Anaerobic Threshold
The intensity above which lactate builds up faster than your body can clear it, and fatigue starts compounding fast.
Anaerobic threshold is the exercise intensity where lactate starts accumulating in your blood faster than your body can clear it. Below that point, effort feels sustainable for a long time because lactate production and clearance stay roughly balanced. Cross it, and fatigue stops rising in a straight line and starts rising fast: the same small increase in pace or power buys you a much shorter time before you have to slow down.
That crossover point is a real physiological event, but pinning it down precisely requires a lab: a graded exercise test with blood lactate samples drawn at increasing intensities, tracing the curve where it bends upward. Almost nobody trains off that number directly. What triathletes actually use are field-testable stand-ins that sit close to the same intensity without needing a syringe.
FTP and threshold heart rate are the practical proxies
On the bike, FTP fills this role: the highest power you could hold for about an hour, estimated from a 20-minute test or a ramp protocol rather than measured in a lab. It isn't a direct measurement of anaerobic threshold, but it lands close enough to the same intensity that cycling training is built almost entirely around it. Threshold heart rate works the same way for running and, less precisely, for swimming: the heart rate you can sustain for roughly an hour of hard, steady effort, tested with a time trial rather than blood draws.
The relationship is real but approximate. FTP and threshold heart rate are correlated with anaerobic threshold, not identical to it. Two athletes can share an FTP and sit at meaningfully different points relative to their actual lactate curve, because the field test estimates a sustainable effort, not the underlying metabolic crossover directly. That gap is exactly why coaches treat FTP and threshold pace as working numbers to retest periodically, not as a fixed physiological fact once measured.
Why the distinction matters for training
Workouts labeled "threshold" are built to sit right around this intensity on purpose: hard enough to push the ceiling on how much lactate your body can process, easy enough to hold for the 15 to 40 minutes a session like that usually asks for. Undershoot it and you get a tempo session wearing a threshold label. Overshoot it and a session that was supposed to build sustainable power turns into a series of short, unsustainable efforts with long recoveries between them. That's a different training stimulus than the one intended.
For a triathlete pacing a race, the same idea applies across all three disciplines at once. Swim, bike and run each have their own threshold, and holding effort just under it, rather than repeatedly crossing back and forth over it, is most of what separates a well-paced long-course race from one that falls apart in the final third.