Coach’s In-depth Guide: Threshold Training
Understanding Lactate and Why It Builds Up
Lactate is not a poisonous waste product; it is a normal carbohydrate molecule produced when your muscles break down glucose during exercise . At lower intensities the body uses mostly fat and a little carbohydrate for fuel, and the by‑product pyruvate is easily used to provide energy in the presence of oxygen . When you run faster and oxygen becomes less available, pyruvate is converted into lactate so that energy production can continue . Lactate itself is an important fuel and signalling molecule; at low intensities it is cleared as quickly as it is produced, but as your pace increases the production rate eventually exceeds the clearance rate and lactate accumulates in the blood . The sensation of “burning” muscles during hard running comes from the associated drop in muscle pH rather than the lactate itself .
What Is Lactate Threshold Training?
Lactate threshold (also called the anaerobic threshold) is the point at which lactate begins to accumulate faster than it can be removed . In lab tests this point often occurs when blood lactate reaches about 4–5 mmol per litre . Running at or just below this threshold point allows you to train hard enough to stress your aerobic and lactate‑clearing systems without tipping into full anaerobic mode. A lactate‑threshold run involves running at this “sweet spot” pace; it feels “weirdly sustainable” it is uncomfortable yet you can hold it for 30–60 minutes .
Training at lactate threshold improves your body’s ability to clear lactate and therefore delays fatigue. Runners who consistently include these sessions in their program see improvements in the pace they can sustain before lactate accumulation forces them to slow down . This kind of training benefits runners at all levels and distances because it raises the ceiling for aerobic performance . It also strengthens the mental skill of sustaining hard but controlled effort, which helps you cope with mid‑race discomfort .
The L1 and L2 Markers
L1 (First Lactate Threshold / Aerobic Threshold)
What it represents: L1 is the exercise intensity at which blood lactate first rises above resting levels . It marks the transition from easy to moderate effort and is sometimes called the aerobic threshold.
Physiological meaning: Below L1 the aerobic system comfortably meets energy demands and lactate clearance matches production. As you approach L1, glycolytic (carbohydrate) metabolism begins to contribute more, but lactate still remains in steady state .
Typical intensity: In incremental tests L1 often occurs at around 2 mmol/L of blood lactate . Well‑trained runners can sustain efforts at L1 for three to four hours, this is roughly what we call marathon pace .
Why train here: Training at or just below L1 (often called Zone 2) enhances fat oxidation, increases mitochondrial density and improves lactate clearance . This builds your aerobic “engine” so that harder efforts feel easier.
L2 (Second Lactate Threshold / Anaerobic Threshold)
What it represents: L2 is the intensity at which lactate accumulation surpasses the body’s ability to remove it . It is sometimes called the anaerobic threshold.
Physiological meaning: Above L2, lactate rises exponentially and the body cannot reach a steady state; below it, lactate production and clearance can balance for a limited time .
Typical intensity: Laboratory tests often identify L2 when blood lactate reaches about 4 mmol/L . Runners can typically sustain this intensity for 30–60 minutes, or what we refer to as ‘about 10 km race pace’.
Why train here: Training near L2 improves your tolerance to lactate and increases the pace you can sustain before lactate accumulates excessively . It enhances both aerobic and anaerobic energy systems and helps you push your threshold higher.
Here is a simple diagram illustrating how heart rate typically increases with exercise intensity or time, with approximate points for the first lactate threshold (L1) and the second lactate threshold (L2):
In this example, heart rate rises gradually at lower intensities and more steeply as intensity increases. The first lactate threshold (L1) occurs at the point where lactate levels begin to rise above resting levels; training at or near L1 improves fat oxidation and overall endurance .
The second lactate threshold (L2) represents the point where lactate accumulation exceeds the body’s ability to clear it, which leads to fatigue; training around L2 helps develop tolerance to higher intensities .
Relevance for Training Plans
Coaches often use L1 and L2 to structure training into zones:
Zone 1 (below L1): Easy running for recovery and general endurance.
Zone 2 (between L1 and L2): Moderate effort; long runs and steady‑state sessions. This zone is capped by your L2 pace .
Zone 3 (above L2): Hard efforts like interval or VO₂ max workouts; used sparingly.
Spending ample time in Zone 2 develops your aerobic base and improves metabolic efficiency . Workouts at L2 raise the point at which lactate accumulates, boosting race performance .
Practical Guidance for Training at L1 and L2
Estimating Your Zones
If you don’t have access to lab testing, you can estimate your L1 and L2 using several cues:
Heart‑rate zones: Zone 2 (L1) typically corresponds to about 70–80 % of your heart rate at L2 , which often equates to roughly 65–80% of your max heart rate . L2 training generally occurs around 85-90% of maximum heart rate.
Perceived effort (RPE): Zone 2 feels like 3–4 out of 10; breathing is controlled and conversation is easy . L2 feels like 5–6 out of 10 or “comfortably hard”; you can speak, but only in short phrases.
Talk test: At L2 you may be unsure whether you can speak comfortably; short sentences are possible but conversation is limited .
Pace guidelines: Zone 2 tends to align with marathon to half‑marathon pace and can be sustained for about 2½ hours . L2 pace is roughly what you could sustain for a 10 km or one‑hour race or 24–30 seconds per mile slower than your 5 k pace .
Training at L1 (Zone 2)
Easy long runs: Spend a significant portion of your weekly mileage at a very easy pace. Keep your breathing relaxed and maintain a conversational pace . Monitoring heart‑rate drift (when heart rate steadily rises at a constant pace) can help ensure you stay in this zone .
Duration: Sessions at L1 can last from 30 minutes to several hours. Because the intensity is low, they promote mitochondrial growth, fat oxidation and lactate clearance .
Benefits for all abilities: Novice runners build endurance without excessive fatigue, while advanced runners use L1 runs to recover between harder sessions and to improve aerobic efficiency.
Training at L2 (Threshold Zone)
Threshold runs: Warm up, then run at L2 pace for 20–40 minutes. Focus on maintaining a steady effort and avoid going too hard; the goal is to hover at the point where lactate production and clearance balance out.
Cruise intervals: Break the threshold work into repeats (e.g., 4 × 5 mins at threshold with 1 minute easy jog). This allows you to accumulate more time at threshold while controlling lactate build‑up.
Pacing: Use heart rate, RPE or GPS watch data to stay on target. True threshold pace is “comfortably hard,” around 25–30 seconds per mile slower than your 5k pace.
Recovery: Because threshold work is taxing, allow easy days before and after. Include plenty of easy running or cross‑training for active recovery .
Integrating Both Zones in a Balanced Program
Polarised approach: Devote ~70–80 % of your weekly volume to easy Zone 1–2 running and ~20–30 % to harder L2 or above. This maximizes aerobic adaptations while giving enough stimulus to raise your threshold.
Progression: Start with more easy work and gradually introduce short L2 segments. Increase the duration or number of threshold intervals as your fitness improves, but do so cautiously to avoid overtraining.
Individual differences: Age, fitness level and daily stress influence your heart rate and perceived effort. Use multiple cues such as heart rate, talk tests and generally how you feel, to adjust your pace.
Cross‑training and strength: Incorporate resistance training and mobility work to improve muscle efficiency and support higher training loads
Final Thoughts
Lactate threshold training is not reserved for elite athletes. Understanding and training around your L1 and L2 markers lets you train smarter, not just harder. By spending plenty of time in the comfortable realm of Zone 2 to build your aerobic base and sprinkling in focused threshold sessions to nudge your L2 upward, you can run faster and further with less fatigue. Keep listening to your body, be patient, and remember that improvements in running come from consistent, well‑balanced training over weeks and months .
STAY STRONG.