Lactate testing explained — how to find your athlete’s thresholds

What is blood lactate?

Every cell that burns glucose produces lactate as a byproduct. At low intensities, muscles clear lactate almost as fast as they produce it — the liver reconverts it to glucose (the Cori cycle), and neighbouring muscle fibres oxidise it directly as fuel. Resting blood lactate sits around 0.8–1.5 mmol/L.

As effort increases, production outpaces clearance. Lactate accumulates, hydrogen ions tag along, and the local pH drops. This acidosis — alongside other metabolic byproducts like inorganic phosphate — contributes to the sensation of fatigue, not lactate itself. The old term “lactic acid” conflates two things: lactate (a useful fuel shuttle) and the acidosis that happens to rise alongside it. Modern physiology treats lactate as a signal, not a villain.

Why does this matter for coaching? Because the rate at which lactate rises relative to intensity tells you exactly where an athlete transitions from predominantly aerobic metabolism to increasingly anaerobic work. Those transition points — thresholds — are the anchors for every training zone you prescribe.

The step test protocol

A lactate step test is straightforward: the athlete performs a series of fixed-duration stages at progressively higher intensities. At the end of each stage you take a capillary blood sample (usually from the earlobe or fingertip) and measure lactate concentration with a portable analyser.

Running protocol

1. Warm up for 10 minutes at an easy conversational pace.
2. Begin the first stage at a comfortable speed — typically 2–3 km/h below the athlete’s estimated threshold pace.
3. Each stage lasts 3–4 minutes (long enough for lactate to reach a near-steady state at that intensity).
4. Increase speed by 0.5–1.0 km/h per stage. Keep the treadmill gradient at 1 % to approximate outdoor air resistance.
5. At the end of each stage, pause briefly (30–45 seconds) to take the blood sample and record heart rate.
6. Continue until blood lactate exceeds 8 mmol/L, the athlete cannot maintain the pace, or you have enough data points above the deflection zone (usually 6–8 stages).

Cycling protocol

1. Warm up for 10 minutes at 100–120 W (or a self-selected easy effort).
2. Start the first stage at roughly 50–60 % of estimated FTP.
3. Each stage lasts 3–4 minutes. Increase power by 20–30 W per stage.
4. Maintain a steady cadence throughout — cadence shifts alter the metabolic demand independent of power.
5. Sample blood and record heart rate between stages.
6. Stop when lactate rises above 8 mmol/L or the athlete can no longer hold the target power.

Consistency is everything. Use the same warm-up, the same stage duration, the same sampling site, and the same analyser across tests. Small variations compound into meaningless data.

Key thresholds: LT1 and LT2

A typical lactate curve starts flat, bends gently upward, and then steepens into an exponential rise. Two inflection points along that curve define the thresholds that matter most.

Aerobic threshold (LT1)

LT1 is the intensity at which lactate first rises above baseline — the first sustained deflection from resting values. It usually corresponds to roughly 2 mmol/L, though this varies between athletes. Below LT1, metabolism is almost entirely aerobic and the athlete can sustain the effort for hours. Training at or just below LT1 builds the aerobic engine without meaningful fatigue.

Anaerobic threshold (LT2 / MLSS)

LT2 approximates the highest intensity at which lactate production and clearance are still in equilibrium — close to the maximal lactate steady state (MLSS). Above this point, lactate accumulates exponentially and fatigue follows within minutes. LT2 often falls around 4 mmol/L, but using a fixed number is a rough approximation. An individual curve fit is far more reliable.

The gap between LT1 and LT2 describes the athlete’s aerobic “bandwidth” — how wide the range of sustainable intensities is. Endurance-trained athletes tend to have a wide gap; less trained athletes have a narrow one. Monitoring how this gap changes over a training block is one of the most useful longitudinal signals a coach can track.

From data to training zones

Once you know LT1 and LT2 (as pace, speed, or power), you can anchor training zones to physiology instead of percentages of max heart rate. A common framework:

• Zone 1 — recovery: well below LT1. Very easy effort, used for warm-ups and active recovery.
• Zone 2 — aerobic endurance: around and just below LT1. The bread and butter of base training.
• Zone 3 — tempo: between LT1 and LT2. Moderately hard, sustainable for 40–60 minutes in trained athletes.
• Zone 4 — threshold: at or near LT2. Sustainable for roughly 20–40 minutes. The classic “comfortably hard” effort.
• Zone 5 — VO2max: above LT2. Intervals of 3–5 minutes at maximal aerobic intensity.

The exact boundaries depend on the model you use, but the principle is the same: thresholds derived from lactate data set zone boundaries far more accurately than age-predicted formulas.

Raceday automates this step. Enter the speed/power and lactate values from each stage, and the platform fits a curve, detects LT1 and LT2 automatically, and generates training zones with corresponding pace and heart-rate ranges — no spreadsheet required.

How often to test

Thresholds shift as fitness changes. Test too often and you waste training time; test too rarely and your zones drift out of alignment with the athlete’s current physiology. A practical cadence:

• Every 8–12 weeks during a structured training block. This captures meaningful aerobic adaptations without over-testing.
• At the start of a new macrocycle to establish fresh baselines after a recovery or off-season period.
• After illness or extended breaks — thresholds can drop faster than perceived effort suggests.

Keep every test result on file. Comparing lactate curves over months and seasons tells a clearer adaptation story than any single snapshot. Raceday stores each test alongside the calculated thresholds and zones so you can overlay curves and track shifts over time.