Cycling Training Zones Explained: What Your Body Actually Does at Each Intensity

Training zones are models we use to group different intensities under the same characteristics. They're useful for understanding what happens in your body at different efforts. But they're not real. They're artificial constructs we've created to simplify the prescription of training.

Your body doesn't switch gears at a threshold. Physiology is a continuum where changes happen progressively as intensity or fatigue increases. The boundaries between zones aren't clear-cut lines; they're fuzzy transitions. At most, we could distinguish three domains: moderate (below the first threshold), heavy (between thresholds), and severe (above FTP). But even these thresholds aren't as clear as we think. They're dynamic, and they behave more like transition zones than the thresholds themselves.

The Two Thresholds That Actually Matter

Before diving into the six zones, understand that the only real physiological boundaries in your body are two thresholds. Everything else is a gradient.

First threshold (aerobic)

The first threshold is the intensity where you start producing more lactate than you can metabolize inside the muscle cell, and blood lactate levels rise slightly above resting values. It also marks the point of maximum fat oxidation. Above it, carbohydrate use increases and fat use decreases.

Practically, this threshold separates the low, cruising rhythms you can maintain for several hours from a medium effort where fatigue accelerates significantly. You can sustain it for 2 to 3 hours. It typically sits at 80-90% of FTP.

In terms of training zones, the first threshold marks the transition between Zone 2 and Zone 3.

Second threshold (anaerobic)

The second threshold is where aerobic metabolism alone can no longer sustain the activity, and anaerobic energy production kicks in. Lactate production rises exponentially. Fats become a minority fuel source: most energy now comes from oxidizing carbohydrates in the mitochondria.

Above the second threshold, oxygen consumption and heart rate don't stabilize. If you continue long enough, you'll reach maximum oxygen consumption and exhaustion.

You can sustain the second threshold for approximately 40-60 minutes maximum. In training zones, it marks the transition between Zone 3 and Zone 4.

The 6 Training Zones

There are countless zone models out there: 3 zones, 5 zones, 7 zones. In NUA, we work with 6 training zones based on the physiological changes that occur in the body as intensity increases. Here's what's actually happening in each one.

Zone 1 — Recovery

How it feels: You can ride like this almost indefinitely without generating fatigue. It's the pace where your body barely notices it's exercising.

What's happening: A small amount of lactate is produced, but it's immediately reused in the mitochondria and never reaches the bloodstream. Energy comes almost entirely from fat oxidation.

When you ride here: Warm-ups, cool-downs, recovery between intervals. Rest days can be swapped for Zone 1 rides as long as you stay under 2 hours and truly stay in this zone. If you have more training time than your plan prescribes, you can add Zone 1 minutes without generating additional fatigue.

Zone 2 — Endurance

How it feels: Easy to moderate. A perceived exertion of 4-6 out of 10. You can hold a conversation. The effort is sustainable but you're generating a solid aerobic stimulus.

What's happening: This is the intensity with the best adaptation-to-fatigue ratio, which is why professional cyclists spend most of their training time here, along with Zone 1. Lactate levels are similar to rest: whatever lactate is produced gets oxidized inside the muscle cell and doesn't enter the bloodstream. You're running on slow-twitch fibers and burning primarily fat. This is also where maximum fat oxidation occurs.

When you ride here: Long endurance rides, base-building sessions. This is where you spend the majority of your training volume. The foundation of all endurance fitness.

Zone 3 — Tempo (Between Thresholds)

How it feels: The perception of exertion is high: approximately 6-7 out of 10 when you start, climbing to 8-9 as you accumulate time at this intensity. It's a rhythm that feels moderate at first and hard at the end, but one you can sustain. You can talk but you don't want to. It takes concentration to hold the pace, and your mind is constrained by the effort.

What's happening: Lactate production increases above resting levels due to greater glucose use as a fuel source. This also brings greater activation of fast-twitch fibers, which demand more oxygen the longer you spend at this intensity.

When you ride here: This is the typical intensity of long mountain passes in gran fondos, breakaways in road racing. The bread and butter of racing at sub-elite level.

Zone 4 — Threshold to VO2max

How it feels: Very demanding. This is the intensity that hurts the most to train. RPE is at least 8-9 out of 10 at the start of intervals, reaching 9-10 at the end. You can speak in monosyllables. You're gasping. You can barely think about anything other than the effort.

What's happening: This is where things get physiologically interesting. Heart rate decouples from power: if you hold power steady, heart rate drifts upward throughout the interval. If you hold heart rate steady, power drops. This cardiac drift means heart rate is unreliable for guiding Zone 4 efforts, so use it with extreme caution above this threshold.

The primary fuel is carbohydrates, so carb-loading in the 24 hours before a Zone 4 session is important.

When you ride here: Short climbs in races, time trials, XCO, cyclocross. Trained with intervals because they maximize time at high oxygen consumption and blood lactate levels.

Important: Zone 4 efforts require adequate recovery and preparation. Don't attempt them fatigued.

Zone 5 — Anaerobic Capacity

How it feels: Maximum or near-maximum. The effort is very hard, 9-10 on a scale of 1 to 10. You can't think about anything other than the effort. It enslaves your body and your thoughts. The sensation is like tasting blood: you're at that limit where you believe you're going all out but you can somehow hold on a few more seconds.

What's happening: Despite the name "anaerobic capacity," this zone isn't purely anaerobic. The aerobic system is activated to its maximum to meet energy demands, and also during the recovery phase of intervals to replenish myoglobin and phosphocreatine stores. Anaerobic energy complements aerobic energy to generate efforts this intense and close to maximum.

This zone generates significant neuromuscular fatigue due to the extreme intensities, so it's trained sporadically.

When you ride here: Short hills, punchy climbs, and rollers under 3 minutes and over 30 seconds.

Zone 6 — Sprint / Maximum Power

How it feels: This doesn't feel aerobically demanding; your heart rate barely rises. But muscularly, it's extremely taxing. Maximum concentration. You're not regulating anything. It's all-out from the first pedal stroke to the last.

What's happening: This is where we work maximum force and power. Zone 6 starts at approximately 70% of your peak power output. This is much more like a gym session than a cycling session. The energy comes from phosphocreatine and fast glycolysis, not aerobic metabolism.

Standing or seated? Standing improves sprint power and works the whole body. Seated improves threshold changes of pace and produces a pedaling pattern closer to normal riding, but with lower watts.

When you ride here: Sprints and attacks. Improvements in this zone depend much more on quality than quantity. It's not about doing many sprints; it's about doing them as intensely as possible. Respect the recovery time between sprints to avoid losing power.

Heart Rate vs Power: Which Should Guide Your Training?

Both are useful, but they measure different things.

Power measures the mechanical work your body produces. It's immediate and objective. Heart rate measures the energetic cost of producing that work. It lags behind and is influenced by many external factors.

Heart rate lag

Heart rate takes 1 to 5 minutes to couple with the power you're producing. At higher efforts it adjusts faster; at lower efforts the delay is longer. As a rule of thumb, use 3 minutes as a reference. This means that if you want to do an interval guided by heart rate, you should gradually increase your pulse over the first 3 minutes to reach the target. Don't try to hit it immediately.

Factors that affect heart rate

• Temperature is the biggest factor. Heat raises heart rate because your body sends blood to extremities for thermoregulation, plus likely dehydration reduces plasma volume. On hot days, increase your target heart rate by 5-10 beats. Cold days, decrease by 5-10 beats.
• Caffeine and hydration can elevate heart rate significantly.
• Fatigue and illness can push heart rate unusually high.
• "My heart rate won't go up." This can be a symptom of fatigue, mental stress, or cold conditions. Also note that when you're fitter, your maximum heart rate is typically 5-10 beats lower than when detrained.
• Fitness level. The same wattage will produce different heart rates on different days based on fatigue, training status, freshness, and temperature. Heart rate and power are correlated but not always in the same way, and not linearly.

When to use which

Quick recovery as a signal

If your heart rate drops quickly after an interval, that's a great sign. It indicates cardiovascular adaptation and fast oxygen debt recovery. If it takes a long time to come down, it may indicate detraining, accumulated fatigue, or illness.

The Three Energy Systems Behind Every Interval

Zones tell you how hard an effort feels. But what actually determines how NUA designs your intervals is which energy systems are working and in what proportion. Your body has three ways to produce energy, and they all activate simultaneously. What changes is the balance between them.

Phosphocreatine (PCr): Your explosive system. Provides immediate, maximum power but depletes in about 10 seconds. Think sprint efforts. PCr recovers quickly (half-life of about 28 seconds), which is why NUA prescribes specific rest periods between sprints: too short and you can't reproduce the power; too long and you lose the training stimulus.

Glycolytic: Your fast energy system. Breaks down glucose rapidly to produce energy, generating lactate as a byproduct. Peaks during efforts of 10-30 seconds and remains significant up to about 2 minutes. This is what burns during hard interval work.

Oxidative (aerobic): Your sustainable system. Uses oxygen to burn carbohydrates and fats. Becomes the dominant energy source beyond about 75 seconds of effort, and provides over 90% of the energy for anything longer than 5 minutes.

Here's how the balance shifts with effort duration at maximum intensity:

NUA uses this data to design every interval session. When you see a workout with 6x30-second sprints with 3-minute recovery, NUA isn't picking numbers arbitrarily. It's calibrating the recovery duration so your phosphocreatine recovers enough to reproduce the target power, while accumulating just enough fatigue to drive adaptation.

For longer intervals (3-5 minutes at zone 4-5), the oxidative system dominates. NUA adjusts the interval duration and recovery to maximize the time you spend at high oxygen consumption, which is the stimulus that improves your VO2max.

This is also why repeated short intervals with short recovery (like 30/30s) feel harder than a single effort of the same total duration. After several reps, your VO2 stays elevated between intervals, shifting even more load onto the oxidative system. NUA accounts for this accumulation effect when prescribing these sessions.

Why NUA Doesn't Use Zones

Our training engine doesn't use training zones to prescribe workouts, because physiology is a continuum. There's nothing magical in the transition from one zone to another. For example, the effort at the top of Zone 2 is more similar to the bottom of Zone 3 than it is to the bottom of Zone 2. Zones are artificial constructs that we use to categorize intensities under the same labels, but they're not physiological.

At most, we could distinguish three domains: the moderate domain (below the first threshold), the heavy domain (between thresholds), and the severe domain (above FTP). But even these thresholds aren't as clear as we think. On one hand, they're dynamic; on the other, they're more like transition zones than thresholds themselves.

NUA uses percentages and percentiles to establish training intensity. When NUA prescribes a workout at 88% of FTP, that's a precise target, not a vague "Zone 3" that could mean anything from 76% to 90%.

This also means NUA auto-detects changes in your thresholds from your ride data and instantly updates the targets of all upcoming sessions. No need to manually reconfigure zones across apps and devices.

This isn't something you need to memorize. NUA handles the prescription for you. But understanding the scale helps you interpret why certain sessions feel harder than others, even when they're technically in the "same zone."