sarcopenia: The age-related loss of muscle mass, strength and function.
Satiety: The feeling of fullness that ends a meal and keeps hunger away afterwards.
SeleniumSerum selenium: A trace mineral built into antioxidant and thyroid enzymes.
Set: A group of consecutive reps performed without resting.
SHBGSex hormone-binding globulin: The carrier protein that decides how much of your testosterone is usable.
Short-chain fatty acidsSCFAs, Butyrate: What gut bacteria make from fibre, and a main fuel for the colon lining.
Skeletal muscleStriated muscle: The voluntary muscle attached to bone that moves the body: the tissue training targets.
Skin barrierStratum corneum, Moisture barrier: The skin's outer defence layer that keeps water in and irritants out.
Sleep cycle: A roughly 90-minute progression through the sleep stages, repeated four to six times a night.
Slow-wave sleepDeep sleep, N3, Delta sleep: The deepest stage of sleep, central to physical recovery and growth hormone release.
Smooth muscle: Involuntary muscle lining organs and blood vessels, working without conscious control.
SodiumNa: The main salt in blood, and the number that tracks your body's water balance.
Steady-state cardioContinuous training: Continuous exercise held at a constant, moderate intensity.
StrengthMaximal strength: The greatest force a muscle or muscle group can produce in a given movement.
Stroke volume: The amount of blood the heart ejects with each individual beat.
Subcutaneous fat: Fat stored directly under the skin: the fat you can pinch.
Superset: Two exercises performed back to back with little or no rest between them.
Surrogate endpointSurrogate marker, Intermediate outcome: A measurable stand-in for the outcome you actually care about: useful, and sometimes misleading.
Sympathetic nervous system: The fight-or-flight branch that mobilises the body for action.
Synbiotics: Products combining live microbes with a substrate that feeds them.
Synovial fluid: The viscous fluid inside a joint capsule that lubricates and nourishes cartilage.
Systolic and diastolicSystole, Diastole: Pressure while the heart contracts, and pressure while it refills between beats.

T23 terms

TDEETotal Daily Energy Expenditure: The total calories you burn in a day, from all activity and metabolism.
Telomere: Protective DNA caps on the ends of chromosomes that shorten each time a cell divides.
TempoRep tempo, Lifting tempo: The speed of each phase of a rep, often written as four digits such as 3-1-1-0.
Tempo runThreshold run: A sustained effort at or near lactate threshold, run to raise that threshold.
Tendon: Tough connective tissue that transmits force from muscle to bone.
Testosterone: A key sex hormone for muscle, bone, mood, energy and libido.
Thermic effect of foodTEF, Diet-induced thermogenesis: The calories burned digesting, absorbing and processing what you eat.
Thyroid hormoneT3, T4, Thyroxine, Triiodothyronine: The hormones that set the body's metabolic rate.
Thyroid peroxidase antibodiesTPO antibodies, Anti-TPO: Antibodies against a thyroid enzyme, marking autoimmune thyroid disease.
Time under tensionTUT: The total time a muscle is loaded during a set.
Timed Up and Go (TUG) test: A simple clinical test that times how quickly a person stands up, walks a short distance, turns and sits back down.
Total cholesterol: All the cholesterol carried in your blood, added together.
Total iron binding capacityTIBC: How much iron your blood could carry if every transport site were filled.
Total proteinSerum total protein: All the protein circulating in your blood, mostly albumin and antibodies.
Training intensityIntensity, Percentage of 1RM: How heavy a load is, usually expressed as a percentage of your one-rep max.
Training to failureMuscular failure: Taking a set to the point where another good rep is impossible.
Training volume: The total amount of work done, often counted as hard sets per muscle per week.
TransferrinSiderophilin: The protein that carries iron around the blood.
Transferrin saturationTSAT, Iron saturation: What percentage of your iron-carrying capacity is actually occupied.
TriglyceridesTG: The main form of fat carried in your blood.
TSHThyroid-stimulating hormone: The brain's signal that controls the thyroid: the first-line thyroid check.
Type I muscle fibreSlow-twitch fibre, Type I muscle fiber: Slow-twitch, fatigue-resistant fibres built for sustained aerobic work.
Type II muscle fibreFast-twitch fibre, Type II muscle fiber: Fast-twitch fibres that produce force quickly and powerfully, but fatigue sooner.

U2 terms

UreaSerum urea, BUN: The waste product of protein breakdown, cleared by the kidneys.
Uric acidUrate, Serum urate: The end product of purine breakdown, and the substance behind gout.

V4 terms

Visceral fatBelly fat (deep): Fat stored deep around the abdominal organs, linked to metabolic risk.
Vitamin B12Cobalamin: A vitamin essential for healthy nerves and red blood cells.
Vitamin D25-hydroxyvitamin D, 25(OH)D: The 'sunshine vitamin', really a hormone, and a very common deficiency.
VO₂max: The maximum rate at which your body can use oxygen, a key marker of aerobic fitness.

W4 terms

Waist-to-height ratioWHtR: Waist circumference divided by height: arguably the best single-number risk screen.
Waist-to-hip ratioWHR: Waist circumference divided by hip circumference: a marker of fat distribution.
Weight regain: What tends to happen after weight loss stops, including after medication.
White blood cell countWBC, Leukocytes: The number of infection-fighting cells in your blood.

Z2 terms

ZincSerum zinc: A mineral essential to immunity, healing and testosterone production.
Zone 2: A moderate aerobic intensity you can sustain while holding a conversation.