What your plan
and watch actually mean.
Short, plain-language definitions for the training and physiology terms that show up in your plan, your watch, and every coaching article, including ours.
Aerobic Base
The underlying aerobic fitness, built mostly through Zone 2 volume, that determines how much pace or power you can hold before flipping into fatigue-producing anaerobic metabolism.
Anaerobic Threshold
The intensity above which lactate builds up faster than your body can clear it, and fatigue starts compounding fast.
ATL (Acute Training Load)
A rolling average of your daily training load over about a week: a measure of how much fatigue you're carrying right now.
Base Phase
The lower-intensity, volume-focused stretch of a training plan (or the off-season itself) where the aerobic engine, strength, and technique get built before race-specific work begins.
Brick Workout
A session that links two disciplines back to back, almost always bike to run, to rehearse the specific discomfort of a race transition.
Critical Swim Speed (CSS)
The pace you could theoretically hold in a maximal-effort swim: swimming's equivalent of FTP, used to set swim training paces and zones.
CTL (Chronic Training Load)
A rolling, weighted average of your daily training load over roughly six weeks: the closest thing to a single 'fitness' number.
FTP (Functional Threshold Power)
The highest average power, in watts, you could hold for about an hour: cycling's core intensity benchmark.
Heart Rate Zones
Dividing training intensity into numbered bands, typically three to seven, based on percentage of max or threshold heart rate, each with a different physiological purpose.
HRV (Heart Rate Variability)
The variation in time between consecutive heartbeats: a marker of nervous-system recovery, not fitness itself.
Lactate Threshold
The intensity where blood lactate stops tracking flat and starts climbing: measured directly in a lab, estimated in the field for everyone else.
Negative Split
Running or racing the second half of an event faster than the first half, the opposite of starting fast and fading.
Overtraining Syndrome
A serious, poorly understood condition of chronic performance decline and fatigue lasting weeks to months despite rest, distinct from ordinary overreaching, which resolves in days.
Periodization
Structuring training into deliberate phases (base, build, peak, taper) instead of training the same way, at the same intensity, every week.
Race Pace
The specific, tested pace or effort you can actually sustain for your target race distance and conditions, not a generic comfortable pace.
Recovery Week
A deliberately lighter week built into a plan every few weeks, scheduled proactively so accumulated fatigue clears before it turns into a bigger problem.
Resting Heart Rate
Your heart rate at true rest, ideally measured first thing in the morning before you get out of bed: a slow-moving recovery signal, not a fitness score by itself.
RPE (Rate of Perceived Exertion)
A subjective 1-10 rating of how hard an effort felt, no hardware required, and it catches things devices sometimes miss.
Running Economy
The energy cost of running at a given pace: how efficiently an athlete converts oxygen into forward motion. Essentially, running efficiency.
Sighting
Briefly lifting your eyes above the water while swimming freestyle to check your direction against a landmark, since there's no lane line to follow in open water.
Taper
The planned drop in training volume before a race that lets fatigue clear while fitness holds. It cuts volume, never intensity.
Training Load
How much stress your training is putting on your body, the general concept behind specific scores like TSS, CTL, and ATL.
Transition Zone (T1/T2)
The area where athletes switch gear between disciplines: T1 is swim to bike, T2 is bike to run, and both count toward your finish time.
TSS (Training Stress Score)
A single number estimating how hard one workout was, combining duration and intensity into one score.
VO2 Max
The maximum rate your body can consume oxygen during exercise: a ceiling on aerobic capacity, measured in ml of oxygen per kg of body weight per minute.
Zone 2 Training
Low, purely aerobic effort, roughly 60-70% of threshold, that builds the aerobic base without piling up much fatigue.