Reading Your Lactate Test Step by Step: From Raw Numbers to Training Zones

You ran the test, you wrote down nine pairs of numbers, you plugged them into a calculator and out came LT1 = 220 W and LT2 = 285 W. Now what?

Threshold numbers without context are useless. The same LT1/LT2 pair can come from a beautifully shaped curve (trust it, train against it) or from a noisy one that should be re-done. This article walks through how to read a real curve so you can tell the difference.

What you're actually looking at

A blood-lactate test produces a 2D plot:

• X axis: intensity — watts for cycling, pace (in sec/km or min/km) for running, sec/100 m for swimming.
• Y axis: blood lactate concentration in mmol/L.
• Each dot is one stage of your step test — typically a 4-minute steady-state effort with a finger-prick blood sample at the end.
• The line through the dots is a polynomial (usually 3rd-order) fitted by software. It smooths out the natural noise in finger-prick measurements (±0.2 mmol/L per sample).

Two key inflections on that line define the rest of the analysis:

LT1 — the first deflection (aerobic threshold)

LT1 is the lowest intensity at which blood lactate starts to consistently rise above resting baseline. Below LT1 your aerobic system is matching demand; above it, anaerobic glycolysis is contributing and lactate begins to accumulate at a manageable rate.

How to spot it on the chart:

• Look for the first stage where lactate is ~1 mmol/L above your resting baseline. If your resting lactate was 0.8, look for 1.8.
• Visually: the curve is roughly flat (or even slightly dipping in the fat-burning zone) and then starts to tick up. The tick is small — sometimes just 0.2–0.3 mmol/L per stage.
• Most well-trained endurance athletes hit LT1 at 65–75 % of LT2 power. If yours is at 50 %, the curve doesn't have a clear first deflection — re-test.

LT2 — the second deflection (anaerobic threshold)

LT2 is where lactate production overwhelms clearance and the curve becomes nearly vertical. Above LT2 you can hold the intensity for minutes, not hours. This is the "30–60 min all-out" effort that FTP / Critical Power / MLSS all try to estimate.

How to spot it:

• Look for the "explosion" — the stage where lactate jumps by 1.5+ mmol/L from the previous one. Often the rise from 2.5 → 4 → 6 mmol/L.
• Mathematically, the D-max method draws a line from the first to the last point and finds where the curve is furthest below that chord. The modified D-max method anchors at LT1 instead — usually more accurate.
• Lactate at LT2 is typically 3–5 mmol/L — never 6+ if the test was done right. If your software reports LT2 at 7 mmol/L, the algorithm is pulled into the explosion zone.

The shape of your curve tells you a story

Type A — long flat baseline, sharp deflection

What it looks like: 4–5 stages at < 1.5 mmol/L, then a single big jump from 2 → 4 → 7 mmol/L in the last two stages.

What it means: You're a well-trained aerobic athlete. Big aerobic engine, good fat oxidation, late onset of anaerobic contribution. LT2 is at a high % of VO2max. Common in cyclists with years of base.

Training implication: Most gains will come from raising the ceiling (VO2max work, threshold intervals), not from more Zone 2.

Type B — gradual smooth rise

What it looks like: Lactate climbs by 0.5 mmol/L every stage from start to finish, no clear inflection.

What it means: Limited aerobic capacity. Your body is relying on glycolysis even at low intensities. Common in newer athletes or after a long training break.

Training implication: Spend the next 8–12 weeks heavy on Zone 2 (60–70 % of LT2 power). The curve will literally flatten as mitochondrial density improves.

Type C — early rise, then plateau, then explosion

What it looks like: Lactate is at 2.5–3 mmol/L early, stays there for several stages, then jumps to 8+ mmol/L at the end.

What it means: Probably one of three things: under-recovered (lactate clearance impaired), under-fueled (low glycogen → less anaerobic capacity), or measurement error early on (sensor warm-up, alcohol contamination).

Training implication: Repeat the test. Same time of day, same warm-up, properly fed and rested. If the same pattern appears, look at recovery, not training.

Red flags that mean re-test

1. Peak lactate < 5 mmol/L. Test ended too easy. The D-max method needs the curve to go vertical to anchor the upper end — without it, LT2 comes out 20–40 W low.
2. A single point sticks 2+ mmol/L above its neighbours.Almost certainly a measurement error (didn't wipe the alcohol, sampled too late, tissue fluid in the drop). The polynomial fit gets distorted around that point.
3. Lactate at the first stage above resting baseline + 0.5.You started too hard — there's no flat aerobic zone to anchor LT1.
4. Power/pace decreased on a later stage. Either a typo in your stage values (very common — "196" instead of "296") or you couldn't hold the prescribed intensity. Either way, sort it out before trusting the curve.

From LT1 and LT2 to actual training zones

Once you trust the two thresholds, zones fall out trivially:

A few quick rules of thumb:

• The bigger the gap between LT1 and LT2, the better your aerobic base. Pros often have 80+ W between them; recreational athletes 30–50 W.
• If LT1 and LT2 are within 20 W, your training should be almost entirely Zone 2. You're glycolytic too early in the curve.
• HR at LT1 ≈ MAF heart rate for steady-state aerobic work.

What to look for when you re-test in 8 weeks

Real adaptation shows up as one (or all) of these:

• The whole curve shifts RIGHT. Same lactate at higher watts → more aerobic power. Best possible outcome.
• The LT1 → LT2 gap widens. Bigger sustainable Zone 2 / Zone 3 buffer.
• The curve flattens in the middle. Less glycolytic contribution at sub-threshold intensities. Often the first sign of a good base block.
• HR at the same lactate drops 5–10 bpm. Cardiovascular efficiency gain even without raw power change.

If nothing changes after 8 weeks of training, the issue is either the training (probably too much Z3 grey zone) or recovery (sleep, fueling, stress). The curve doesn't lie.

One last thing — trust the trend, not the single test

Even a perfect test has ±5 W of error on LT2. If your power jumps from 280 → 285 → 290 W across three tests, the trend is real (+10 W). If it jumps to 305 W in one test and 275 in the next, something changed between them — protocol, conditions, recovery state.

Always test under identical conditions, and always look at 2–3 tests together before deciding to bump your zones.