How Scoring Works

Why generic predictions fall short

Most race-time calculators and fitness trackers predict your times with a single, fixed formula applied identically to every athlete — the same population-average exponent whether you're a sprinter-built 5K runner or an ultra-endurance specialist, and the same "easy pace" credit whether your heart rate that day was textbook-controlled or drifting toward your max. That's a reasonable default with zero data on you — but it stops being the best available estimate the moment real evidence about your own physiology exists.

Split Index starts from the same published, standard formulas everyone else does — Riegel for race projections, DOTS/IPF GL for strength, ACWR for training load — but personalizes every one of them to your own logged history as soon as there's enough evidence to trust it, and is explicit about exactly when it's still falling back to a population default versus using your own data. The rest of this page is that methodology in full, not marketing copy — every constant and threshold named below is the real one the engine runs.

Why easy runs are scored differently

A race, tempo, or interval session is meant to test your absolute pace — so it's scored against a fixed pace-vs-benchmark table. An easy, recovery, or long session is designed to be run slow and controlled, at a deliberately low heart rate. Judging both on the same absolute scale means a well-executed easy run — one that's doing exactly what it's supposed to do — reads as a mediocre score. Split Index scores these session types differently: on how appropriate your effort was, not on raw pace.

Your personalized heart-rate zones

If you provide your resting heart rate and max heart rate(in onboarding or Settings), your easy/recovery/long sessions in running, rowing, swimming, cycling, and SkiErg are scored against zones built entirely from your own physiology — not a population average. Walking is excluded, since it's typically done at low, non-zone-driven intensity.

The zones are built like this:

• Base = your max HR − your resting HR. This is your aerobic floor.
• Each zone spans 20% of your resting HR, stacked upward from base to max HR — five zones in total.
• Target = base + 30% of your resting HR. This sits in the lower half of zone 2 — a textbook, well-paced easy effort.

Worked example — max HR 207, resting HR 50:

• Base = 207 − 50 = 157
• Zone 1: 157–167 · Zone 2: 167–177 · Zone 3: 177–187 · Zone 4: 187–197 · Zone 5: 197–207
• Target = 157 + (0.30 × 50) = 172

Credit and penalty

Landing right at your target heart rate means you've executed a genuinely good easy effort — so it earns a solid credit floor on its own, not a neutral, unrewarded score. From there, your average heart rate for the session shifts that credit further up or down:

• At target — a flat credit floor, earned just for executing a well-paced, well-controlled easy effort.
• Below target — the lower your average HR (down to base), the more credit is added on top of that floor, up to +10%more. A lower heart rate at the same pace and distance means you're working less hard to produce the same result — genuine efficiency.
• Above target — the higher your average HR (up to max HR), the more that floor is eroded, by up to −10%. Drifting toward your max HR on a session tagged "easy" usually means overexertion, or the session wasn't really easy.
• Both effects clip at the base and max-HR boundaries — going even lower or higher doesn't change the adjustment further.

Elevation, temperature, distance, and pace still play a (smaller) role separately — a hilly or hot session still earns its own modest credit, on top of the heart-rate-zone adjustment above.

Guarding against noisy readings

Heart-rate monitors aren't perfect, and different activities can read differently for the same effort — rowing, for example, often reads lower than running at the same felt exertion. If your average HR reads at or below your base value, Split Index cross-checks it against your own typical easy-effort pace for that sport before granting the full bonus. If your pace doesn't back up an unusually easy effort, the credit is scaled back instead of applied in full — a single low reading can't award maximum credit on its own.

Without heart-rate data

If a session has an average HR reading but you haven't set up personalized zones yet, it's scored against your own recent easy-effort history instead — still a real, evidence-based signal, just less precise than your personalized zones.

If there's no heart-rate data at all for a session — no reading logged, and no history to compare against — Split Index assumes you executed it right at your target heart-rate zone (a well-paced, well-controlled easy effort) and scores it accordingly. This is a guess, not a measurement, so it's clearly flagged on the session, and it may not be accurate for that specific run.

TRIMP

TRIMP (Training Impulse) is a single number blending how long a session lasted and how hard your heart was working throughout it — a rough measure of a workout's total training "cost," rather than a score of how well it went. A short, easy jog and a much longer one at the same intensity produce very different TRIMP values even though both might score similarly on effort quality.

As a rough guide: under 50 reads as light, 50–100 as moderate, 100–150 as hard, and above 150 as very hard. A 45-minute easy run might land around 60–80 (moderate); a hard interval session can push past 150. It uses the published Banister TRIMP formula (1991), weighted for your sex and heart-rate reserve.

Efficiency factor

Efficiency factor is how much speed (or power, for erg/bike sessions) you're getting per heartbeat — a rough proxy for aerobic fitness at a given effort. It has no fixed "good" or "bad" number: Split Index doesn't compare your efficiency factor against anyone else's, only against your own history. Trending upward over weeks at a similar effort means your aerobic fitness is genuinely improving; trending downward can be a sign of fatigue, illness, or detraining.

Erg/bike sessions show this as watts per beat-per-minute; pace-based sessions (running, rowing, swimming without a power meter) show it as meters-per-minute per beat-per-minute — the two aren't on the same scale, so don't compare a power-based reading against a pace-based one.

Decoupling

Decoupling compares your heart rate in the first half of a session against the second half, at a similar pace or power. A small amount of upward drift (your heart rate creeping higher for the same output) is normal, especially in heat or on longer efforts. A larger drift is a sign of fading aerobic durability or accumulated fatigue — even on a session that felt evenly paced throughout.

Lactate threshold and VO2max

Neither of these is a lab measurement — no consumer wearable measures actual blood lactate or gas exchange, Garmin included. Both are estimated from your own logged training the same way every mainstream platform does it.

Lactate thresholduses your own sessions explicitly tagged "Threshold" or "Tempo" — sustained efforts of 15 to 70 minutes. Races are deliberately excluded: a 5K sits near your VO2max effort, a marathon sits well below threshold, and mixing the two in would corrupt the estimate. The average heart rate and pace across your most recent (up to 3) qualifying sessions becomes the reading — more sessions means higher confidence, not a different number.

VO2maxuses Jack Daniels and Jimmy Gilbert's published VDOT formula, applied to your most recent logged race, or your predicted 5K time if you haven't logged a race recently. VDOT corrects for how long an effort was sustained, which is why it's calculated from a genuine hard effort rather than an easy run — feeding it an easy pace would read as a much lower fitness level than you actually have.

Your two 1RMs: all-time best and current

"My one-rep max" means two different things depending on when you ask, so you get both rather than one number pretending to be both.

All-time best is the heaviest single rep your logged sets have ever implied. It is a high-water mark: a bad session, a deload or six months off cannot lower it. Only beating it moves it.

Current 1RM is what your recent training says you could do today. Each session counts by how recently it happened, with a three-week half-life — a session from six weeks ago carries about a quarter of the weight of one logged today. So it rises when you train well and falls when you train worse, which is exactly what an all-time best must never do. Three weeks is our estimate rather than a measured constant: it sits inside the two-to-four weeks after which detraining studies start to see measurable strength loss, and roughly matches the length of a training block.

Each session is read by its best set, so warm-ups and back-off sets never make a good day look like a bad one. Neither number is stored — both are recalculated from your logged sets, so correcting a mis-typed session corrects them too.

Strength Index: DOTS and IPF GL

A raw total (squat + bench + deadlift) rewards bodyweight above almost everything else — a heavier lifter and a lighter lifter can move very different loads for the same underlying strength. DOTS and IPF GL are published, sport-standard formulas that adjust your total against your own bodyweight, so it can be compared fairly against lifters of any size, or tracked meaningfully over time as your bodyweight changes.

They're two independent formulas on two different scales — a DOTS score and a GL score for the same lift are not directly comparable to each other. Track each on its own axis over time rather than treating one as a conversion of the other.

Accessory and isolation lifts (anything outside squat/bench/deadlift) aren't part of either formula — they're scored instead against ExRx bodyweight-ratio tiers, a separate published standard for judging relative strength on lifts DOTS/GL don't cover.

Race predictions

Predictions across distances use Riegel's formula — T2 = T1 × (D2/D1)^k — the standard, published approach for translating a known performance at one distance into a projected time at another. The exponent k represents how much your pace naturally falls off as distance increases; a generic population value (roughly 1.06) is the reasonable default for an athlete with no history yet.

Once you've logged enough races and hard efforts across a couple of different distances, Split Index fits kto your own pace curve instead — somewhere in a realistic 1.03–1.10 range — so the ladder reflects how your own endurance actually degrades with distance, not a stranger's average. A real race result at or near a benchmark distance is trusted close to fully; an inferred projection from a different distance or session type is blended in more cautiously, so one unusual session can't swing your prediction on its own.

Predictions stay in a "calibrating" state until there's enough evidence logged to be worth showing — a single session isn't enough signal to project from confidently.

Injury risk (ACWR)

The Injury Risk Index is built on the Acute:Chronic Workload Ratio— your rolling 7-day training load divided by your rolling 28-day average weekly load. It's widely used in sports science as a proxy for whether recent training has spiked well above what your body has adapted to, which is when soft-tissue injury risk rises.

• Below 0.8 — Undertraining: recent load is well under your own baseline.
• 0.8–1.3 — Optimal: the well-supported sweet spot for building fitness without spiking risk.
• 1.3–1.5 — Caution: load is climbing meaningfully above your baseline.
• Above 1.5 — Danger: a spike large enough that the research literature links it to materially higher injury risk.

This needs a real baseline to mean anything — with less than ~2 weeks of consistent logging, the 28-day window is mostly empty and the ratio collapses toward a fixed, meaningless extreme rather than reflecting genuine overreaching, so it's hidden until there's enough history behind it. If you log a morning HRV reading, it nudges the index further — a suppressed HRV relative to your own baseline raises the index a little beyond what load alone would say, since incomplete recovery is itself a risk signal independent of training volume.

This is a training-load accountability tool, not a medical diagnosis — it reflects relative risk against your own history, not an absolute probability of injury.