LT1 (first lactate threshold) or VT1 (first ventilatory threshold) is the intensity where your body starts producing lactate faster than it can clear at rest — but still slowly enough that it doesn't accumulate. This is the ceiling of genuine 'easy' riding. Most amateur cyclists ride above LT1 on their easy days without realising it, which is why their Zone 2 training doesn't produce the expected adaptations. Lab testing or a talk test can identify LT1.