Many endurance athletes spend their training weeks oscillating between Z2 and Z3 without ever really committing to one. Easy sessions become a bit too fast, hard sessions lose their freshness, and after a few weeks fatigue sets in. Progress slows down — sometimes it even comes to a complete stop. We’ll show you how to put Z2 back at the center to build your engine, and how to use Z3 with intention, without letting it become your default zone.
What exactly is Z2?
Z2 refers to a low, sustainable intensity that is primarily aerobic. It’s the ideal zone for accumulating volume while protecting recovery. In running, it’s often called aerobic base or easy endurance. In cycling, endurance. In swimming, a controlled aerobic pace that allows you to maintain good technique.
The simplest reference point is the talk test. In Z2, you can speak in full sentences. Breathing is noticeable but controlled. The effort feels steady, and you finish the session with the sense that you could have continued.
Why is Z2 so important?
Z2 is where you build your engine. The more you progress, the more speed or power you can produce at a lower physiological cost. That’s exactly what makes the difference in endurance sports: being efficient, lasting longer, and stacking training weeks without burning out.
Concretely, regular Z2 training promotes several highly valuable adaptations:
- Improved cardiorespiratory efficiency. At the same pace or power, the effort “costs” you less.
- Development of the aerobic metabolism. You use oxygen more efficiently and can sustain effort over time.
- Better fat utilization at submaximal intensities, helping to preserve carbohydrates during long sessions.
- Increased muscular endurance capacity (including higher mitochondrial density) and better tolerance to training volume.
- Improved capillarization. Oxygen delivery to the muscles becomes more efficient, reducing the cost of movement.
- More robust recovery. You handle heavy training weeks better and arrive fresher for key sessions.
This is not just a trend. Reviews of endurance training research show that successful athletes spend a large proportion of their training time at low intensity, because it is an effective and sustainable long-term approach (Seiler, 2010).
What is Z3 really for?
Z3 corresponds to a moderate intensity. Too high to truly feel “easy,” but not high enough to count as a genuine high-intensity session. It’s often found around tempo pace. In the moment, it can feel very rewarding: you’re moving well and feel like you’ve done a solid workout. And that’s exactly why it can become a trap if you spend too much time there.
That said, Z3 absolutely has its place. It’s useful when you want to develop your ability to sustain a strong effort close to race pace, or when you’re working on rhythm and pace stability. The key is that Z3 remains a tool used with intention—not your default intensity.
The grey zone and the Z3 plateau
The grey zone is when you almost never train truly easy, but also rarely train really hard. Over the weeks, fatigue accumulates without fully triggering the adaptations that come either from high volume at low intensity or from well-targeted high intensity. Eventually, progress stalls.
This pattern is well known in endurance sports. In trained athletes, intensity distributions that emphasize more volume clearly below threshold have been associated with greater performance gains than approaches focused more heavily on near-threshold intensities (Esteve-Lanao et al., 2007).
Other studies suggest that a more structured distribution—with lots of “easy” and a smaller proportion of “very hard”—can lead to better improvements than training that is overly centered on tempo or threshold work (Stöggl & Sperlich, 2014).
How to tell if you’re slipping into the grey zone
You don’t need a lab to notice it. The signs are usually very concrete: your “easy” sessions already require focus to hold the pace. You can only speak in short phrases. During sessions that should be relaxed, you see a clear heart-rate drift. And when it’s time for quality workouts, your legs already feel heavy. The most frustrating part is the feeling of training a lot without breaking through to the next level.
If you recognize yourself here, the solution isn’t necessarily to “train harder.” More often, it’s about restoring clarity in your intensities so that every session does exactly the job it’s meant to do.
How to break the plateau and restart your progress
Start by protecting your easy days. A true Z2 is a Z2 that is chosen and accepted. You keep the intensity steady, stay relaxed, and accumulate time. It’s this consistency, week after week, that builds a strong engine.
Next, give Z3 a clear role. If you use it, do so intentionally: a well-defined tempo block placed at the right moment in the week, rather than Z3 creeping into every session out of habit. Z3 is effective when it’s dosed. It becomes costly very quickly when it turns automatic.
Finally, leave room for your true quality sessions. They require freshness to create a meaningful adaptation signal. And that’s exactly what Z2 helps you achieve—by getting you to those sessions ready, rather than already fatigued.
2PEAK keeps your zones well calibrated
Many issues come down to one very concrete point: your zones are not set in stone. They evolve with your fitness, your fatigue, your progress, and sometimes even with the season. If they’re poorly calibrated, you can drift into Z3 without realizing it—or hold yourself back by thinking you’re in Z2 when the intensity is actually too low.
That’s where 2PEAK becomes a real advantage. 2PEAK uses a five-zone model and provides tools to calculate and regularly update your zones. You can also activate automatic zone determination, both for running and cycling, to keep your intensities aligned with your actual fitness level at the right time.
When your zones are accurate, everything becomes simpler. Z2 returns to being a solid foundation. Z3 becomes a precise tool. And your key sessions land exactly where they should— with better-controlled load and more coherent recovery.