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.
Lactate threshold is the exercise intensity where blood lactate stops holding roughly steady against rising effort and starts climbing instead. Below it, your muscles produce lactate and your body clears it at close to the same rate, so effort feels sustainable. Above it, production outruns clearance, and the same small increase in pace or power buys a much shorter time before you have to back off.
Physiologists generally describe two inflection points on a lactate curve, not one: a lower rise above resting levels (LT1, or the aerobic threshold) and a higher point where lactate stops leveling off altogether (LT2, or the maximal lactate steady state). In everyday triathlon conversation, "lactate threshold" almost always points at the second, higher one.
How it's measured in a lab
The direct method is a graded exercise test: rising intensity in stages of a few minutes each, with a small blood sample drawn at the end of every stage. Plotted against intensity, the result is a curve that stays flat, then bends upward. Where exactly that bend counts as "threshold" is genuinely contested. A widely used convention, the onset of blood lactate accumulation (OBLA), uses a fixed 4 mmol/L cutoff, but researchers have shown that point doesn't land in the same physiological spot for every athlete or protocol, which is why individualized alternatives exist and don't always agree with OBLA or each other.
The relationship to anaerobic threshold and FTP
This is the part worth being precise about. Anaerobic threshold is frequently used as a near-synonym for lactate threshold's higher-intensity sense, and in casual coaching talk, treating the two as the same rarely causes real confusion. Both describe the same underlying crossover between sustainable and unsustainable effort. Where the terms diverge is that "lactate threshold" sometimes gets used, more precisely, for the lower LT1 point too, while "anaerobic threshold" almost always refers to the higher one.
FTP is different again: a cycling-specific field estimate built to sit near this same intensity, not a direct measurement of it. Research on how closely FTP agrees with a lab-measured threshold is mixed: some studies find strong correlation with real individual scatter, others find disagreement large enough that the authors don't consider FTP a reliable stand-in for every athlete tested. Treat FTP, and running's threshold-pace equivalent, as good practical estimates of lactate threshold, not the physiological number itself.
Field tests you can use
Almost nobody trains off a lab lactate curve day to day. On the bike, a 20-minute maximal time trial multiplied by roughly 0.95 estimates FTP, a proxy for lactate threshold. For running, a 30-minute time trial, with the final 20 minutes averaged for heart rate and pace, does the same job. Swimming's version, critical swim speed, is usually estimated from a fixed-distance time trial instead.
None of these field numbers are fixed facts. Fitness changes, so they're typically retested every 6 to 8 weeks or after a focused block. What matters is a consistent number tracked against your own trend, not the exact mmol/L value a lab you don't have access to would show.