What are LT1 and LT2?

LT1 (first lactate threshold, ~2 mmol/L) is the intensity where lactate first rises above resting levels. LT2 (second lactate threshold, ~4 mmol/L) is the intensity where lactate accumulation accelerates — often called the "anaerobic threshold" or "OBLA." LT2 pace roughly corresponds to your 1-hour race effort.

How accurate is this simulator?

This is a simplified educational model. Real lactate kinetics depend on muscle fibre type, glycogen status, temperature, hydration, and many other factors. The simulator captures the general shape and relative differences between workout structures but should not replace actual blood lactate testing.

What does the fitness level setting change?

Fitness level determines lactate clearance rate. Beginners clear at ~0.25 mmol/L per minute while elite athletes clear at ~0.55 mmol/L per minute. This reflects real physiological differences in monocarboxylate transporter density and mitochondrial capacity.

Why does lactate keep rising even during rest segments?

It does not — rest segments reduce the production rate to a basal level (0.1 mmol/L/min) while clearance continues. If you see a slight rise during early rest, it is because the clearance rate has not yet overcome the residual production. Lactate typically begins falling within 1–2 minutes of rest.

Can I simulate a full marathon?

Yes, the simulator supports workouts up to 300 minutes. However, for very long workouts at moderate intensity, real-world lactate stabilises at steady-state much earlier than the full duration would suggest. The simulation will show this plateau effect.

What is the estimated recovery time?

After the workout ends, the simulator estimates how many minutes of rest are needed for lactate to fall back to 2 mmol/L (just above resting). Elite athletes recover much faster than beginners due to higher clearance rates.

Lactate Accumulation Simulator — Free Blood Lactate Kinetics Tool

Input Guide

How each input affects the lactate simulation.

LT2 Pace

time

Your anaerobic threshold pace in min:sec/km. This is the boundary between Zone 2 (moderate) and Zone 3 (hard). Running faster than this causes rapid lactate accumulation.

Options

2:00–15:00 per km

Approximate LT2 as your 1-hour race pace or the pace where conversation becomes impossible.

LT1 Pace

time

Your aerobic threshold pace in min:sec/km. This is the boundary between Zone 1 (easy) and Zone 2 (moderate). Must be slower than LT2.

Options

2:00–15:00 per km

LT1 is typically 15–25% slower than LT2. It is your comfortable conversation pace.

Fitness Level

select

Determines lactate clearance rate: beginner (0.25), intermediate (0.35), advanced (0.45), elite (0.55) mmol/L cleared per minute per mmol/L present.

Options

BeginnerIntermediateAdvancedElite

If you have been running consistently for 2+ years and train 5+ days/week, choose Advanced.

Workout Segments

number

Each segment is either a run (with pace and duration) or a rest (recovery with duration only). Segments are processed in order to build the time series.

Options

1–20 segments, 1–120 min each

Include warm-up and cool-down as separate easy-pace segments for a realistic simulation.

Simulate Blood Lactate Kinetics Across Your Workout

Blood lactate is a key biomarker of exercise intensity. This simulator models lactate production and clearance minute-by-minute through a structured workout using Euler-method numerical integration. Define your LT1 and LT2 thresholds, build a workout with run and rest segments, and watch how lactate accumulates and clears in real time — all calculated locally in your browser.

What Is a Lactate Accumulation Simulator?

This tool models blood lactate concentration ([La⁻]) over time during a structured workout. It uses a 3-zone production model based on your pace relative to LT2, combined with first-order clearance kinetics scaled by fitness level. The result is a minute-by-minute time series showing lactate accumulation, zone transitions, peak and average concentrations, and estimated recovery time.

Why Simulate Lactate Accumulation?

Understand how different interval structures affect lactate accumulation patterns
Optimise rest intervals — too short and lactate spirals, too long and the training stimulus drops
Compare lactate responses across easy runs, tempo sessions, VO₂max intervals, and race simulations
Visualise why the same workout feels different at beginner vs. elite fitness levels
Learn how lactate clears during rest periods and after your workout finishes
Plan workouts that target specific lactate zones for metabolic adaptation

Who Uses a Lactate Simulator?

Coaches designing interval sessions

Optimise work-to-rest ratios by simulating how lactate accumulates during hard efforts and clears during recovery jogs.

Athletes preparing for threshold workouts

Preview how a 20-minute tempo run or 4×5-minute cruise intervals will stress your lactate system before you run them.

Exercise physiology students

Visualise lactate kinetics concepts from textbooks — production rate, clearance rate, steady-state, and accumulation — with an interactive model.

Runners comparing fitness levels

See how the same workout produces dramatically different lactate responses for a beginner vs. an elite athlete due to clearance rate differences.

Under the Hood

The mathematical model behind the lactate simulation.

Intensity Ratio

R = LT2_pace / current_pace. When R > 1 the athlete is running faster than LT2. Three production zones are defined: Zone 1 (R < LT1_ratio), Zone 2 (LT1_ratio ≤ R < 1), Zone 3 (R ≥ 1).

Production Rate

Zone 1: P = 0.3 mmol/L/min. Zone 2: P = 0.3 + 0.8 × ((R − LT1_ratio) / (1 − LT1_ratio))^1.5. Zone 3: P = 0.3 + 0.8 + 1.2 × (R − 1)^3. Rest: P = 0.1 mmol/L/min.

Clearance Rate

First-order kinetics: C = k_clear × L_current, where k_clear varies by fitness level (beginner 0.25, intermediate 0.35, advanced 0.45, elite 0.55).

Euler Integration

L(t+1) = max(FLOOR, L(t) + (Production − Clearance) × dt), where dt = 1 minute and FLOOR = 0.8 mmol/L. Starting lactate = 1.0 mmol/L.

Privacy

All calculations run entirely in your browser using JavaScript. No data is sent to any server.

Example Scenarios

Common workout types and their expected lactate responses.

Easy Recovery Run (35 min)

Warm-up + 30 min at 6:30/km + cooldown. For an intermediate runner with LT2 at 5:00/km, lactate stays in Zone 1, peaking around 1.3 mmol/L with minimal time above LT1.

35 min · Peak ~1.3 mmol/L · 0 min > LT2

5×1K Threshold Intervals

10 min warm-up, 5 × 4 min at LT2 pace with 2 min rest, 10 min cooldown. Peak lactate reaches ~5–6 mmol/L during the final interval with progressive accumulation.

48 min · Peak ~5.5 mmol/L · ~18 min > LT2

VO₂max 6×800m Repeats

Warm-up, 6 × 3 min at 3:30/km with 3 min jog, cooldown. Lactate spikes above 8 mmol/L in Zone 3 then partially clears during jog recovery.

56 min · Peak ~9 mmol/L · ~18 min > LT2

Research & References

The lactate kinetics model is informed by the following peer-reviewed research.

Billat, V.L. et al. (2003). The concept of maximal lactate steady state: a bridge between biochemistry, physiology and sport science. Sports Medicine, 33(6), 407–426.
Beneke, R. et al. (2011). Blood lactate diagnostics in exercise testing and training. International Journal of Sports Physiology and Performance, 6(1), 8–24.
Brooks, G.A. (2018). The science and translation of lactate shuttle theory. Cell Metabolism, 27(4), 757–785.
Faude, O., Kindermann, W., & Meyer, T. (2009). Lactate threshold concepts: how valid are they? Sports Medicine, 39(6), 469–490.
Goodwin, M.L. et al. (2007). Blood lactate measurements and analysis during exercise: a guide for clinicians. Journal of Diabetes Science and Technology, 1(4), 558–569.

Lactate Accumulation Simulator

Workout Setup

Input Guide

LT2 Pace

LT1 Pace

Fitness Level

Workout Segments

Simulate Blood Lactate Kinetics Across Your Workout

What Is a Lactate Accumulation Simulator?

Why Simulate Lactate Accumulation?

Who Uses a Lactate Simulator?

Coaches designing interval sessions

Athletes preparing for threshold workouts

Exercise physiology students

Runners comparing fitness levels

Under the Hood

Intensity Ratio

Production Rate

Clearance Rate

Euler Integration

Privacy

Example Scenarios

Easy Recovery Run (35 min)

5×1K Threshold Intervals

VO₂max 6×800m Repeats

Research & References

Frequently Asked Questions

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Deepen Your Knowledge

Lactate Threshold Training Explained

Choosing Your Training Model

Training Zones Explained

Workout Setup