The research behind it
Fitstrology is playful on the outside and careful on the inside. Here's what the numbers are made of, and what they're not.
1. Heart rate variability (HRV) and readiness
HRV is the tiny variation in time between heartbeats. When you're well recovered your nervous system is relaxed and HRV tends to be higher; after a hard week, poor sleep, illness or alcohol it usually drops. Coaches have used morning HRV to decide when to push and when to back off, and athletes who trained by their daily HRV did as well as, or better than, those on a fixed plan.
Fitstrology uses your overnight HRV from Apple Health as one input to Charge, compared with your own recent average.
Kiviniemi AM, Hautala AJ, Kinnunen H, Tulppo MP. Endurance training guided individually by daily heart rate variability measurements. European Journal of Applied Physiology, 2007.
Shaffer F, Ginsberg JP. An overview of heart rate variability metrics and norms. Frontiers in Public Health, 2017.
2. Resting heart rate
A lower resting heart rate generally goes with better cardio fitness, and large studies link a higher resting heart rate with higher long-term risk. Day to day, a resting heart rate that's a few beats above your usual can mean you're under-recovered, stressed or coming down with something.
It feeds both Charge (today vs your normal) and Body Age (your weekly level).
3. Sleep: how long, and how regular
Everyone knows hours of sleep matter. Less well known: regularity (going to bed and waking at similar times) matters too. In a large UK Biobank study, people with more regular sleep had lower mortality risk, and regularity was a stronger predictor than duration.
Sleep feeds Charge, and your bedtime pattern decides your Body Sign's Rhythm: Early Lark, Night Owl, Steady Bear or Wildcard Moon.
Phillips AJK, Clerx WM, O'Brien CS, et al. Irregular sleep/wake patterns are associated with poorer academic performance and delayed circadian and sleep/wake timing. Scientific Reports, 2017.
4. Cardio fitness (VO₂max) and "fitness age"
VO₂max, how much oxygen your body can use at full effort, is one of the strongest single markers of long-term health. Apple Watch estimates it as "Cardio Fitness" during outdoor walks and runs. Reference data from tens of thousands of lab tests (the FRIEND registry) show what's typical for each age and sex, which lets you say "your fitness is typical of someone X years old".
That's the backbone of Body Age, alongside resting heart rate, sleep, strength training and body composition. Each driver adds or takes off years, and the app shows you which ones.
Mandsager K, Harb S, Cremer P, et al. Association of cardiorespiratory fitness with long-term mortality among adults undergoing exercise treadmill testing. JAMA Network Open, 2018.
Nes BM, Vatten LJ, Nauman J, Janszky I, Wisløff U. A simple nonexercise model of cardiorespiratory fitness predicts long-term mortality. Medicine & Science in Sports & Exercise, 2014.
5. Why "your normal" beats the average
HRV in particular varies hugely between people: one person's great day is another's terrible one. That's why Fitstrology compares you with your own rolling 28-day baseline rather than a population chart. It needs about a week to learn you, and gets sharper after that.
6. Training: little and often, progressively
Coach's pick spreads work across muscle groups through the week and uses double progression: hit the top of the rep range on every set and the weight goes up next time; otherwise keep it and add a rep. It never guesses a first weight. You choose it, and it learns from what you log. Rate an exercise Hard and the weight holds; rate it Hurt and it's left out for two weeks.
And the star signs?
Pure fun. Your sun sign adds a playful line to the forecast and never changes a score, a workout or a safety message. We'd never pretend otherwise.