Lactate threshold
Lactate threshold is the exercise intensity above which lactate accumulates in the blood faster than the body can clear it. Above it, fatigue builds quickly; below it, you can keep going for a long time. It is one of the strongest predictors of endurance performance.
LT1 and LT2
Physiologists describe two thresholds. The first (LT1, or aerobic threshold) is where lactate first rises above resting levels — the top of easy, conversational training. The second (LT2, often just called lactate threshold) is the highest intensity where lactate stays steady — roughly the effort you can hold for about an hour. FTP, threshold pace and CSS are practical stand-ins for LT2.
How it's measured and trained
The precise way is a lab test with blood samples at increasing intensities. Field tests estimate it well enough for training. Both thresholds respond to training: plenty of easy volume raises LT1, and threshold intervals and tempo work raise LT2.