Activity Load Factors

Strain drives real recommendations in your app — how hard to train tomorrow, how much you should sleep tonight, whether you’re building fitness or digging a hole. A number that important shouldn’t be a mystery. This document lays out the reasoning behind our Strain metric and the complete table of scaling factors. You may not agree with every number, and that’s fine — but you’ll always be able to see the choice we made and the logic behind it.

Strain is a single number from 0 to 21 representing how much total load you’ve placed on your body, mapped onto a logarithmic scale so the numbers stay intuitive:

The scale is logarithmic: each additional point is harder to earn than the last. Going from 2 to 4 is easy; going from 16 to 18 takes enormous effort. Strain also accumulates across your whole day, not just workouts — a stressful afternoon or a long day on your feet adds to it, because your body doesn’t distinguish “exercise stress” from “life stress” when it comes time to recover.

Under the hood, Strain is computed from load points, which are then converted onto the 0–21 curve. Everything in this document is about getting those load points right before they hit the curve.

This is the problem we set out to fix.

Heart rate measures how hard your cardiovascular system is working to move oxygen. For steady aerobic exercise — running, cycling, swimming, rowing — that’s almost the whole story. Heart rate and true physiological load rise and fall together. For those activities a heart-rate-based score is genuinely accurate, and we don't change it at all — they sit at a factor of exactly 1.0.

But many of the most demanding things you can do barely register on heart rate, or register far below their real cost:

• Isometric load — gripping, framing, holding position under tension. When a muscle contracts hard and holds, it restricts its own blood flow, so your heart rate rises less than the effort demands. Grappling, climbing, and heavy carries are full of this.
• Eccentric & impact load — the braking, lowering, landing, and collisions that cause micro-damage and next-day soreness. This is mechanical stress on muscle and connective tissue; your heart barely sees it. Think downhill skiing, plyometrics, or a hard tackle.
• Neuromuscular (CNS) load — maximal strength and explosive power are paid for primarily by your nervous system, not your aerobic system. A near-maximal lift can leave you wrecked at a heart rate you’d consider a warm-up.
• Anaerobic bursts — repeated all-out efforts (a scramble, a sprint, a flurry) cost more than your heart rate reflects in the moment, because heart rate lags the true metabolic demand.
• Competitive / sympathetic stress — live competition and combat carry an adrenal and psychological toll with no proportional heart-rate signature.

This isn’t just our opinion — it’s well documented. Session-RPE (a validated measure of an athlete’s whole-body load) tracks closely with heart-rate-based load during easy training (r ≈ 0.70), but the agreement falls apart as intensity rises — down to r ≈ 0.31 for high-intensity work — and it’s worst in intermittent sports, exactly the combat and team sports where heart rate misses the most.^[1] Separately, exercise-physiology standards note that during isometric (static) effort, cardiac output rises less than in dynamic exercise because the contracting muscle limits its own blood flow — so heart rate systematically under-reads static load.^[2]

There’s a metabolic cross-check too. In the Compendium of Physical Activities — the standard reference for the energy cost of activities — competitive boxing is rated at 12.3 METs and grappling / martial arts around 10.3 METs, on par with or above vigorous running.^[3] These are genuinely top-tier demanding activities. Yet because they’re intermittent (maximal bursts separated by brief resets), a continuously averaged heart rate lands them in the mid-range, and a heart-rate-only strain model scores them below an easy run. That gap is what we correct.

For every activity in our library we assign one transparent multiplier — the Activity Load Factor (ALF) — that scales the heart-rate-derived load to account for the toll heart rate can’t see.

• 1.0 is the anchor. It means “heart rate fully captures this activity.” Steady-state running, cycling, swimming, and rowing live here. If you only do steady cardio, your Strain is unchanged.
• Above 1.0 corrects upward for isometric, eccentric, neuromuscular, anaerobic, or competitive load. Jiu-jitsu is 1.70; wrestling is 1.80.
• Below 1.0 corrects downward for activities where heart rate is elevated for reasons that aren’t training load — most notably heat. A sauna raises your heart rate without taxing your muscles, so it shouldn’t bank training strain; it sits at 0.50.

Two design choices are worth stating plainly:

• We scale load, not the score. The factor is applied to the underlying load before the logarithmic conversion to 0–21 — never to the final number. This keeps the diminishing-returns curve intact: the same factor adds a lot of strain at the bottom of the scale and a little at the top, which is how strain should behave.
• Steady cardio is deliberately untouched. The entire spectrum is anchored to the activities where heart rate already works. We are not inflating everything — we are correcting the specific activities where the standard approach is known to be blind.

We didn’t pick these numbers by feel. Each factor is built from six load dimensions — five that heart rate under-counts, and one (heat) that it over-counts. Each activity is rated 0–1 on each dimension, and the ratings combine with fixed weights:

Activity Load Factor =
    1.00
  + 0.20 × Isometric load
  + 0.20 × Eccentric / impact load
  + 0.20 × Neuromuscular (CNS) load
  + 0.20 × Anaerobic-burst load
  + 0.15 × Competitive / sympathetic load
  − 0.50 × Thermal inflation
  (bounded to a 0.4 – 2.0 range)

One subtlety that explains everything: each dimension scores only the load that is decoupled from heart rate — the part HR doesn’t already reflect. That’s why running, despite its real impact, stays at 1.0: a runner’s impact load rises and falls with heart-rate cost, so heart rate already accounts for it. A factor only climbs when an activity loads your body in a channel heart rate can’t see.

Every activity in our library has an explicit factor. Anything we don’t yet recognize defaults to 1.0 (the heart-rate-only behavior), so an unknown activity is never penalized or inflated.

Your day’s Strain isn’t just your workouts — it includes the cardiovascular load of ordinary life. When you log an activity, we identify that block of time, and the load you earned during it is weighted by the activity’s factor before it folds into your day total. The rest of your day is counted normally, at a factor of 1.0.

So a day with an hour of jiu-jitsu carries that session’s true weight, while your commute and your desk hours count as exactly what they are. We don’t multiply your whole day by your hardest activity — only the minutes you actually spent on it.

Transparency means owning the limits, too.

Strain is a living metric, and we version it so changes are legible. Activity-aware Strain (the system described here) is v2.0. Scores recorded before this version reflect our earlier, heart-rate-only method, so you may notice that strenuous-but-low-heart-rate activities are scored more heavily going forward than they were in your older history. As we collect more data — and more feedback from athletes like you — we’ll keep tuning the spectrum and will note material changes here.

If a number looks wrong to you for an activity you know well, tell us. The whole point of publishing this is that the metric should be defensible in the open.