Lactate Threshold and Pace: The Science Behind Your Tempo Runs

Lactate threshold is one of the most discussed concepts in endurance running and one of the most misunderstood. Coaches talk about "threshold pace." Athletes do "tempo runs." Training plans mention "LT work." But the actual physiology behind these terms — what lactate is, why it accumulates, and where the threshold sits in relation to VDOT — is rarely explained clearly.

This guide traces the concept from the physiology to the formula, shows where threshold pace sits in the Daniels framework, and explains how to estimate yours from race time without a laboratory.

What lactate actually is

A common misconception: lactate causes muscle soreness and fatigue. It does not. Lactate is a metabolic byproduct of carbohydrate metabolism, continuously produced and continuously cleared. At rest and at easy running pace, production and clearance are balanced. Lactate levels in blood remain near resting values (approximately 1–2 mmol/L).

As exercise intensity increases, carbohydrate metabolism accelerates. Lactate production rises. The body clears lactate via several pathways — conversion back to glucose (Cori cycle), use as fuel in cardiac muscle and slow-twitch fibres, and export to the liver. At moderate intensities, clearance keeps pace with production. Blood lactate stays stable at 2–4 mmol/L.

At a critical intensity, production begins to exceed clearance capacity. Blood lactate rises progressively. This is the lactate threshold.

The threshold that matters for marathon training is what researchers call LT2 (the second lactate threshold) or the lactate turnpoint — the intensity where blood lactate begins its rapid, progressive rise. Below LT2, effort is sustainable for long periods. Above it, lactate accumulates until you slow or stop.

LT2 is distinguished from LT1 (the first lactate threshold or aerobic threshold), where lactate first begins rising above true baseline, typically occurring at an easier effort around 65–75% VO2 max. LT1 corresponds roughly to a comfortable conversational pace — the zone boundary between Zone 1 and Zone 2 in most HR zone systems.

Where threshold sits in the VDOT framework

Jack Daniels positions Threshold (T) pace in his training system as approximately 86–88% of VO2 max. This is operationally defined as the pace you could sustain in a maximal 30–60 minute race effort.

For most trained runners, this corresponds closely to their current 10K pace or slightly slower, depending on how well-trained they are for sustained efforts. An elite runner's threshold pace may be as high as 90% of VO2 max; a recreational runner's may be closer to 82%.

The VDOT-derived T pace formula:

From your VDOT (derived from any race using the calculator below), the Daniels tables give T pace. For VDOT 50, T pace is approximately 4:14 min/km — the pace sustainable for 20–40 minutes in a threshold workout, not necessarily for a full 10K race.

The calculator shows your VDOT and the corresponding Daniels training paces, including Threshold (T) pace.

Estimating your lactate threshold pace from race performance

Without a lab test, the most practical estimate comes from race performance.

Method 1: Daniels T pace from VDOT Enter a recent race result in the calculator above. The T pace in the output is a validated estimate of your threshold pace based on the Daniels-Gilbert equations.

Method 2: Recent 10K pace For runners with adequate 5K–10K racing, threshold pace is typically:

• Approximately equal to current 10K race pace, or
• 5–15 seconds per km slower than 10K pace

For a runner with a 45-minute 10K (4:30/km race pace), threshold pace is approximately 4:30–4:40 min/km.

Method 3: Friel's 30-minute time trial Run 30 minutes all-out solo (not a race). Average pace for the full 30 minutes is approximately your threshold pace. This is more directly measured than VDOT-derived T pace but requires a proper effort.

Method 4: Heart rate at threshold Using the LTHR approach from Joe Friel: run a hard 30-minute effort and record average HR for the final 20 minutes. This is your lactate threshold heart rate (LTHR). Zone definitions are then calculated as percentages of LTHR rather than HRmax.

How training improves threshold

Lactate threshold is highly trainable — among the most trainable performance factors in distance running. Two mechanisms:

1. Improved lactate clearance capacity: Training increases the density of mitochondria in muscle cells and the capacity to oxidise lactate as a fuel. The heart, slow-twitch fibres, and liver become more efficient at extracting and clearing lactate from the blood.

2. Increased lactate threshold relative to VO2 max: A well-trained runner sustains a higher fraction of their VO2 max before the threshold is crossed. An untrained runner's LT2 may occur at 60–70% VO2 max; an elite marathoner's may occur at 85–90%.

The training stimulus that specifically improves LT is sustained effort near the threshold. Daniels' recommended threshold workout formats:

• Tempo runs: 20–40 minutes continuous at T pace. Note: this site explains the physiology of tempo pace but does not prescribe how to use it — that is training advice for a coach.
• Cruise intervals: Repeated segments (e.g., 4 × 10 minutes) at T pace with brief recovery.

High aerobic volume (Easy pace running) also improves threshold over time, though less acutely than targeted threshold training.

Lactate threshold vs VDOT: the relationship

Lactate threshold and VDOT (VO2 max proxy) are related but different. VDOT represents aerobic ceiling; threshold represents what fraction of that ceiling can be sustained.

Two runners with the same VDOT (same 10K time) can have very different threshold performance depending on how well-trained their aerobic base is. The runner who races 10Ks frequently but does little long-distance training may have a lower threshold-to-VDOT ratio than a marathon specialist.

Improving VDOT raises the ceiling. Improving threshold raises the fraction of the ceiling you can sustain at race pace. Both matter; for distances beyond 10K, the threshold fraction is increasingly important.

Limitations

Individual variation in threshold-to-VDOT relationship: The Daniels formula gives T pace as approximately 86–88% of VO2 max, which is accurate for typical trained runners. Highly speed-trained runners may find their actual threshold is lower relative to their VDOT (they're fast but not well-trained in sustained aerobic efforts). Highly endurance-trained runners may find their threshold is higher.

Threshold varies with fitness: As your aerobic base improves or deteriorates with training cycles, your threshold shifts. VDOT-derived T pace should be recalculated after significant fitness changes.

The 30-minute race test has its own errors: Friel's LTHR test is a useful field estimate but depends on being able to run a genuine hard 30-minute effort solo, without drafting, on a measured flat course. Under and over-effort both produce incorrect LTHR estimates.