Glossary
Every term in this guide, defined in plain language. If a concept has a full page, the entry links to it. Terms are grouped alphabetically - use your browser's find function if you are hunting for something specific.
Where the running world uses two words for one thing (threshold pace / tempo pace, easy pace / conversational pace), the entry says so, because a lot of confusion comes from coaches using different vocabulary for the same physiology.
A
Adaptation - The change your body makes in response to training stress: more mitochondria, more capillaries, stronger tendons, better fuel use. Adaptation happens during recovery, not during the workout. See Recovery and Adaptation.
Aerobic - Energy production that uses oxygen. It is the dominant system for anything longer than about two minutes, which means it is the dominant system for essentially all distance running, including the 5K. See The Aerobic System.
Aerobic base - The accumulated aerobic fitness built by consistent, mostly easy mileage over months and years. It is the platform every other kind of training sits on. Sometimes just called "base."
Anaerobic - Energy production without oxygen. Powerful but short-lived, it dominates efforts under roughly 60-90 seconds. Distance runners use anaerobic energy mainly in the finishing kick and in short, fast reps.
Anaerobic threshold - An older name for lactate threshold. Somewhat misleading (the effort is not actually anaerobic) but still widely used. See Lactate Threshold.
B
Base phase - The first block of a training cycle: build mileage, run mostly easy, add strides and light hills, avoid heavy race-specific work. See Progression and Periodization.
Bonk (also "hitting the wall") - The sudden, dramatic collapse in pace that happens when muscle and liver glycogen run out, typically after 18-22 miles of marathon running for an underfuelled runner. It feels like the power has been switched off, not like ordinary fatigue. Prevented by aerobic training (better fat use), carbohydrate loading, and fueling during the race. See Fueling and Nutrition.
Build phase - The block after base, where race-specific quality work (threshold, intervals, race-pace long runs) is layered onto the mileage you built.
C
Cadence - Steps per minute. Recreational runners commonly land somewhere between 160 and 180; elites are often above 180. Cadence naturally rises with pace. It is a useful diagnostic (a very low cadence often accompanies overstriding) but chasing an arbitrary "180" number is not a goal in itself.
Cooldown - Easy jogging after a hard session, typically 10-15 minutes. It clears the session, adds cheap aerobic mileage, and eases the transition back to rest.
Cross-training - Non-running aerobic work: cycling, swimming, elliptical, aqua-jogging. Useful for maintaining aerobic fitness when injured, and for adding load without impact. It does not fully substitute for running (see Specificity).
Cruise interval - A threshold-pace repeat broken into chunks with short rest, for example 4 x 1 mile at threshold pace with 1 minute jog between. Jack Daniels' term. The short rest keeps the whole session near threshold while making the work more manageable than a single continuous tempo. See Jack Daniels' Running Formula.
D
Deload / down week - A planned reduction in volume (usually 20-30%) every third or fourth week, letting accumulated fatigue clear so adaptation can surface. See Progression and Periodization.
Detraining - The loss of fitness when training stops. Aerobic fitness begins to decline measurably after roughly two weeks off and drops substantially by four to six weeks. Discussed as "reversibility" in Training Principles.
DOMS (Delayed Onset Muscle Soreness) - Muscle soreness that peaks 24-72 hours after unfamiliar or eccentric-heavy work, such as downhill running or a first weights session. It is normal, it is not injury, and it is not a measure of workout quality. Sharp, localized, one-sided pain is a different thing entirely - see Injury Prevention.
E
Easy pace - Comfortable, conversational, sustainable-for-hours effort. Usually 60-90 seconds per mile slower than 5K pace and roughly 65-79% of max heart rate. Most of your running lives here. If you cannot speak in full sentences, you are not running easy. See The Easy/Hard Rule.
Economy - See Running economy.
F
Fartlek - Swedish for "speed play": unstructured surges within a run, for example "1 minute hard, 2 minutes easy, repeat 8 times" or simply pushing between lamp posts. Less rigid than track intervals, which makes it a good way to add intensity without the mental load of a formal session. See Types of Running Workouts.
Fat oxidation - Burning fat for fuel. Fat is nearly unlimited in supply but burns slowly; the better your aerobic system, the more of your race pace you can fund from fat, which spares glycogen. This is one of the main reasons easy mileage matters for the marathon.
Foot strike - Which part of the foot lands first (heel, midfoot, forefoot). It varies with pace and terrain. The evidence that deliberately changing your foot strike prevents injury or improves performance is weak; overstriding (landing far in front of your center of mass) is the more meaningful problem.
G
Glycogen - Carbohydrate stored in muscle and liver. It is the fuel that lets you run fast. Total stores are limited (roughly 90-120 minutes of hard running), which is why marathon runners must fuel mid-race and why hard sessions feel awful when you are underfed. See Fueling and Nutrition.
Gray zone - The moderate-intensity no-man's-land between easy and threshold. Too hard to recover from, too easy to drive threshold or VO2max adaptation. Where most junk miles live. See The Easy/Hard Rule.
H
Heart rate zones - Intensity bands defined as percentages of maximum or reserve heart rate, usually five of them, from recovery (Zone 1) to VO2max (Zone 5). Useful for enforcing easy days, less useful for short, fast reps where heart rate lags behind effort. See Heart Rate Zones and the zones table.
Hill repeats - Hard, short efforts run up a hill (typically 30 seconds to 3 minutes) with a jog-down recovery. They build power, strength and running economy with less impact stress than flat sprinting, which is why they are often called "speedwork in disguise."
HRmax (maximum heart rate) - The highest heart rate you can reach. Age formulas (220 minus age) carry an error of roughly ±10-12 beats, so field-testing your own is far more accurate. See Heart Rate Zones.
HRrest (resting heart rate) - Your heart rate at complete rest, best measured before getting out of bed. A rise of 5-10 beats sustained over several days is one of the cheapest available warnings of accumulated fatigue or illness.
I
Interval - A repeated hard effort separated by recovery. Strictly, "interval" refers to the recovery period, but in practice runners use it for the whole session. Classic VO2max intervals are 3-5 minutes at roughly 3K-5K pace, for example 5 x 1000m. See Types of Running Workouts.
J
Junk miles - Miles run at moderate effort that are too hard to aid recovery and too easy to drive adaptation. Note the nuance: genuinely easy miles are never junk, no matter how slow. It is the moderate ones that are junk. See The Easy/Hard Rule.
L
Lactate - A byproduct of fast carbohydrate metabolism, and also a fuel your body reuses. It is not the cause of muscle soreness (a myth that survives despite being disproven) and it is not a waste product. Blood lactate is measured because it is a convenient marker of metabolic intensity.
Lactate threshold (LT) - The intensity above which lactate production outruns clearance and blood lactate climbs steeply. Practically: the fastest pace you can hold in a steady state, roughly one hour of racing for a trained runner. It is the single most trainable determinant of distance performance. See Lactate Threshold.
LT1 / LT2 - The two-threshold model. LT1 (aerobic threshold) is the first small rise in blood lactate above baseline - roughly the top of easy running. LT2 (anaerobic threshold, or "the" lactate threshold) is the point where lactate accumulates rapidly - roughly tempo/threshold pace. Polarized training means spending most time below LT1 and your hard work above LT2, with little in between.
Long run - The week's longest run, usually 20-30% of weekly mileage. It builds capillary density, mitochondria, fat oxidation, tendon resilience, and the mental capacity to keep going. It is the least skippable run in the week. See Types of Running Workouts.
M
Marathon pace (MP / M pace) - The pace you intend to race a marathon at. As a training intensity it is comfortably hard: harder than easy, easier than threshold. Used in long-run finishes and marathon-specific blocks.
Mileage (mpw) - Miles per week, the standard unit of training volume. "40 mpw" means forty miles in a week. Volume, more than any single workout, is what drives aerobic fitness - which is why it is also the number to increase most carefully.
N
Negative split - Running the second half of a run or race faster than the first. It is the pacing pattern that produces most personal bests and nearly all distance world records, because starting conservatively delays glycogen depletion and the drift into unsustainable intensity. See Race Day Execution.
O
Overreaching - A short-term overload that leaves you flat for days to a couple of weeks. Planned ("functional") overreaching followed by a taper is a legitimate tool. Left unchecked, it slides into overtraining.
Overtraining syndrome (OTS) - The chronic state past overreaching: performance drops, resting heart rate rises, sleep degrades, mood and motivation crater, illness and injury become frequent. Recovery takes weeks to months, not days. It is far easier to avoid than to fix. See Recovery and Adaptation.
P
Periodization - Organizing training into sequenced phases (base, build, peak, taper, recovery) so that fitness rises over the cycle and peaks at the goal race. See Progression and Periodization and the periodization guide.
Polarized training - An intensity distribution in which roughly 80% of running is easy and roughly 20% is genuinely hard, with very little moderate work. Repeatedly observed in elite endurance athletes across sports. Often shortened to "80/20." See The Easy/Hard Rule.
Progression run - A run that starts easy and finishes faster, for example 8 miles with the last 3 at marathon pace. It teaches you to run strong on tired legs and rehearses the negative split, at a lower total cost than an interval session.
R
Recovery run - A short, deliberately very slow run the day after a hard session. Its job is circulation and easy aerobic volume, not fitness. If you are tempted to speed it up, you have missed its purpose.
Repetition pace (R pace) - Faster than interval pace - roughly mile-race pace - run as short reps (200-400m) with full recovery. The target is running economy, speed and neuromuscular coordination, not the aerobic system. Jack Daniels' terminology. See Jack Daniels' Running Formula.
RPE (Rating of Perceived Exertion) - Your subjective effort on a scale, usually 1-10 (or the older 6-20 Borg scale). Easy runs are 3-4, threshold is about 7, VO2max intervals are 8-9, all-out is 10. RPE is free, needs no batteries, and self-corrects for heat, hills, sleep and stress - which is why experienced runners still use it after they have all the gadgets.
Running economy - How much oxygen you burn to hold a given pace. Two runners with identical VO2max can differ by minutes over a half marathon because one is more economical. It improves with years of mileage, strides, hills, and strength training. See VO2 Max and Running Economy.
S
Split - The time for a segment of a run or race, for example a mile split or a 5K split. See also Negative split.
Strides - 15-30 second accelerations to roughly mile pace, relaxed and controlled, with full recovery between. Typically 4-8 at the end of an easy run, once or twice a week. They preserve leg speed and improve economy at almost no fatigue cost, and they are the single most underused item in most amateur training.
Supercompensation - The mechanism behind training: stress temporarily lowers your capacity, and during recovery your body rebuilds slightly above the previous level. Train again at the top of that rebound and fitness rises; train again too early and you dig a hole. See Recovery and Adaptation.
T
Taper - The planned reduction in volume in the last 1-3 weeks before a race (roughly 40-60% off peak volume, while keeping some intensity) so accumulated fatigue clears and fitness surfaces. Cutting intensity entirely leaves you flat; cutting volume is the point. See Race Day Execution.
Tempo run - A sustained run at threshold effort, classically 20-40 minutes, described as "comfortably hard": you could speak a few words, not a sentence. Confusingly, some coaches use "tempo" for anything moderately fast. In this guide, tempo means threshold. See Types of Running Workouts.
Threshold pace (T pace) - The pace corresponding to lactate threshold, roughly what you could race for an hour. Both "tempo pace" and "threshold pace" are used for it. See Lactate Threshold.
Time trial - A solo, all-out test over a set distance (commonly 1 mile, 3K, or 5K) used to estimate current fitness and set training paces when you have no recent race. It is the cheapest substitute for a lab test. See VDOT and Pace Tables.
V
VDOT - Jack Daniels' single-number fitness score, derived from a recent race performance rather than a lab test. Given a VDOT, the tables hand you your easy, marathon, threshold, interval and repetition paces. It is effectively a "performance VO2max" that already accounts for economy. See Jack Daniels' Running Formula and VDOT and Pace Tables.
VO2max - The maximum rate at which you can take in and use oxygen, in ml/kg/min. It sets the ceiling on your aerobic engine, is highly trainable in beginners, and plateaus in trained runners - at which point threshold and economy are what continue to improve. It is a ceiling, not a prediction: plenty of runners beat athletes with higher VO2max values. See VO2 Max and Running Economy.
W
Warmup - 10-20 minutes of easy jogging plus drills and strides before a hard session or race. It raises muscle temperature and gets your aerobic system online so the first rep is not wasted getting up to speed.
Workout (vs. run) - In training vocabulary these are not synonyms. A run is aerobic mileage - easy, recovery, or long. A workout is a structured hard session with a specific target (tempo, intervals, hills, race-pace work). Most weeks contain many runs and only one or two workouts. When a plan says "2 quality sessions per week," it means two workouts, and everything else stays genuinely easy.