The research behind your number.

Your Strength Index starts with the heaviest set you can actually do and estimates the single rep you could grind out from it. We use the Epley formula, 1RM = weight × (1 + reps ÷ 30), across your squat, bench, deadlift and overhead press, for sets of one to twelve reps. Epley began life as a gym wall chart, not a study, but every validation since has found it holds: predicted and measured 1RMs correlate above r = 0.95, and the error stays near ±5% when reps are low.^1,2 Past six reps the estimate drifts high, which is why we cap the input.

We then divide that best lift by your bodyweight and grade the ratio against sex-specific standards, from untrained to elite. Relative strength is the honest comparison: a 140 kg deadlift means something different at 60 kg than at 100 kg, and the normative ranges we grade against are drawn from large competition datasets, including one analysis of more than 800,000 entries.^3,4,5

Our design choice. Dividing straight by bodyweight (ratio scaling) is simple and slightly favours lighter lifters compared with allometric scaling, which corrects for the fact that strength doesn’t rise linearly with size. We chose ratio scaling for legibility, and we mention the trade-off rather than hide it. The Epley formula is a validated heuristic, not a law.

The Engine Index is your VO2max, the peak rate your body can use oxygen, graded against age and sex norms. Of everything we measure, this is the construct with the deepest evidence base. The American Heart Association calls cardiorespiratory fitness a clinical vital sign; higher VO2max tracks strongly with lower mortality, and the relationship is graded, with no plateau where more fitness stops helping.^6,7,8 The age and sex reference ranges we grade against come from established fitness registries.⁹

Our design choice. We don’t put you in a lab. hybridAF estimates VO2max from your running (pace, heart rate and duration), which is a field estimate, not a direct gas-exchange measurement. The science linking VO2max to health is strong; the number we show is a well-established approximation of it, and it improves as we see more of your training.

Fitness Age answers a simpler question than VO2max: your aerobic fitness is typical of someone how old? It’s the same measurement translated into a number people feel. The idea comes from the HUNT Fitness Study in Norway, which mapped VO2max onto age across tens of thousands of people, and it’s the basis for NTNU’s widely used fitness calculator.^10,11 We grade it using the same age and sex mortality data behind the Engine Index.⁷

Our design choice. Fitness Age is a communication device, not a new measurement. The original HUNT model was non-exercise (built from questionnaire data); ours adapts the same idea from your running. Treat it as a motivating translation of your VO2max, not a second, independent reading.

The Hybrid Score fuses the two pillars into one number. Both are graded 0–100 on the same percentile ladder, and we combine them with a harmonic mean rather than an average. That choice matters: a harmonic mean is dragged toward the smaller of the two. Be a strong lifter with no engine, or a runner who can’t squat, and the score punishes the gap. You only score well by being good at both.

The reasoning is grounded. Muscular strength independently predicts mortality, and so does cardiorespiratory fitness, and the people who are high in both carry the lowest risk of all.^12,13,14,15 A score that rewards balance rather than a single spike reflects that finding. The ingredients are evidence-based.

Our design choice, stated plainly. No published instrument validates a harmonic mean of strength × engine as a health or performance metric. We built it because it captures something real, that lopsided athletes are worse off than balanced ones, but the composite is our engineering, not a clinical tool. The pillars are validated; the way we blend them is a considered opinion.

Your Recovery score (0–100) reads how ready your nervous system is, mostly from heart rate variability, specifically rMSSD, the beat-to-beat measure that best reflects parasympathetic (rest-and-recover) activity. The signal that matters isn’t your HRV against the population; it’s your HRV against your own rolling baseline. A value that’s low for you is the alarm. This rolling-baseline approach is the method the research endorses.^16,17 Resting heart rate adds a weaker secondary read, and sleep rounds it out.^18,19

Our design choice. We weight the three inputs 45% HRV, 30% resting heart rate, 25% sleep. Those weights are a heuristic: a sensible ordering of signal strength, not a blend anyone has validated. The parts are strongly evidenced; the exact recipe is our prior, and we tune it as we learn from real athlete data.

Strain compresses a day of internal load into a single number from 0 to 21, built from heart rate and active energy. The principle underneath it is old and solid: load is intensity multiplied by duration, and the cost of effort rises non-linearly: the last hard interval taxes you far more than the first.^20,21,22That’s why the scale is logarithmic rather than linear: it stretches out the top end where the real damage accumulates.

Our design choice. The 0–21 range itself is a presentation choice, in the style athletes already recognise. And any strain derived from heart rate inherits a known blind spot: it under-weights short, sharp, intermittent work (think heavy lifting or sprint intervals) where the cost outruns what heart rate alone reports. Read Strain as a strong directional signal, not a precise fuel gauge.

Readiness turns your recovery into a decision: green, yellow or red. When it’s low, the app moves your hard sessions off that day and protects it for easier work. This is where the numbers do something. Training that adapts to daily readiness, HRV-guided rather than fixed on a calendar, performs at least as well as a rigid plan, and it produces fewer non-responders and fewer athletes who stall or regress.^23,24,25,26

The honest read. The gains here are about individualisation, not raw ceiling. Several studies find the effect on peak aerobic capacity is small or not statistically significant. The win is that more people improve and fewer break down, not that the best-case number goes higher.²⁷ Most of this research is in endurance athletes, so we apply it to strength work with appropriate caution.

Almost every number here is a percentile. A raw lift or a raw VO2max means little on its own, so we grade it against normative data for your age and sex and tell you where you land: untrained to elite, or 0 to 100. That’s what makes two very different athletes comparable on one screen.

The AI coach reasons from the same evidence base rather than improvising. It reads your scores, your recovery and your calendar, and reasons over a curated knowledge base of the research summarised on this page, so its advice stays anchored to what’s measured and what’s published.