A Guide to Lactate Testing

What Is Lactate Testing?

Lactate testing measures the concentration of lactate in a swimmer's blood — in millimoles per litre (mmol/L) — at different swimming speeds. A small blood sample is taken from the fingertip or earlobe and read by a handheld analyser in about 10 seconds, so it can be done poolside between reps without breaking up a session.

The point isn't the lactate number in isolation — it's what happens to that number as speed increases. Plotted across a set of increasing efforts, lactate traces a curve that reveals two things every coach actually cares about: the fastest pace a swimmer can hold aerobically for a long time, and the fastest pace they can sustain before fatigue accumulates rapidly. Those two paces anchor every training zone a program uses.

The Science Behind It

During exercise, muscles break down glucose for energy through glycolysis. One of the products of that process is lactate. At rest and at low intensities, the body produces and clears lactate at roughly matched rates, so blood levels stay low — typically 0.5–1.5 mmol/L. As swimming speed increases, glycolysis speeds up and lactate production begins to outpace clearance, so blood levels start to climb.

Lactate is fuel, not a waste product

The older "lactic acid causes muscle fatigue" model has been superseded by what exercise physiologists call the lactate shuttle — first proposed by physiologist George Brooks. Lactate produced in one part of a muscle cell (or in one muscle fibre) is transported via proteins called monocarboxylate transporters (MCTs) to be used as fuel elsewhere — in a neighbouring fibre, the heart, or the same cell's own mitochondria. Lactate is, in effect, a currency the body moves around to keep working muscle supplied with energy, not a toxin it needs to flush out.

What actually changes at higher intensities is the balance between production and clearance. Below a swimmer's threshold, the shuttle system keeps up — lactate is produced and used at matched rates, and levels stay low and stable. Above threshold, production outstrips the muscle's ability to shuttle and oxidise it, so lactate accumulates in the blood. That accumulation point — not lactate itself — is what a step test is designed to find.

Thresholds & What the Numbers Mean

A step test produces a series of lactate readings at increasing speeds. Two points on that curve matter most, generally referred to as LT1 and LT2:

LT2 is also widely known as OBLA — the Onset of Blood Lactate Accumulation. The 4.0 mmol/L figure is a useful fixed reference point, but it's a population average, not a universal law: a given swimmer's true threshold can genuinely sit anywhere from about 2.5 to 5.5 mmol/L depending on training history and individual physiology. That's exactly why testing an individual swimmer matters more than applying a textbook number.

V4mM — the single most useful number

Because the fixed 4.0 mmol/L reference is so widely used, coaches often talk about a swimmer's V4mM — their swimming speed at 4 mmol/L blood lactate — as shorthand for "threshold pace." It's the most heavily used number from a step test because it correlates strongly with middle-distance performance and is simple to track over a season: as a swimmer's aerobic fitness improves, V4mM speed increases, meaning they can hold a faster pace before crossing that 4 mmol/L line.

Running a Step Test

The most common field protocol for swimmers is an incremental step test — a series of repeats at the same distance, each one faster than the last, with a blood sample taken during the rest after every step.

Equipment

A 7-step protocol

1. Pick the swimmer's main competitive stroke and a consistent distance per step — 200m is typical (100m or 400m both work, adjust rest accordingly).
2. Start noticeably below race pace — roughly 15–20 seconds/100m slower than the swimmer's Critical Swim Speed, as an easy aerobic effort.
3. Increase pace by a fixed increment each step — around 3–4 seconds/100m per step works well for most swimmers.
4. Allow a fixed rest between steps, typically 3–4 minutes, so lactate has time to reach the bloodstream and stabilise before sampling.
5. Take the blood sample 30–60 seconds into the rest period, from the fingertip or earlobe. Record the split time and the lactate reading together.
6. The final step should be at or near race pace — a maximal or near-maximal effort — to capture peak lactate as well as the threshold points.
7. A full 7-step test typically takes 45–50 minutes per swimmer including rest periods.

Worked example

A 7-step test for a middle-distance freestyler might look like this:

This exact dataset is loaded into the interactive plotter below — see section 08 for how it's read.

Reading the Curve

Plot swimming speed on the x-axis and blood lactate on the y-axis. A healthy curve stays flat and low through the early, easy steps, then bends upward at two points — a gentle "elbow" around LT1, and a much sharper one at LT2/V4mM, beyond which lactate rises steeply for each further increase in speed.

• A right shift after a training block — reaching 2.0 and 4.0 mmol/L at faster paces than before — confirms aerobic adaptation. The engine got bigger.
• A left shift — hitting those same lactate values at slower paces than a previous test — signals fatigue, insufficient recovery, or an eroded aerobic base. It's a flag to check training load and recovery before assuming fitness has dropped.
• A flat curve with an elevated starting point (lactate already high on step one) usually means the swimmer arrived under-recovered or in a high-fatigue state, not that the test itself is wrong. Don't draw conclusions from a single odd-looking test without checking how rested the swimmer was.
• Peak lactate on the final, maximal step is a useful secondary number, especially for sprinters. A peak above roughly 12–14 mmol/L on an all-out effort suggests strong glycolytic capacity. A peak below about 8 mmol/L can mean limited anaerobic development — or simply that the swimmer didn't commit fully to the last step. Combine the number with what you observed.

Applying It: Swimming & Open Water

Pool training zones from V4mM

Once you know a swimmer's V4mM pace, it becomes the anchor for their training zones rather than a guess based on feel or a generic percentage of best time.

Without a test, it's easy to assign "easy aerobic" work that's actually creeping into threshold intensity — it fatigues the swimmer without producing the aerobic adaptation that was the point of the session. A step test closes that gap with an individual number instead of a generic guess.

Open water perspectives

The same underlying physiology applies in open water, with a few practical differences. Research on elite open-water swimmers has found threshold speeds around 1:02/100m pace for men and 1:08/100m pace for women at the top of the sport — useful as a scale reference, not a target for developing swimmers. More usefully for pacing strategy: successful open-water swimmers tend to race with an even pace through the first three-quarters of a race, then produce a modest 1.5–3% speed increase in the final quarter — and a conservative, controlled start is consistently associated with better finishing results than going out hard from the gun.

Blood lactate testing itself is harder to run mid-swim in open water than on a pool deck between reps, so many open-water coaches use a step test in the pool to establish threshold pace, then apply it to open-water pacing plans — adjusting for current, chop, and drafting on race day rather than retesting lactate in the field. A simpler in-water proxy worth knowing alongside lactate testing is Critical Swim Speed (CSS) — calculated from two time trials at different distances (e.g. a 400m and a 200m) using the pace difference between them — which approximates threshold pace without needing any blood sampling at all, and is a useful cross-check against a lactate-derived V4mM.

When to Test

Lactate testing adds useful data, but it also adds fatigue and time — it doesn't need to happen often to be valuable. Three to four tests across a season is typically enough to track whether training is producing the intended adaptation.

Where possible, track both V4mM (LT2) and the LT1 pace across tests. A rightward shift in both points to broad aerobic development; a shift in V4mM alone without a matching LT1 shift suggests more specific threshold adaptation without a wider aerobic base — useful information for adjusting the next training block.

Plot Your Own Curve

Enter a swimmer's step-test data below — pace per 100m and the lactate reading for each step — to plot the curve and estimate LT1 and V4mM (LT2) pace automatically. It loads with the worked example from section 04; edit any field to plug in real data from your own testing.