understanding zone training
Each heart rate zone represents a different level of effort and intensity, and as you exercise in these zones, your body adapts in specific ways to improve performance.
(Pro tip: if you are new to high-intensity training and don’t know your maximum heart rate, you can estimate it by doing 220 minus your current age. For example, we will use a 35-year-old runner, who should have a max heart rate of ~185 beats per minute. This method isn’t very accurate, but use it if you don’t know your max HR).
Here’s the physiology behind each zone and the adaptations:
Zone 1: Very Light (50-60% of Max HR) (35-year-old: 93-111 bpm)
Physiology: In this zone, your body relies mostly on aerobic energy systems, using oxygen to break down fats for energy.
Adaptations:
Improved blood flow: Blood flow to muscles improves, aiding in recovery and nutrient delivery.
Increased capillary density: Tiny blood vessels (capillaries) grow around muscle fibers, improving the ability to deliver oxygen and nutrients to muscle cells.
Enhanced recovery: Increases circulation and helps clear waste products (like lactic acid) from muscles after more intense workouts.
Fat utilization: Your body becomes better at burning fat as fuel, which is useful for long-duration activities like endurance running.
Zone 2: Light (60-70% of Max HR) (35-year-old: 112-130 bpm)
Physiology: Your body is still using aerobic systems, but now you are tapping into both fat and carbohydrate stores for energy. The intensity is moderate, so the body can keep up with the oxygen demand.
Adaptations:
Increased mitochondrial density: Mitochondria (the "powerhouses" of your cells) increase in number, improving your ability to produce energy from oxygen and nutrients.
Better fat oxidation: The body gets more efficient at using fat for fuel. This helps spare glycogen (carbohydrate stores), which is valuable for longer runs or events.
Aerobic capacity: Your heart becomes more efficient at pumping oxygenated blood throughout your body, increasing your overall endurance.
Improved cardiovascular efficiency: The heart gets stronger and can pump more blood per beat, increasing stroke volume.
Zone 3: Moderate (70-80% of Max HR) (35-year-old: 130-148 bpm)
Physiology: At this intensity, you begin to use a mix of aerobic and anaerobic energy systems. While oxygen is still being used, the demand for energy exceeds what can be provided solely by aerobic metabolism, and lactic acid begins to accumulate in the muscles.
Adaptations:
Increased lactate threshold: The body becomes better at clearing lactic acid and delaying its accumulation, allowing you to maintain a moderate pace for longer periods.
Improved VO2 max: VO2 max is the maximum amount of oxygen your body can use during intense exercise. Training in Zone 3 helps increase this, improving your ability to sustain moderate intensities over time.
Greater cardiovascular output: Your heart becomes even more efficient at delivering oxygenated blood to muscles, improving stamina.
Zone 4: Hard (80-90% of Max HR) (35-year-old: 149-166 bpm)
Physiology: This zone is primarily anaerobic. Your body can no longer rely solely on oxygen to produce energy and switches to using stored glycogen (carbs). Lactic acid begins to accumulate more quickly as the body produces energy without enough oxygen.
Adaptations:
Increased anaerobic capacity: Your muscles improve their ability to generate energy without oxygen (anaerobic), which is useful for sprinting or high-intensity efforts.
Muscular strength and power: Muscle fibers adapt to handle more intense efforts, increasing their ability to exert force. This zone enhances your ability to do fast, powerful movements.
Improved lactate tolerance: You improve your ability to buffer and tolerate the buildup of lactic acid, which means you can sustain harder efforts for longer before fatigue sets in.
Faster recovery: Your body learns to recover quicker between high-intensity efforts by improving the efficiency of clearing out metabolic waste products like lactic acid.
Zone 5: Maximum Effort (90-100% of Max HR) (35-year-old: 167-185 bpm)
Physiology: This zone is all about anaerobic energy production. The intensity is so high that the body cannot supply oxygen fast enough, so it relies almost entirely on glycogen stores and anaerobic processes.
Adaptations:
Increased muscle power: Muscles adapt to produce maximum force over short periods. This is beneficial for sprinting and explosive efforts.
Improved neural recruitment: Your nervous system becomes more efficient at recruiting muscle fibers, especially fast-twitch fibers, which are responsible for explosive movements.
Increased explosive strength: This zone improves your ability to generate power quickly, useful for short bursts of effort like sprints or hills.
Muscle buffering: Your body’s ability to handle the buildup of metabolic waste, such as lactic acid, improves. However, this zone is not sustainable for long because lactic acid will accumulate faster than it can be cleared.
Summary of Adaptations by Zone:
Zone 1 (Recovery) – Improved circulation, fat burning, and recovery.
Zone 2 (Endurance) – Increased mitochondrial density, fat utilization, and cardiovascular efficiency.
Zone 3 (Tempo) – Improved lactate threshold, increased VO2 max, and stamina.
Zone 4 (Anaerobic) – Increased anaerobic capacity, lactate tolerance, and muscle strength.
Zone 5 (Maximum Effort) – Improved power, neural recruitment, and explosive strength.
Training across these different zones helps develop a well-rounded running performance, with endurance, strength, and speed all improving at the same time.
Runners! You should be spending most of your time (about 80% of your time) training in Zone 1/2, and some time (20%) in zone 4/5.