Lactate threshold (LT1 & LT2)

Lactate threshold refers to the exercise intensities at which blood-lactate behavior changes, and there are two of them. LT1, the aerobic threshold, is where lactate first rises above resting levels (~2 mmol/L); LT2, the anaerobic threshold or maximal lactate steady state, is the highest intensity at which lactate production and clearance still balance (~4 mmol/L). Together these two points carve your intensity into the three physiological zones that sit underneath all endurance training.

LT1 (aerobic threshold)          ≈ first rise above baseline, ~2 mmol/L
LT2 (anaerobic threshold / MLSS) ≈ ~4 mmol/L, highest lactate steady state
    (2 and 4 mmol/L are conventional anchors, not universal constants)

The two thresholds define three domains. Below LT1 is easy endurance: lactate stays near baseline and fat dominates as fuel. Between LT1 and LT2 is the moderate-to-heavy band where lactate is elevated but stable. Above LT2, lactate climbs inexorably toward exhaustion. The familiar 2 and 4 mmol/L figures are reference conventions, not physiological laws — an individual's true thresholds can sit meaningfully above or below them, which is exactly why lab testing measures the inflection rather than assuming a number.

Because most riders won't draw blood mid-ride, lactate threshold is often anchored by heart rate — LTHR, the lactate-threshold heart rate, commonly estimated from the average HR of a hard 20–30 minute time trial. Joe Friel's heart-rate zones are built as percentages of LTHR (%LTHR), which is a different anchor from %HRmax. LTHR is more stable day to day than power output, but it still drifts with heat, dehydration, and fatigue.

Lactate threshold is not the same construct as FTP, even though the two usually sit close. FTP is defined operationally (the power you can sustain for about an hour); LT2 is defined by lactate kinetics, and maximal-lactate-steady-state testing is its gold standard. The practical upshot is that FTP is the convenient field proxy for the threshold that actually matters physiologically — good enough to set zones from, without a blood-lactate meter.