Training guide
Heart rate zones explained
Heart rate zones split your effort into ranges, from easy to maximal. They are drawn from a reference number: maximum heart rate, heart rate reserve, or a threshold found in a test. The same word, zone 3, can mean different heart rates in different tools.
The easiest reference is also the roughest. Age-based formulas estimate maximum heart rate for a group, and individuals vary widely around the average.
How zones are built
Three common ways to set zones are a percentage of maximum heart rate, a percentage of heart rate reserve (maximum minus resting), and a percentage of threshold heart rate. Research on endurance athletes often uses three zones whose boundaries come from the first and second ventilatory thresholds measured in a lab. Consumer apps often use five zones, and some use seven.
In a study of junior cross-country skiers, three zones were defined from lab thresholds. Across 318 endurance sessions, heart rate analysis put about 75% of training time in zone 1, 8% in zone 2 and 17% in zone 3. Session effort ratings gave nearly the same split.
Estimating maximum heart rate
The old rule of 220 minus age was never established in a sample with enough older adults. A meta-analysis of 351 studies and 18,712 subjects found maximum heart rate relates to age as 208 minus 0.7 times age, and a laboratory study of 514 healthy people found nearly the same line, 209 minus 0.7 times age. The relation did not differ between men and women or with habitual activity level.
These are averages for groups. An individual can sit well above or below the line, which is why a measured maximum or a test result is more reliable than a formula for setting zones.
What your AI can read in Memory.fit
Training zones in Memory.fit hold FTP, threshold pace, critical swim speed, threshold heart rate, maximum heart rate and resting heart rate, and you can edit them. When a maximum or resting heart rate is not set, Memory.fit shows the highest recorded heart rate or the 30 day median resting heart rate as a muted suggestion you can accept. It never applies one by itself.
Your AI reads these with get_athlete and writes them with set_thresholds when you ask. Each workout's average heart rate arrives with it, so your AI can see how your sessions were spread across your zones.
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.
“What are my heart rate zones in Memory.fit, which reference numbers are they built on, and when were those set?”
Tools:
get_athleteset_thresholds“Find my highest heart rate in Memory.fit over the last 12 months and check it against my saved maximum.”
Tools:
get_workoutsget_athlete“Split my last 6 weeks of sessions in Memory.fit by my heart rate zones and show the share of time in each.”
Tools:
get_workoutsget_trends“Update my threshold heart rate in Memory.fit to the value from my last test, and keep a note saying where it came from.”
Tools:
set_thresholdssave_note
Questions
In short.
It is a rough group average and was never validated in a sample with enough older adults. A meta-analysis found 208 minus 0.7 times age fits group data better, and individuals still vary widely.
Research often uses three zones from lab thresholds. Apps use five or seven. The count matters less than knowing how your tool sets the boundaries.
The age and maximum heart rate relation did not differ between men and women in the meta-analysis. Individual differences were large.
They may use different reference numbers, such as maximum heart rate, heart rate reserve or threshold, and different percentages.
No. It stores the numbers you or your AI enter and shows suggestions from your data that you can accept.
Sources · Checked November 10, 2026
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