Lactate threshold, specifically LT2, is the single most trainable lever for endurance performance. For most athletes, improving LT2 pace or power yields faster race times than any other physiological change, including VO2 max gains. Understanding it, measuring it, and training it specifically makes the difference between athletes who plateau and those who keep improving.
What LT1 and LT2 are
Blood lactate rises predictably with exercise intensity. Two inflection points define how the body handles that rise.
LT1 (Aerobic threshold) is where lactate first rises above resting levels, roughly 2 mmol/L. Below LT1, the body clears lactate as fast as it produces it. This is Zone 2: an effort where conversation flows easily and you could continue for hours. Training here builds the aerobic base that every other physiological quality depends on.
LT2 (Anaerobic threshold) is where lactate begins accumulating faster than it can be cleared, roughly 4 mmol/L. This is approximately your one-hour race pace: hard, controlled, sustainable for 30-60 minutes but unmistakably demanding. Speaking becomes difficult. Breathing is deliberate and audible.
The gap between LT1 and LT2 reflects aerobic capacity. A world-class marathoner may have LT2 sitting at 93% of VO2 max. A recreational athlete might hit the same ceiling at 75-80%. That gap is trainable with consistent, structured threshold work.
| Threshold | Blood Lactate | Heart Rate (approx.) | Effort feel |
|---|---|---|---|
| Below LT1 | <2 mmol/L | <75% max HR | Easy, conversational |
| LT1-LT2 | 2-4 mmol/L | 75-87% max HR | Moderate, controlled |
| At LT2 | ~4 mmol/L | 87-92% max HR | Hard, sustainable |
| Above LT2 | >4 mmol/L | >92% max HR | Very hard, time-limited |
How to find your threshold
Lab testing remains the gold standard. A graded exercise test with fingertip blood samples at each stage produces a precise lactate curve. Most sports physiology labs charge £80-150 for a full protocol. Results show precisely where LT1 and LT2 fall in terms of pace, power, and heart rate.
The 30-minute field test is the practical alternative. Run or cycle at the hardest sustainable effort for exactly 30 minutes. Average heart rate for the final 20 minutes approximates LTHR (Lactate Threshold Heart Rate). Power-based cyclists use average power over those 20 minutes as a proxy for threshold power.
One important caveat from a 2026 study comparing threshold methods: critical speed, derived from time trials at multiple distances, consistently yields faster estimates than laboratory lactate testing across trained, highly trained, and world-class runners. The two should not be treated as interchangeable when setting training zones.
How to train it
Threshold work belongs in a programme once or twice per week. More frequent sessions without adequate recovery accumulate fatigue without proportional adaptation. The specific stimulus for raising LT2 is sufficient threshold time per session, not simply more sessions per week.
Two formats deliver the most consistent results:
Sustained tempo: 20-40 minutes continuous at LT2 pace or LTHR. Psychologically demanding, but trains lactate clearance under prolonged metabolic stress.
Cruise intervals: 3 x 12 minutes or 2 x 20 minutes at LT2 pace or LTHR, with 2-3 minutes easy recovery between efforts. Total threshold time per session: 36-60 minutes.
| Protocol | Duration | Intensity | Recovery |
|---|---|---|---|
| Sustained tempo | 20-40 min | LT2 pace / LTHR | None |
| Cruise intervals (short) | 3 x 12 min | LT2 | 2 min easy |
| Cruise intervals (long) | 2 x 20 min | LT2 | 2-3 min easy |
The bulk of weekly volume, roughly 75-80%, should stay below LT1. This is the foundation that polarised and pyramidal training models share: high-quality threshold work placed on top of genuinely easy aerobic volume, not in place of it.
The grey zone mistake
The most damaging threshold training error is chronic grey zone training: too hard for real aerobic adaptation, too easy to drive LT2 up. It is the default pace most athletes fall into when training by feel without structure. Slightly harder than easy, never quite reaching threshold.
Grey zone sessions accumulate fatigue while producing modest physiological return. Athletes in this pattern tend to feel perpetually tired without improving. Easy runs feel moderate; intended threshold sessions feel impossible.
The fix is deliberate contrast. Easy sessions held genuinely below LT1, conversational pace and low RPE. Threshold sessions hit precisely at LT2. The physiological signal from well-executed threshold work is sharp and specific. Blunting it with grey zone intensity reduces its effect.
How PROTR tracks threshold training
PROTR reads LTHR from Apple Health and uses it to define heart rate zones for every endurance session automatically. Whether a run was genuinely Zone 2 or strayed into the grey zone is visible immediately from session data. TSS is calculated from LTHR for every run, ride, and swim, feeding into the PMC chart so threshold training accumulates alongside easy volume in one coherent picture. PROTR is free to download.