WHOOP Strain vs Other Wearables: Comparing Load
WHOOP strain is the most talked-about training-load number in wearables, and the most commonly misread one — usually by being treated as a universal unit that other brands merely copy with a different name. They do not. This page sets WHOOP's approach beside what Fitbit, Garmin, Apple, Oura and Polar actually publish about their own load metrics, and is explicit about which comparisons are meaningful and which are not.
We earn nothing from this page. There are no affiliate links, no sponsorship and no paid placement, and we have no commercial relationship with any vendor named here. How we evaluate what we publish is set out in our editorial standards.
The short answer
WHOOP is unusual in accumulating strain continuously across the entire day on a 0–21 logarithmic scale, combining cardiovascular load from time in heart-rate zones with accelerometer-derived muscular load from logged strength sessions. The other major platforms measure something categorically different: Garmin estimates EPOC from heart rate, Polar computes Banister TRIMP, Apple multiplies a 1–10 effort rating by duration, and Google's Fitbit Cardio Load is proprietary and calculated from first-party data only. None of these numbers is interchangeable with a WHOOP strain score, and none of the vendors publishes enough of its formula to make them so.
How WHOOP actually calculates strain
WHOOP Strain is defined by WHOOP as a measure of total cardiovascular and muscular exertion across a workout or a whole day, reported on a scale from 0 to 21. The scale is anchored at the mathematical limit: 21 represents operating at your maximum heart rate for a full 24 hours, which is why WHOOP states that nobody ever reaches it. The scale is inspired by Borg's Rating of Perceived Exertion, and the displayed curve is logarithmic — climbing from 10 to 11 costs less effort than climbing from 20 to 21.
Three properties of the calculation matter more than the number:
- It is a whole-day accumulator, not a per-workout metric. WHOOP counts every heart-rate elevation over the day — workouts, commuting, walking, illness, stress — because the stated philosophy is that fatigue does not care whether the elevated heart rate came from a session or a hard day at work. Nothing else on this page works that way.
- It is personalised. WHOOP computes strain from your own heart-rate metrics and maximum heart rate, so two people completing the same workout receive different scores. A 90-minute hike that registers 10 or 11 for an average person may register 5 or 6 for a highly conditioned athlete. That is the design intent, not an inconsistency.
- HRV is not an input. WHOOP has stated plainly that HRV feeds Recovery and not Strain, on the grounds that HRV fluctuates too much within a day to be a stable load signal; it is measured overnight instead. Strain responds to heart rate; recovery responds to HRV, resting heart rate, sleep and respiratory rate.
Strength training is the interesting case. WHOOP describes muscular load as measured from movement using the accelerometer and gyroscope, combined with what it describes as biomechanics and mathematics, capturing both volume and intensity; that muscular load is then combined with cardiovascular load to produce a single strain score. In practice, this is why a heavy lifting session produces strain on WHOOP in a way it does not on a heart-rate-zone-only platform. WHOOP also notes a consequence of the display that trips people up: because the day number is shown on a logarithmic scale, the strain from individual workouts does not add arithmetically to your day strain.
What WHOOP does not publish is the formula itself. The 0–21 scale, the zone boundaries, the personalisation approach and the two input families are documented; the weighting and the exact curve are not. That is normal among consumer wearables, and it is the single biggest reason comparisons across brands stop being meaningful.
The comparison table
| Platform | Load metric | Basis | Window | Strength-session handling |
|---|---|---|---|---|
| WHOOP | Strain, 0–21 logarithmic | Time in heart-rate zones + accelerometer-derived muscular load | Continuous, whole day, plus per workout | Muscular load from logged Strength Trainer sessions folded into the same number |
| Fitbit / Google Health | Cardio Load | Proprietary; Google documents it as calculated from first-party data only | Weekly | No announced strain or strength-load metric on Fitbit Air |
| Garmin | Training Load (acute + chronic) | Estimated EPOC summed with decaying weights | Rolling 7-day acute load alongside a longer chronic window | Strength sessions are recorded; the load estimate is heart-rate driven |
| Apple Watch | Training Load | Effort rating (1–10) × duration | Last 7 days compared with the previous 28 | Effort is rated, auto-estimated from average heart rate, and user-adjustable |
| Polar | Training Load Pro: Cardio, Muscle, Perceived | Banister TRIMP per second; power output for muscle load; user RPE | Per session, plus Strain (7-day) vs Tolerance (28-day) | Muscle load requires power measurement, so lifting is not covered by it |
| Oura Ring | None published | — | — | Oura publishes readiness, sleep and activity scores, not a load metric |
How each wearable handles load
WHOOP: a continuous daily accumulator
WHOOP's strain is designed so that the day is the unit. The practical consequence is that a rest day after a hard session can still show meaningful strain from ordinary life, and a hard session on a moving day can push you toward strain that two hard workouts would have produced on another day. For athletes who periodise by daily readiness, that is the intended behaviour: the number answers "how much did today cost my body", not "how hard was my workout". The trade-off is that strain becomes hard to interpret if you are not also watching recovery, which is why WHOOP pairs the two.
Fitbit and Google Health: Cardio Load without a published formula
Google Health carries a Cardio Load metric, and Google's own support documentation states that Google Health currently uses first-party data only to calculate your sleep score and Cardio Load. Two things follow. First, connecting a Garmin or an Oura into Google Health will not improve your Cardio Load — it populates history, not the headline metric. Second, the formula is unpublished, so there is no way to audit it, reproduce it, or compare it against WHOOP's on method.
Fitbit Air, at launch, had no announced strain or training-load metric at all — the spec row in our Fitbit Air vs WHOOP comparison reads "not announced" for exactly this reason. What the platform does expose is active zone minutes: time in heart-rate zones, which is a legitimate input for computing your own load score, and is the same quantity WHOOP's cardiovascular load is built from in spirit. If you want a strain-style number from Fitbit data, the defensible route is the published-methods one in our guide to building a load score from the Google Health API, not reverse-engineering Google's.
Garmin: EPOC, and a load gauge rather than a load number
Garmin's Training Load is built on excess post-exercise oxygen consumption. Garmin documents that training-load data is based on mathematical modelling of heart rate to plot the accumulation of a physiological measure called EPOC, and that acute load is a weighted sum of your EPOC values over recent days, with the gauge telling you whether current load is low, optimal, high or very high. The optimal range is individual — it is based on your own fitness level and training history and moves as your training changes.
Garmin also splits the load by intensity distribution with Training Load Focus, which reports how much of your recent training sat in low-aerobic, high-aerobic and anaerobic categories and nudges you toward balance. WHOOP has no direct equivalent of the focus split; its zone bands serve a loosely similar interpretive purpose.
Apple Watch: you rate the effort, the watch does the arithmetic
Apple's Training Load is the most transparent mechanism of the group, and the one most dependent on the user. Apple documents that each workout receives an effort rating on a 1 to 10 scale, that the estimate is produced automatically for most cardio workouts using a model trained on heart rate and workout data, that you can adjust it, and that training load is then the effort multiplied by duration. The result is a comparison of your last 7 days of training against the previous 28, classified from well below to well above.
The documented transparency cuts both ways, and this is worth understanding rather than repeating as marketing: because the effort estimate is driven by average heart rate for the session, a workout whose heart rate oscillates — intervals being the obvious case — produces an average that understates the effort of the hard portions equally with the easy ones. The adjustable rating is Apple's answer to that, and it is a reasonable one, but it means the metric is only as good as your honesty about the number. Independent testing of the feature at launch noted the interval problem explicitly.
Polar: the most documented, including its own limits
Polar is the vendor on this list that publishes the most, so it gets the longest entry. Training Load Pro reports three separate load values: Cardio Load, described as a training impulse (TRIMP) calculated from heart rate and session duration after every workout; Muscle Load, determined from power output and available for sports where power is measured; and Perceived Load, which is your own RPE rating for the session. Short-term load is called Strain and long-term load Tolerance, and the status shown to you is the ratio between them.
Polar's own documentation then states the limitation that WHOOP's design exists to avoid, which is unusually honest: the first main limitation of Banister TRIMP is using heart rate to quantify training load in sports with short bouts of very high intensity, anaerobic work, with strength training named as an example. In other words, the vendor that documents its method most thoroughly also documents why that method is weakest for the exact reader who lifts. Polar's Muscle Load does not rescue this, because it requires power measurement and so is available for running and cycling, not for the squat rack.
Oura Ring: no load metric at all
Oura publishes Readiness, Sleep and Activity scores, and does not publish a strain or training-load metric. The readiness score is a recovery measure built from HRV, resting heart rate, temperature and sleep; it answers whether you are ready, not what you did. Ring form factor and comfort make Oura a strong sleep tracker, but if training load is the number you care about, there is nothing to compare here — and an Oura membership is required for most of the app's features beyond the three daily scores anyway.
Why the numbers are not comparable across platforms
Three separate reasons stack up, and it is worth keeping them distinct.
- Different quantities in different units. A WHOOP strain of 14, a Garmin acute load of 400, an Apple 7-day load 25% above your 28-day baseline, and a Polar cardio load of 90 are four different measurements. There is no exchange rate between them, and anyone implying one is guessing.
- Different windows. WHOOP asks "what did today cost"; Garmin and Polar ask "what did the last week cost relative to the last month"; Apple asks "how does this week compare to my baseline". A number that is high on one timescale is unremarkable on another.
- Neither is validated against a standard. As we established when rebuilding these scores from raw data, no vendor publishes a formula, so there is no reference implementation and no ground truth. Two scores can both be defended and still disagree, because they are answering slightly different questions with different weighting.
On the acute:chronic ratio
Garmin's acute-versus-chronic framing, Polar's Strain-versus-Tolerance ratio and Apple's 7-versus-28-day comparison are all cousins of the acute:chronic workload ratio. As we note in our Strain-score guide, the ratio originates with Foster's monitoring framework but its interpretation as a causal injury predictor has been substantially challenged — most prominently by Gabbett, whose argument is that the apparent relationship is largely explained by fitness already being low when athletes get injured. We report these ratios where vendors expose them and do not present them as validated risk models, because they are not.
Which one should a lifter actually use?
If your training is mostly barbell work, the ranking changes for one specific reason: whether the metric can see your lifting at all.
- WHOOP first, on muscular load. It is the only platform here that folds accelerometer-derived muscular load from logged strength sessions into the same daily number as cardiovascular load. Its ceiling is that the sensor package is an accelerometer, not a force plate — it estimates the work, and it only sees sessions you actually log in Strength Trainer.
- Garmin if you also run, cycle or row. EPOC-based load handles endurance work well and the gauge is genuinely useful across sports, but heart-rate-derived load is weakest exactly where lifting is concerned.
- Polar for transparency, with the caveat documented. If you want to know exactly what your device computes and are prepared to rate sessions yourself, Polar publishes the most; just note that its own whitepaper concedes the strength-training blind spot.
- Apple for control, not for automation. The effort rating being adjustable is a feature for a lifter who trains by feel, and a weakness if you want no-input tracking.
- Fitbit Air not for load. No announced strain metric, no strength logger, and Cardio Load that only trusts first-party data. It is a strong cheap sleep and recovery tracker — see our subscription-free wearables guide — but load is not the job it does.
Whatever you wear, the load metric only tells you what the session cost. Separately tracking whether your lifts are actually progressing is the other half of the picture, and no wearable does it for you — our guide to what WHOOP Strength Trainer records and what it leaves to you covers that gap in detail.
Frequently asked
How does WHOOP handle strain tracking compared with other wearables?
WHOOP accumulates strain continuously across the whole day on a 0–21 logarithmic scale, driven by time spent in heart-rate zones plus accelerometer-derived muscular load from logged strength sessions. Garmin estimates EPOC from heart rate into a weekly acute load, Polar computes Banister TRIMP per session, Apple multiplies a 1–10 effort rating by duration and compares 7 days against 28, and Google's Fitbit Cardio Load is proprietary and calculated from first-party data only. The daily accumulation is the part WHOOP does that the others do not.
Is WHOOP strain the same as training load?
It is one species of it. WHOOP strain is a personalised cardiovascular-load score normalised so 21 means maximum heart rate for 24 hours. Garmin Training Load and Polar Cardio Load are training-load measures computed per workout or per rolling window in different units, so a strain of 14 and a training load of 600 cannot be placed on a shared scale.
Does Fitbit have a strain score like WHOOP?
Not an equivalent one. Google Health carries Cardio Load, calculated from first-party data only and with no published formula. Fitbit Air launched with no announced strain or training-load metric, and the API exposes active zone minutes rather than a strain number. Active zone minutes are a valid input for computing your own load score, but they are not the same product.
Which wearable has the best training load metric for strength training?
WHOOP, on one specific ground: it folds accelerometer-derived muscular load from logged strength sessions into the same daily number as cardiovascular load, and it is the only mainstream platform here that does. Polar documents the opposite finding for heart-rate-based TRIMP, naming strength training as a case it handles poorly.
Can I compare my WHOOP strain with a friend's Garmin load?
No. They are different quantities in different units, on different windows, with different inputs. Even within WHOOP, two people doing the same workout score differently because strain is personalised to your own maximum heart rate and fitness.
Does higher strain mean I should not train tomorrow?
Not by itself. Strain is a load number with no ceiling of its own; the interpretation comes from pairing it against recovery. WHOOP's own framing is that the Strain Target, not the raw strain, is the thing to plan against — and even that guidance depends on the recovery score you woke up with.
Related
- Recreating WHOOP on Google Fitbit — the published methods behind any strain or recovery score, and where a Fitbit reimplementation stops short
- Fitbit Air vs WHOOP — the full comparison, including the strain row where Fitbit Air has no entry
- WHOOP Strength Trainer: log sets, reps and track progress — what the strength logging actually records
- Does WHOOP recovery predict your lift performance? — read this before acting on any load or recovery number
- Best fitness tracker for recovery 2026 — if the buying decision is still open
Sources
- How does WHOOP Strain work? — WHOOP. Source of the 0–21 scale, the four zone bands, cardiovascular and muscular load, and the personalisation statement
- WHOOP Strain: 15 questions on score, recovery and load — WHOOP. Source of the 21-as-24-hours-at-max-HR anchor, the muscular-load sensor description, and the statement that HRV does not factor into strain
- How WHOOP Strain works and what your daily score really means — WHOOP. Source of the logarithmic-display and non-additive-workout-strain points
- Ask the expert: what is training load? — Garmin. Source of the EPOC-based description and 7-day accumulation
- Acute load — fénix 7 owner's manual, Garmin. Source of the weighted-sum-of-EPOC definition and individual optimal range
- Training load focus — fénix 7 owner's manual, Garmin. Source of the low/high aerobic and anaerobic split
- Track your training load on Apple Watch — Apple. Source of the 7-versus-28-day comparison and the classification scale
- Training Load Pro — Polar support. Source of the Cardio/Muscle/Perceived definitions, the TRIMP description and the Strain-versus-Tolerance ratio
- Polar Training Load Pro white paper — Polar (J. Schröderus). Source of the Banister-TRIMP method detail and the documented strength-training limitation
- Connect third-party devices and apps to the Google Health app — Google Health Help Center. Source of the first-party-data-only calculation for Sleep Score and Cardio Load
- Introducing the all-new Fitbit Air — Google, 7 May 2026. Source of the Fitbit Air sensor list and the absence of any announced strain metric
- Oura Membership — Oura Member Care. Source of the three daily scores available without membership
- Borg, G. (1998). Borg's Perceived Exertion and Pain Scales. Human Kinetics — the RPE tradition the WHOOP scale references
- Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology (1996). Heart rate variability: standards of measurement, physiological interpretation and clinical use. Circulation 93(5):1043–1065
- Karvonen, J., Kannel, M. & Salo, J. (1981). Effects of age and body weight on cardiovascular response during exercise. Heart 87(5):441–451
- Edwards, S. (1991). Heart rate reserve. Medicine & Science in Sports & Exercise 23(2 Suppl.):S498–S503
- Banister, E. W., Calvert, T. W., Horner, F. T. & Brooks, R. J. (1991). An integrated model of endurance athlete training. International Journal of Sports Medicine 12(1):5–10
- Foster, C. (1998). Monitoring training in athletes with reference to overtraining syndrome. Medicine & Science in Sports & Exercise 30(9):1164–1168
- Gabbett, T. J. (2016). The training-injury prevention paradox: should athletes be training smarter and harder? British Journal of Sports Medicine 50(5):273–280