In last week’s article, The New Evolution of Speed, we explored the science of internal load and why metabolic stability; staying in the "sweet spot" is the true engine of progress. But to apply those scientific principles to daily training, we need a common language for intensity. In practice, this shift means we no longer chase exhaustion as proof of effectiveness; we chase repeatability.
Before we dive into the "gears" of high performance, we must acknowledge the foundation. Regardless of your model; Zone 2 (Aerobic Base) remains the non-negotiable bedrock. Volume is still the king of endurance; the following shifts are about how we layer quality on top of that foundation.
The Foundation: Understanding the 7-Zone Model
To dose training stress precisely, we use a framework based on Joe Friel’s intensity zones, mapped against both internal metabolic markers (Lactate) and external output (Critical Speed).
Zone 1 (Recovery): Very easy (< 1.5 mmol/L). Active recovery between hard days.
Zone 2 (Aerobic Base): The foundation (1.5–2.0 mmol/L). Build endurance and fat-burning efficiency.
Zone 3 (Tempo): "Extensive" aerobic work (2.0–3.0 mmol/L). Lower end of the heavy domain; roughly 90–94% of Critical Speed.
Zone 4 (Threshold / CS): "Intensive" aerobic gear (~3.0–4.0 mmol/L, individual MLSS-dependent). This is your Critical Speed (CS)—the highest steady-state pace you can maintain without a rapid "drift" in lactate or fatigue.
Zone 5 (VO2 Max): High-intensity "ceiling" work (> 5.0 mmol/L), roughly 3km to 5km race effort.
Zone 6 [5b] (Anaerobic Capacity): High-intensity bursts (30–60s) for power and lactic capacity.
Zone 7 [5c] (Neuromuscular Power): Maximum "all-out" effort (sprints/hills) under 20 seconds.
Note on Notation: While traditional Friel models use "5b" and "5c," we label them as distinct Zones 6 and 7. They target mechanical power and neuromuscular snap, requiring different recovery strategies than the "metabolic storm" of Zone 5.
1. The Traditional Model vs. The “Modern” Model
For decades, the Traditional Model focused on "Raising the Ceiling" via a binary approach: lots of Zone 2 and hard Zone 5 intervals. The logic is simple: work your heart and lungs at their absolute limit. This builds "mental callusing" but often ignores the metabolic middle ground.
The Modern Model (championed by researchers like Alejandro Casado and coaches like Marius Bakken) focuses on "Filling the Engine." We prioritize controlled Zones 3 and 4, using Critical Speed (CS) as our primary anchor to manage lactate levels. Instead of 15 minutes in Zone 5, we do 45–60 minutes in the Z3/Z4 "Heavy Domain." This is a "surgical" approach designed to maximize efficiency with minimal systemic fatigue.
2. The "Hollowed Out" Zone 4 & Zone 5
We effectively "hollow out" Zone 5 and the higher end of Z4; not by removing them entirely, but by using them sparingly and strategically.
Working Below (CS / Zone 4): Builds your "aerobic floor." The key is staying sub-critical (95-100% of CS), which corresponds to the high-efficiency lactate range of 2.5–3.5 mmol/L.
Working Above (Zones 6 & 7): Develops the "chassis" (tendons, stiffness, force application). This is "free speed"; high power with zero acid. It prevents the "wet noodle" chassis that leads to plateaus. While much of the recent hype centers on controlled "double threshold" sessions, very little focus is placed on sessions like the Ingebrigtsen 20 x 200m hill repeats. For most recreational runners, this type of mechanical stimulus is severely underused.
Why keep any Zone 5? As Steve Magness points out, we don't eliminate Z5 because we still need to recruit high-threshold motor units and teach the brain to stay composed in a "metabolic storm." We just no longer use it as our primary workhorse.
3. Critical Speed: The Athlete’s Practical Dashboard
While Lactate (mmol/L) is the internal signal telling us if the engine is overheating, Critical Speed (CS) is your external dashboard. It is the mathematical boundary between "sustainable" and "unsustainable" effort.
How to Find Your CS (Testing Best Practices)
Since we aren't using daily blood tests, we calculate CS using a three-point mathematical model. This requires three max-effort time trials on fresh legs:
1000m (All-out max effort).
1500m (All-out max effort).
3000m (All-out max effort).
By using three distances instead of two, we create a more robust "line of best fit." This statistical redundancy helps filter out daily variations (bad sleep, heat, or pacing errors on a single test) and provides a much truer metabolic map of your engine.
All tests must be performed on separate days, fully recovered, or the calculation will underestimate your true Critical Speed. The relationship between these three data points defines your CS; roughly the pace you could race for 45–60 minutes. For more details on testing and critical speed see this guide.
CS Training Rules
The "Sub-CS" Volume Play (Lactate 2.0–3.5): Most of our work is done at 95–98% of CS. This allows for massive quality volume because you never "tap too far into" your anaerobic reserves or spike your lactate into the red.
The "Over-CS" Exposure (Lactate > 4.0): When we work at 102–105% of CS, we are intentionally draining your "Anaerobic Tank" (D'). This is used sparingly to build race-specific toughness.
4. The Performance Spectrum: A 10km Analysis
The 30-Minute Runner (Sub-Elite / Elite)
Pace: 3:00/km
The Nuance: These runners have a narrow gap between CS and VO2 Max. Precision is mandatory.
Verdict: They would use CS (and probably lactate testing) to ensure morning sessions stay strictly at ~2.5 mmol/L, protecting their legs for the afternoon session.
The 40-Minute Runner (Competitive Amateur)
Pace: 4:00/km
The Nuance: "Life Stress" is the biggest variable. A Z5 "soul-crusher" can spike cortisol levels in a way that derails an entire week.
Verdict: Shifting to CS-based intervals (96-98% CS / ~3.0 mmol/L) allows them to stack "quality minutes" without the cortisol spikes of traditional VO2 max repeats.
The 50-Minute Runner (Recreational / Club)
Pace: 5:00/km
The Nuance: Simplicity is enough for the heart, but precision may be needed for safe peripheral adaptations (tendons, connective tissue).
Verdict: Using CS ensures they don't accidentally "race" their workouts. Training near CS (~3.0-3.5 mmol/L) is mechanically safer and builds the structural foundation needed to reach the 40-minute mark.
5. Understanding the "6-Minute Max" (The Gordo Byrn Approach)
Coaches like Gordo Byrn prescribe intervals at "the maximum pace you can sustain for 6 minutes." In the CS model, this is a point on the curve above your Critical Speed. We use this strategically for:
Maximum Stimulus: The strongest signal for heart stroke volume.
Mental Callusing: Teaching the brain to handle the chaos above the CS boundary.
The Efficiency Wedge: If your 6-minute max pace is 3:30/km, then your 10km race pace (near CS) of 4:00/km feels mechanically effortless.
In other words, the 6-minute max becomes a spice; not the base of the meal.
Summary: The New Training Hierarchy
Feature | Traditional (Simple) | Emerging (Precise / CS-Based) |
Philosophy | "No Pain, No Gain" | "Maximum Signal, Minimum Stress" |
Primary Work | Zone 5 (VO2 Max) | Zones 3–4 (95-100% of CS) |
External Metric | Prescribed Race Paces | Critical Speed (3-Point Test) |
Internal Signal | High Acidity (>6.0 mmol) | Metabolic Stability (2.0–4.0 mmol) |
Top-End Speed | 800m–1200m reps | 20s–30s "Free Speed" Bursts |
Fatigue Cost | High and visible | Lower but cumulative |
Final Words
Since you’re a regular reader of this newsletter, I know you’re fascinated by the nuances of training physiology, and rightly so! But it’s important not to let that complexity cloud your actual approach. As Steve Magness puts it with almost haiku-like simplicity: 'Mostly easy, occasionally hard, vary it up, and very seldom, go see God.' Let that be your north star next time you lace up.
We aren't asking you to work less hard; we are asking you to be more intentional. Critical Speed gives us the line in the sand, while Lactate explains why that line matters. Stay just below it to build the engine; step above it to sharpen the sword. Stop chasing exhaustion; start chasing repeatability.
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