Heart Rate Zone Training: Zones 1-5 Tracking
Heart rate zone training gives structure to what would otherwise be an arbitrary sense of workout intensity. By anchoring effort to measurable physiology, you can target specific adaptations — building aerobic base, raising lactate threshold, or improving VO2 max — rather than simply training at whatever intensity feels hard. Fitiv Pulse tracks time in each zone automatically across every workout, giving you a precise picture of your weekly intensity distribution.
What are Heart Rate Zones?
Heart rate zones are ranges of heart rate, typically expressed as percentages of maximum heart rate, that correspond to distinct physiological states. Each zone elicits different metabolic responses, recruits different energy systems, and produces different adaptations with consistent training.
The five-zone model, which Fitiv uses, is the standard in endurance sports science and is used by organizations including the British Cycling Federation and USA Triathlon:
| Zone | Name | % of Max HR | Primary Fuel | Primary Adaptation | |------|------|-------------|--------------|-------------------| | 1 | Recovery | 50-60% | Fat | Active recovery, blood flow | | 2 | Aerobic Base | 60-70% | Mostly fat | Mitochondrial density, fat oxidation | | 3 | Tempo | 70-80% | Mixed fat/carbohydrate | Lactate clearance, aerobic efficiency | | 4 | Threshold | 80-90% | Mostly carbohydrate | Lactate threshold elevation | | 5 | VO2 Max | 90-100% | Carbohydrate (anaerobic) | Cardiac output, VO2 max |
These thresholds are approximations. Individual variation is significant, and zones based on a percentage of maximum HR are less precise than zones anchored to lactate testing or ventilatory threshold measurement. However, for most athletes training without lab access, HR-based zones provide an effective and actionable framework.
How Heart Rate Zones are Calculated
Fitiv calculates your zone boundaries from your maximum heart rate. The app uses your recorded peak heart rate from past workouts as the primary estimate, updating automatically as new maxima are recorded. You can also enter your maximum HR manually if you know it from a laboratory test or a verified field test.
The most common formula for estimating maximum HR — 220 minus age — has a standard deviation of approximately ±10-12 bpm and is unreliable for individual use. Fitiv treats it as a starting point only and strongly weights observed peak HR from actual workouts as soon as that data becomes available.
For athletes who have undergone lactate testing or know their ventilatory threshold, Fitiv allows manual zone boundary entry to anchor zones to functional thresholds rather than maximum HR percentages.
Zone 2: The Foundation of Aerobic Fitness
Zone 2 training — 60-70% of maximum heart rate — is the most important zone for most athletes and the most chronically underused. At Zone 2 intensity, your body primarily metabolizes fat, which requires abundant mitochondria in slow-twitch muscle fibers. Extended Zone 2 training stimulates mitochondrial biogenesis (the creation of new mitochondria), increases fat oxidation capacity, and improves the efficiency of your aerobic energy system.
The practical consequence is that athletes with a large aerobic base can sustain higher absolute speeds at lower relative effort. Their Zone 3 pace becomes what was once their Zone 4 pace, because the underlying aerobic machinery has improved.
Zone 2 training requires genuine discipline. The correct effort feels almost too easy — conversational pace, low perceived exertion. Most self-coached athletes drift into Zone 3 during intended Zone 2 sessions without realizing it, which reduces the stimulus for the specific adaptations Zone 2 produces.
Zone 3 and Zone 4: Threshold Development
Zone 3 (70-80% max HR) is sometimes called "no man's land" in endurance coaching — it is hard enough to accumulate significant fatigue but not intense enough to produce the strongest lactate threshold adaptations. Heavy Zone 3 training can lead to chronic fatigue without proportional fitness gains.
Zone 4 (80-90% max HR) corresponds roughly to lactate threshold — the highest sustained effort at which lactate production and clearance are balanced. Training at and slightly above lactate threshold raises the threshold ceiling, allowing you to race faster at aerobic effort. Classic threshold workouts — 20-40 minute tempo efforts, Lactate Threshold intervals — target this zone.
Zone 5: VO2 Max Development
Zone 5 (90-100% max HR) represents maximum aerobic effort. At these intensities, the cardiovascular system is working at or near its maximum oxygen delivery capacity. Intervals in Zone 5 — typically 3-8 minutes in duration with equal or longer recovery — produce improvements in maximal cardiac output and VO2 max.
Zone 5 work is highly stressful and requires adequate recovery. Most serious endurance athletes perform Zone 5 work 1-2 times per week at most, with the balance of training in Zones 1-2.
How Fitiv Tracks Heart Rate Zones
Fitiv records beat-to-beat heart rate throughout every workout via Apple Watch or a connected Bluetooth HR monitor, then calculates continuous zone assignment for each second of the workout. Post-workout, you see exact time spent in each zone, displayed as both raw time and percentage of total workout duration.
This granularity matters because interval workouts involve frequent zone transitions. A single "Zone 4 interval session" typically includes substantial time in Zone 3 (during incomplete recovery between intervals) and brief excursions into Zone 5 (at the start of each interval). Seeing the actual zone distribution tells you whether the session executed as intended.
Fitiv also aggregates zone time across the week, allowing you to track your weekly intensity distribution. Sports science research consistently supports a polarized distribution for endurance athletes: approximately 80% of training time in Zones 1-2, and 20% in Zones 4-5, with minimal time in Zone 3. Fitiv's weekly zone summary makes this distribution visible and actionable.
Why Heart Rate Zone Training Matters for Athletes
Zone-based training is not merely theoretical — it is the foundation of how elite endurance athletes structure training across all sports. The physiological rationale for zone training is well-established: specific intensities produce specific adaptations, and unfocused training that blends zones produces blended (suboptimal) adaptations.
For recreational athletes, zone tracking primarily serves as a check against the most common training error: training at a moderate intensity that is too hard to produce aerobic base adaptations and too easy to produce threshold or VO2 max adaptations. Real-time zone feedback during workouts and post-workout zone analysis help athletes identify and correct this pattern.
Frequently Asked Questions
Q: Should I use a chest strap or Apple Watch for zone training? A: For steady-state activities like running and cycling, Apple Watch optical HR is generally adequate for zone tracking. For high-intensity interval training with rapid HR transitions, chest straps (Polar H10, Wahoo TICKR) respond faster and produce more accurate zone time data. The electrical detection used by chest straps has a shorter lag time than optical PPG, which matters when you're doing 30-second intervals.
Q: How do I know if my max HR is accurate in Fitiv? A: Check your max HR setting in the app against the highest HR you've ever recorded in any workout. If you've worn a monitor during a truly maximum effort — an all-out sprint, a race finish, or a max HR test — that number is likely more accurate than any formula. Fitiv updates your max HR automatically when a new peak is detected during a workout.
Q: What percentage of my training should be in Zone 2? A: Research on elite endurance athletes across multiple sports consistently shows approximately 75-80% of total training time at Zone 1-2 intensity. This polarized approach, combined with 15-20% at Zone 4-5 intensity, produces better long-term adaptation than time-matched moderate-intensity training. Most recreational athletes train too much in Zone 3 and not enough in Zones 1-2 or 4-5.
Q: Can I use heart rate zones for strength training? A: HR zones are less meaningful for strength training because heart rate during lifting reflects effort but not metabolic load in the same way as continuous aerobic exercise. Fitiv tracks HR during strength sessions but does not calculate zone-based training load for lifting — it uses volume load (sets × reps × weight) instead, which is a more appropriate training load metric for resistance training.