QT2 Training Zones – Run

Zone Key

The chart above maps running intensity across four primary training zones, defined by your relationship to:

  • Critical Speed (CS)
  • Running Critical Power (rCP)
  • Threshold Heart Rate (TH HR), and
  • Rate of Perceived Exertion (RPE).

Each zone targets a distinct energy system and muscle fiber type, producing specific physiological adaptations when trained consistently.

ZR — Recovery

Targets: Below ~75% CS / rCP  |  ~80% TH HR  |  RPE < 5

The recovery zone sits below the aerobic threshold and primarily recruits Type I (slow-twitch) muscle fibers. Efforts here are fueled almost entirely by fat oxidation and require minimal cardiovascular stress. ZR runs accelerate recovery between hard sessions by promoting circulation and clearing metabolic byproducts without adding meaningful training load. If you feel like you could hold a full conversation and go for an extended period of time, you're in ZR.

Z1 — Aerobic Base

Targets: ~75–85% CS / rCP  |  ~80–86% TH HR  |  RPE 5–6

Z1 is the foundation of endurance fitness. Still dominated by Type I fibers, Z1 builds mitochondrial density, fat-burning efficiency, and aerobic capacity over time. The vast majority of your annual training stress should accumulate here. Intensity feels comfortable and controlled - you can speak in full sentences but are working with purpose. This zone is where aerobic base is built, not borrowed.

Z2 — OGC (Oxidative–Glycolytic Crossover)

Targets: ~85–95% CP / rCP |  ~86–93% TH HR  |  RPE 6-8

Z2 straddles the transition between pure aerobic and mixed-fuel metabolism. This is your tempo zone. At this intensity, Type IIA fibers (fast-twitch oxidative) begin to be recruited alongside Type I fibers. The body shifts from predominantly fat oxidation toward an increasing reliance on glycogen. The LT1 (first lactate threshold) sits at the lower boundary of this zone - pushing above it means lactate begins to accumulate faster than it clears. Z2 work sharpens your ability to sustain higher outputs while staying aerobically efficient.

Z3 — Threshold

Targets: ~95–100% CS / rCP |  ~93–100% TH HR  |  RPE 8–9

Z3 is defined by efforts at or approaching your Critical Speed - the highest intensity you can sustain in a quasi-steady state before glycolytic demand overwhelms aerobic supply. Type IIA glycolytic fibers are heavily recruited here. Lactate production and clearance reach a precarious balance in this zone, and time at this intensity is necessarily limited. Threshold work directly improves Critical Speed and raises the ceiling on every zone below it. 

D’ — Anaerobic Reserve (Above CS)

Targets: >100% CS / rCP |  >100% TH HR  |  RPE 9+

D’ (pronounced “D-prime”) represents your finite anaerobic work capacity - the total amount of work you can perform above Critical Speed before exhaustion forces output below CS. Type IIX (fast-twitch glycolytic) fibers dominate at these intensities. D’ is depleted rapidly during surges and short all-out efforts, and is replenished only when intensity drops back below Critical Speed. Training above Critical Speed develops neuromuscular power and increases the size of your D’ reservoir, making you more resistant to cracking during high-intensity race situations.

Note

  • Train mostly in Z1-Z2. 80–90% of weekly training stress should sit at or below the OGC to build durable aerobic capacity without accumulating excess fatigue.
  • Anchor zones to your current Critical Speed. Zones shift as fitness improves - re-test regularly to keep your training targets accurate.