Core Principles

Sub-Threshold Focus

The primary goal is to shift the lactate curve rightward, allowing higher speeds at lower lactate concentrations. Quality training primarily occurs just below the second lactate threshold (LT2), often termed “sweetspot” or high Zone 3/low Zone 4 (in a 5-zone model). This intensity provides significant aerobic stimulus with manageable fatigue, forming the bulk (~20-25% of weekly time) of the training load. High-intensity VO2max work is generally avoided or minimized in this phase due to its high cost-to-benefit ratio for general preparation.

Intensity Control

This is crucial. Avoid exceeding LT2 during quality sessions. Precise control allows for frequent, repeatable quality work. Erring slightly slower is safer than going too fast. Monitoring methods include:

Lactate Meter (Ideal): Target levels are typically between 2.0-4.0 mmol/L, ensuring you remain below LT2. Many practitioners recommend aiming for the lower end, potentially capping efforts around 2.5-3.0 mmol/L, to ensure sustainability and avoid creeping over the threshold, though individual responses vary. Consistent monitoring helps ensure levels don’t consistently exceed LT2 towards the end of the final reps. Requires understanding of personal lactate curve and consistent testing protocol. Lactate testing provides the most direct measure of internal load.
Heart Rate (Proxy): Use a reliable Lactate Threshold Heart Rate (LTHR) estimate (e.g., Friel’s 30-min test). Aim for sub-threshold zones, perhaps capping effort around 90-92% of Max HR or just below LTHR. Be mindful of HR lag, drift (especially in heat/humidity), and external factors (sleep, stress). The correlation between HR and lactate can vary significantly day-to-day and between individuals; HR is often better used as a ceiling than a precise target, especially as fitness increases or in variable conditions. LTHR itself is generally stable for trained runners.
Pace (Proxy): Use paces derived from current race fitness (e.g., recent 5k/10k time trials or races, ideally within 4-8 weeks). Paces need adjustment based on interval duration (faster for shorter reps, slower for longer reps) and conditions (heat, humidity, wind, terrain). VDOT or Tinman calculators can provide starting points, but individual responses vary. Consistency of terrain (track, flat road, treadmill) improves pace reliability.
Power (Proxy): Running power meters (like Stryd or watch-based estimates) offer another intensity gauge, more responsive than HR and potentially accounting for grade. Critical Power (CP) estimates can correlate well with LT2. Accuracy and consistency across devices/conditions (especially wind) are still evolving and may vary between individuals.
Perceived Exertion (RPE): Sessions should feel “comfortably hard” (e.g., RPE 5-6/10) – challenging but sustainable, allowing conversation in short bursts. You should feel capable of doing another rep but adhere to the plan. RPE can be skewed by previous training habits or external factors.

Easy Recovery

Non-quality runs must be genuinely easy to facilitate recovery. Aim for efforts below 70% of Max HR or ~65% of Maximal Aerobic Speed (MAS). Monitor for low cardiac drift after the initial warm-up period. Slow down as needed, even walking hills; recovery is paramount. This allows for the high frequency of quality sessions.

Consistency & Repeatability

The system’s effectiveness stems from consistent application over months and years, leading to a higher sustainable training load, which correlates strongly with performance improvements. Workouts are designed to be repeatable weekly with minimal burnout or injury risk if intensity is controlled.