Training guide
What is zone 2 training?
Zone 2 is easy, steady aerobic effort. In most five-zone systems it is the second zone, a pace you could hold for a long time while still speaking in full sentences. Different tools draw the zone in different places, so the word does not mean one fixed heart rate.
In research, the same idea is often described by lactate: low-intensity work is training done at or below about 2 millimoles per liter of blood lactate. Elite endurance athletes spend most of their sessions there (see the sources below).
What zone 2 means, and why the number depends on the tool
Heart rate zones are ranges of effort. Some systems use five zones, some use three. In a three-zone model built on two physiological thresholds, zone 1 sits below the first threshold, zone 2 between the thresholds, and zone 3 above the second. In a five-zone model, zone 2 is the second step up from rest. Both models are called zone 2, and they are not the same range.
That is why a zone 2 heart rate from a watch, a calculator and a lab test can disagree. A percentage of maximum heart rate is an estimate. A lab test with lactate or breathing gas is a measurement. Maximum heart rate itself is often estimated from age; a large analysis of 18,712 subjects found the average relation is 208 minus 0.7 times age, with wide spread between individuals.
- Five-zone systems: zone 2 is usually the second easiest zone, often a share of maximum heart rate, threshold heart rate or power.
- Three-zone research models: zone boundaries come from ventilatory or lactate thresholds.
- Your own watch or app picks its own boundaries, so check which method it uses.
What the research says about easy training
A review of training in nationally and internationally competitive endurance athletes found a common pattern: about 80% of sessions at low intensity, and about 20% dominated by hard work such as intervals. A study of junior cross-country skiers found 71% of sessions at or below 2 millimoles per liter of lactate.
A separate laboratory study compared professional endurance athletes with less fit people. It found fat burning during exercise was higher in the professionals and blood lactate was lower, and the two were inversely related. The authors present lactate and fat oxidation as an indirect way to assess metabolic flexibility. That paper is about measurement, not a training prescription.
What your AI can read in Memory.fit
Memory.fit stores your heart rate zones in Training zones: threshold heart rate, maximum heart rate and resting heart rate, plus FTP, threshold pace and critical swim speed. Your AI reads them and can write them with set_thresholds when you ask. Each session's average heart rate comes with the workout, so your AI can count how much of a week was easy.
Memory.fit does not decide your zones or tell you how much easy training to do. Your AI uses your data to answer that question with you.
In your AI
The AI you already use, with your data in Memory.fit.
- ChatGPTChatGPT plans your week, fuels your long sessions and writes it all back to Memory.fit.Set up ChatGPT
- ClaudeClaude writes your plans back to Memory.fit, and asks you before it replaces or removes something you have.Set up Claude
Grok BotGrok Bot plans and reviews from your real training, and can re-plan a week after a bad night's sleep.Set up Grok Bot
MuseMuse plans sessions from your real training and writes them into Memory.fit's Plan.Set up Muse
DotsOpenAI says a dot's background research uses your connected apps read only, so plans are written when you ask in the chat.Set up Dots- GeminiType @ in a Gemini chat and pick Memory.fit to ask about your training. Gemini asks you to allow each session it plans, and planning a whole week is not reliable yet.Set up Gemini
- GrokGrok reads your training, sleep and fuel, and can plan around them.Set up Grok
- Perplexity ComputerPerplexity Computer reads your training in Memory.fit. Whether it writes a plan back isn't checked yet.Set up Perplexity Computer
- Claude CodeClaude Code reads your training and writes the sessions it plans back to Memory.fit, asking before each change unless you allowed it.Set up Claude Code
CueYour Cue agents plan your week and write your fuel plans from your real training, sleep and recovery.Set up Cue
What to send
Copy one, send it to your AI.
“Look at my last 8 weeks in Memory.fit. How much of my training time was at easy heart rates, and how did I define easy?”
Tools:
get_workoutsget_trends“Check my training zones in Memory.fit and tell me which heart rate numbers they are based on and when they were last set.”
Tools:
get_athleteset_thresholds“Plan my next 4 weeks in Memory.fit with most sessions easy, and put them on my watch.”
Tools:
set_training_plansend_session_to_devices“Compare my heart rate to pace or power on my easy runs and rides in Memory.fit over the last 3 months.”
Tools:
get_workoutsget_session_compliance
Questions
In short.
It depends on the method. Five-zone systems often set it as a share of maximum or threshold heart rate. Research models use lactate or ventilatory thresholds. Check which method your watch or app uses.
Not exactly. Research often describes low-intensity training as at or below about 2 millimoles per liter, and that is close to the top of the easy zone in a three-zone model. Five-zone systems vary.
You can estimate it from maximum or threshold heart rate, power or pace, but an estimate is not a measurement. A lab test with lactate or breathing gas measures it.
No. Memory.fit stores the thresholds you or your AI enter and the heart rate data from your workouts. Your AI helps you read them.
Sources · Checked November 10, 2026
- About 80% of sessions at low intensity (2 mM lactate or below) in competitive endurance athletes
- Three zones from ventilatory thresholds; 71% of sessions at or below 2 mM lactate in junior skiers
- Fat oxidation and blood lactate inversely related in athletes and less-fit people
- Maximum heart rate averages 208 minus 0.7 times age (18,712 subjects)
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