A 50 minute Tuesday run with 10 minutes in Zone 2, 30 minutes in Zone 3 and 10 minutes in Zone 4 scores a raw load of exactly 150 in Titan. That load becomes 7.8 Exertion points, and with 8,000 steps across the day the Exertion ring reads 8.5 out of 10. For someone who averages a daily load of 100, the same run lifts Titan's short-term training load by about 7 percent and its long-term load by about 1 percent. On the WHOOP strap worn on the other wrist, the same morning becomes a single number on a 0 to 21 scale. Apple Watch files the run into a 7 day window that it compares against the previous 28 days, and TrainingPeaks would score it in TSS.
These are four readings of one heart rate trace, and none of them converts into another by a simple multiplier. This article explains how each one is built, where they share ancestry, and the one structural difference that decides whether you can treat them as interchangeable. The last sections cover what a WHOOP owner gets inside Titan today, which is less than many people expect, and how to wear WHOOP with an Apple Watch without counting the same workout twice.
01How WHOOP builds a strain score
WHOOP's developer documentation defines Strain as "a measurement of the amount of stress on your body" on a scale from 0 to 21, and says the scale is "based on the Borg Scale of Perceived Exertion" (WHOOP, WHOOP 101). Borg's scales of rating of perceived exertion are answered by the athlete. WHOOP's number comes from the strap, so the Borg reference describes how the scale should feel more than what goes into it.
Three properties follow from WHOOP's own description.
The scale compresses at the top. WHOOP states that "it takes more stress to move from a score of 16 to 17 than 4 to 5." Strain behaves like a logarithm of the work done. A second hard session late in the day adds fewer points than the first, and the gap between 18 and 19 represents far more stress than the gap between 8 and 9.
Strain accumulates all day. WHOOP scores Strain "continuously for members throughout the day and every workout." A walk to the train and an evening run feed the same dial. Each workout also carries its own Strain score.
It resets with sleep. WHOOP assigns data to a Physiological Cycle that runs from sleep to sleep, "rather than calendar days" (WHOOP, Cycle data model). Every cycle starts the dial again from zero.
WHOOP groups the scale into four bands on the same developer page.
| WHOOP band | Strain | WHOOP's description |
|---|---|---|
| Light | 0 to 9 | Room for active recovery with minimal stress |
| Moderate | 10 to 13 | Moderate stress that helps maintain fitness |
| High | 14 to 17 | Increased stress that helps build fitness |
| All Out | 18 to 21 | Significant stress that may be hard to recover from |
WHOOP does not publish the formula. The same page says Strain "is also specific to your baseline," so two people doing identical work will see different numbers, and the same workout on consecutive days will score differently as your body changes. That makes Strain a relative measure, scaled to you, whose coefficients only WHOOP can see. There is also no universal good strain. WHOOP pairs Strain with its morning Recovery score and says a higher Recovery means your body is "more primed" to take on Strain that day, so the right number depends on the morning.
02The TRIMP model behind all of these numbers
Every system in this article descends from one idea. In 1975 Eric Banister and colleagues proposed that a training session could be reduced to a single training impulse, or TRIMP, computed from duration and heart rate, and that performance could be modeled as the difference between a fitness response and a fatigue response to those impulses. The companion paper with the full systems model is Calvert and colleagues (1976, IEEE Transactions on Systems, Man, and Cybernetics). Banister weighted each minute by where heart rate sat between rest and maximum, with an exponential factor so that hard minutes counted for much more than easy ones.
Later variants kept the structure and simplified the weighting. A widely used one replaces the exponential curve with time in five heart rate zones, each minute multiplied by its zone number, and it appears alongside Banister's original and session RPE in reviews of load quantification (Borresen and Lambert, 2009, Sports Medicine). Carl Foster's session RPE method multiplied a perceived effort rating by session minutes (Foster, 1998, Medicine & Science in Sports & Exercise). Foster also gave the word strain a specific meaning years before WHOOP used it. In his paper strain is weekly load multiplied by monotony, the daily mean load divided by its standard deviation, and among his 25 athletes a high share of illnesses followed individual thresholds that were mostly thresholds of strain.
Andrew Coggan and Hunter Allen took the same lineage into TrainingPeaks, where TSS replaced heart rate with relative intensity and duration. TrainingPeaks credits Banister's heart rate TRIMP directly, and its Performance Manager keeps Banister's fitness and fatigue split, with the gain factors removed and exponentially weighted averages in their place (TrainingPeaks, The Science of the Performance Manager). The full history, including the long argument over acute to chronic ratios, is in how Titan measures training load.
The research consensus on what these numbers can do is sober. The 2016 Doha consensus on athlete load monitoring separates internal load, the physiological response such as heart rate, from external load, the work done such as distance or power (Bourdon and colleagues, 2017, International Journal of Sports Physiology and Performance). WHOOP and Titan both measure internal load. Impellizzeri and colleagues (2019, International Journal of Sports Physiology and Performance) describe the psychophysiological response to exercise, the internal load, as what mediates adaptation, which makes heart rate a sound base. Halson (2014, Sports Medicine) found that no single marker has strong enough evidence to stand alone, and Borresen and Lambert (2009) judged the accuracy of the fitness and fatigue performance models poor, possibly because the models lack a measure of how each athlete responds. A systematic review of 56 studies found that athletes' own ratings of well-being tracked acute and chronic load more sensitively than objective measures did (Saw and colleagues, 2016, British Journal of Sports Medicine). Any load number, WHOOP's or Titan's, is a record of heart rate work. How you feel is a second, independent check.
03How Titan computes training load
Titan documents every weight and constant in its training load model, and it uses the zone-summation form of TRIMP.
Each day Titan takes your workout minutes in each heart rate zone and multiplies them by the zone number. A Zone 1 minute counts 1, a Zone 5 minute counts 5. Only heart rate recorded inside a workout counts. The Tuesday run above scores 20 from Zone 2, 90 from Zone 3 and 40 from Zone 4, a daily load of 150. The zones come from your max heart rate, which Titan takes as the higher of your highest Apple Health reading in the last 30 days and 220 minus your age, unless you set a custom max. By default Zone 1 starts at 53 percent of max and Zone 5 at 93 percent (heart rate zones).
A workout with no heart rate still counts. Titan multiplies its minutes by an activity factor, 1.0 for endurance, 0.95 for mixed intensity, 0.9 for strength, 0.65 for mobility, 0.95 for other activities with distance, 0.8 for other activities without distance and 0.5 for an Other workout renamed as sauna or steam. A 60 minute strength session with no heart rate earns a load of 54.
Titan then keeps two exponentially weighted averages of daily load. Short-term load uses a 7 day time constant and moves about 13 percent of the way toward each new day's load. Long-term load uses a 42 day time constant and moves about 2.4 percent per day. Short-term load stands for fatigue and long-term load for fitness. Those are the same default constants TrainingPeaks uses for acute and chronic training load, and Titan's balance, long-term minus short-term, is the same subtraction TrainingPeaks calls training stress balance.
The load ratio divides short-term by long-term load and falls into four zones.
| Titan zone | Load ratio | Titan's guidance |
|---|---|---|
| Low | Below 0.8 | Raise load gradually if you feel healthy |
| Optimal | 0.8 up to 1.4 | The two loads are balanced, build load here |
| High | 1.4 up to 1.6 | Add volume gradually and put recovery first |
| Risk | 1.6 and above | Taper to get back into the optimal range |
04How Titan turns load into a daily Exertion score
The same daily load drives the Exertion score, Titan's closest analogue to WHOOP Strain. Exertion runs from 0 to 10, accumulates through the day and resets the next day.
Workout points follow a curve that flattens as it rises, 10 × (1 − e^(−load / 100)). A load of 50 earns 3.9 points, 100 earns 6.3, 150 earns 7.8 and 300 earns 9.5. Like WHOOP, the scale compresses at the top, and moving from 8 to 9 takes a long, hard session. Unlike WHOOP, you can compute every value by hand.
Steps add up to 1.2 points. Below 700 steps they add nothing, the contribution rises in a straight line to 12,000 steps, and 8,000 steps adds about 0.78. The Tuesday total is 7.77 plus 0.78, which rounds to 8.5. A day with no workout still floors at 1.0 if you took 700 or more steps or burned 90 kcal or more of active energy, which is why a rest day shows a small number on both apps.
| Titan Exertion band | Score |
|---|---|
| Low | Under 2.5 |
| Moderate | 2.5 to under 5.5 |
| High | 5.5 to under 7.5 |
| Very High | 7.5 and above |
Two differences from Strain matter in practice. Titan counts heart rate only during recorded workouts, so a tense afternoon adds nothing to Exertion, while WHOOP scores the whole day. Titan also uses fixed zone weights against your max heart rate, while WHOOP scales Strain to a personal baseline it does not disclose.
05How Titan sets an Exertion target
WHOOP asks you to read Strain against Recovery. Titan does that arithmetic for you and shows a range on the Exertion screen (Exertion target).
The range starts from your training goal. Peak Training is 7.0 to 9.0, Building 5.5 to 7.5, Maintaining 4.0 to 6.0, Tapering 2.5 to 4.5, and Injured or Resting 0.0 to 2.0. Each is 2.0 points wide. Recovery then slides the range 0.02 points for every point away from a Recovery score of 60, capped at 0.8 in either direction. If your average Exertion over the last three scored days sits more than 0.5 above the base range, the target drops another 0.35, and if it sits more than 0.5 below, the target rises 0.35. The target never leaves 0 to 9.8. When your Exertion is inside the adjusted range and Recovery is 65 or higher, a stretch target opens and lifts the top to 9.5.
On the Tuesday in question, a Building goal with Recovery at 70 moves the range up 0.2, to 5.7 to 7.7. The run's 8.5 lands Above Target. A WHOOP owner would reach a similar conclusion by comparing the Strain dial to the Recovery color. Titan writes the comparison down as numbers.
06How Apple Watch, TrainingPeaks and Garmin frame load
Apple Watch
Apple's training load "compares the intensity and duration of your workouts over the last 7 days to what you've done over the previous 28 days" and reports the result "on a scale from well below to well above" (Apple, Track your training load). You can edit the effort rating on a workout. Apple does not publish a formula or a raw number. The structure is still the acute versus chronic comparison Titan uses, with a 28 day chronic window and categories in place of a ratio.
TrainingPeaks
TrainingPeaks defines 100 TSS as "an all out, 100%, 60-minute workout," and a workout can never earn more than 100 TSS per hour (TrainingPeaks, What is TSS). Daily TSS feeds acute training load with a default 7 day time constant and chronic training load with a default 42 day constant, both exponentially weighted averages. Of the systems here, TrainingPeaks and Titan are the closest relatives. They differ in the per-session score, TSS against zone-weighted minutes, and share the averaging.
Garmin
Garmin describes training load as a measure based on excess post-exercise oxygen consumption (EPOC), predicted from heart rate data as an activity progresses using physiological modeling and machine learning (Garmin, Training Load). This article does not quote Garmin's thresholds or window lengths, so check the current manual for your watch before comparing them with Titan's.
07A daily total and a rolling average answer different questions
This is the difference that decides whether the numbers are interchangeable. WHOOP Strain and Titan Exertion are both daily totals. Titan training load, Apple's training load and TrainingPeaks' acute and chronic loads are rolling averages that compare a recent window with a longer one. The two kinds of number answer different questions, and the first kind cannot answer the second question at all.
| System | Headline number | Built from | Window | Resets or rolls | Formula published |
|---|---|---|---|---|---|
| WHOOP Strain | 0 to 21, compressed at top | All-day stress on the body, scaled to you | One Physiological Cycle | Resets each cycle | No |
| Titan Exertion | 0 to 10, compressed at top | Zone-weighted workout minutes plus steps | One day | Resets daily | Yes |
| Titan training load | Two loads and a ratio | Zone-weighted workout minutes | 7 day and 42 day EWMA | Rolls forward | Yes |
| Apple Watch training load | Well below to well above | Workout intensity and duration, editable effort | Last 7 days vs previous 28 | Rolls forward | No |
| TrainingPeaks | TSS, ATL, CTL, TSB | Relative intensity and duration | 7 day and 42 day EWMA defaults | Rolls forward | Definitions only |
A worked case shows the gap. Say you have trained at a steady daily load of 100 for months, about 50 minutes in Zone 2 a day, and then do three days at 300, each 100 minutes in Zone 3. Titan's load ratio reads 1.21 after day one, 1.37 after day two and 1.49 after day three. The first two days sit in Optimal. The third crosses into High. On a daily scale, all three days look alike. Each earns 9.5 workout points in Titan, and on WHOOP each would be a separate high reading judged against that morning's Recovery. Nothing on either daily dial shows that day three follows two others like it.
WHOOP does carry some history elsewhere. Its API reports a sleep need component labeled "need from recent strain" (WHOOP, API reference), and its Recovery score responds to accumulated fatigue through HRV and resting heart rate. That history lives inside other scores. Strain itself starts from zero every cycle, and WHOOP offers no published counterpart to a 7 day against 42 day ratio.
The reverse limit also holds. A single day at 300 moves Titan's ratio from 1.00 to only 1.21, because the averages are built to smooth single days. If you want to know whether today was hard, read the daily number. If you want to know whether this week is out of proportion to the last six, read the rolling one. Pushing the ratio up into High on purpose during a planned block is a normal part of overreaching, and dropping it into Low before a race is the point of a taper.
08Where the systems agree and where they part
They agree on more than their vocabulary suggests. All of them trace to Banister's heart rate training impulse. All of them score a session from both how hard and how long you worked, and all of them treat training intensity as worth more than training volume. Titan's weights make the trade explicit. A Zone 4 minute counts four times a Zone 1 minute, so 30 minutes in Zone 4 outscores 60 minutes in Zone 1. Banister's exponential factor and TrainingPeaks' definition of 100 TSS as an all-out hour push in the same direction.
They part on three points. The first is transparency. Titan publishes its zone weights, activity factors, time constants, Exertion curve and target rules, and TrainingPeaks publishes its definitions and constants. WHOOP and Apple publish scales and descriptions, with no formula. The second is what counts. WHOOP scores the whole day, while Titan's load and Exertion count heart rate only inside recorded workouts, plus steps for Exertion. The third is the time structure described above, one resetting total against two rolling averages.
The practical upshot is that a WHOOP Strain of any value cannot be converted into a Titan Exertion or load figure. The two apps weight different minutes, scale them differently and reset on different boundaries. WHOOP groups data by Physiological Cycle and Titan by day, so a session after midnight can land on different days in the two apps.
09What crosses from WHOOP into Titan
Titan has no direct connection to WHOOP today. A WHOOP connection, described as bringing in recovery and strain, is listed in Titan as coming soon (wearables). Until it ships, everything WHOOP contributes arrives through Apple Health (using WHOOP with Titan).
Once you turn on the Apple Health connection in the WHOOP app, WHOOP can write workouts, active energy, heart rate, sleep with stages, resting heart rate, respiratory rate, blood oxygen and, if you enable it, steps. WHOOP's own Strain and Recovery scores are not among those types, so they never reach Titan. Titan rebuilds its own Exertion and training load from the heart rate samples and workouts WHOOP writes, using the zone math above.
WHOOP does not write HRV to Apple Health. WHOOP measures HRV as RMSSD and Apple Health stores SDNN. That one gap decides which Titan scores a WHOOP-only owner gets.
| Titan score | Needs HRV | Works with WHOOP data alone |
|---|---|---|
| Recovery | Yes | No |
| Stress | Yes | No |
| Battery | Yes | No |
| Sleep | No | Yes |
| Exertion | No | Yes |
| Net Energy | No | Yes |
With WHOOP alone, then, the Exertion target has no Recovery input. Titan skips the Recovery adjustment when Recovery is missing and works from your training goal and recent Exertion.
The heart rate itself is trustworthy enough to build zones from. Against ECG, WHOOP heart rate showed a bias of 0.39 percent or less, well inside the day-to-day variation of heart rate, while its RMSSD agreement approached or exceeded the smallest worthwhile change (Bellenger and colleagues, 2021, Sensors, with a 2022 correction). In a 53 person sleep laboratory study of six devices, WHOOP 3.0 scored sleep against wake with 86 percent agreement and sleep stages with 60 percent agreement against polysomnography (Miller and colleagues, 2022, Sensors). Over 16 weeks with Olympic water polo players, the day-to-day coefficient of variation of WHOOP-derived lnRMSSD averaged 5.4 percent within each week, inside the range seen with other measurement methods (Bellenger and colleagues, 2022, Sensors). Those studies used earlier straps. Current WHOOP hardware is WHOOP 5.0 and WHOOP MG, so treat those figures as a guide to the current straps.
10Wearing WHOOP and Apple Watch together
Many WHOOP owners also wear an Apple Watch. WHOOP has no screen, so it is comfortable for sleep and all-day wear. The watch shows pace, heart rate and maps during a session and writes HRV to Apple Health. Together they fill each other's gaps in Titan. Apple Watch supplies the HRV that Recovery, Stress and Battery need, and WHOOP can supply sleep if you prefer it there. The broader case for pairing wearable devices is in the wearable data quality guide.
The risk with two devices is counting one run twice. If both devices record the same session, Apple Health can hold two workouts with overlapping heart rate, and each carries zone time. WHOOP can also import workouts from Apple Health and export them back, which duplicates them. Titan's HealthKit Sources settings handle this per metric. Tap the avatar on Today, open You > Settings > Health & Connected Apps, and find HealthKit Sources in the data sources section. For Workouts and Heart Rate, set a Primary Source, restrict Allowed Sources if needed, and pick a Duplicate Handling rule of Prefer Primary, Most Recent or Highest Quality. For most people wearing both, Apple Watch as the Workouts source and WHOOP as the Sleep source is a clean split. The Sleep Tracker setting in Settings > Recovery & Sleep has no WHOOP option, so choose WHOOP for sleep under HealthKit Sources > Sleep.
Watch your max heart rate after adding a second device. Titan takes the highest reading in Apple Health over the last 30 days as a candidate max, so a single artifact spike from either device can lift every zone boundary. A Heart Rate Zones Updated card appears on Today whenever your thresholds change. If the new max looks wrong, set a custom max under Settings > Training & Heart Rate.
11Reading WHOOP workouts on the Training load chart
The one screen a WHOOP and Apple Watch household needs is the Training load chart in Trends, documented in training load and load ratio. Tap the Trends tile on Today, which shows your latest Training Load Ratio and a 7 day sparkline of daily load. Trends opens on the Training group at 60 Days. Long-term load is the orange line and short-term load the yellow one. Drag across the chart to read both loads and the ratio for any day, and check where that ratio sits against Low, Optimal, High and Risk.
Before you read it, set your Workouts and Heart Rate sources as described in using WHOOP with Titan so each session enters the chart once. If past load looks wrong after you fix duplicates in Apple Health, Clear Exertion Cache recalculates Exertion and training load history. Keep reading WHOOP Strain for the shape of a single day. Read the Titan ratio for how this week compares with the last six.
12References
- Apple. Track your training load on Apple Watch. Apple Watch User Guide. https://support.apple.com/guide/watch/track-your-training-load-apde4c07a6cf/watchos
- Bellenger CR et al. (2021). Wrist-based photoplethysmography assessment of heart rate and heart rate variability, validation of WHOOP. Sensors 21(10):3571. https://doi.org/10.3390/s21103571
- Bellenger CR et al. (2022). Correction to Bellenger et al. 2021, wrist-based photoplethysmography assessment of heart rate and heart rate variability, validation of WHOOP. Sensors 22(8):3090. https://doi.org/10.3390/s22083090
- Bellenger CR et al. (2022). Evaluating the typical day-to-day variability of WHOOP-derived heart rate variability in Olympic water polo athletes. Sensors 22(18):6723. https://doi.org/10.3390/s22186723
- Borresen J et al. (2009). The quantification of training load, the training response and the effect on performance. Sports Medicine 39(9):779-795. https://doi.org/10.2165/11317780-000000000-00000
- Bourdon PC et al. (2017). Monitoring athlete training loads, consensus statement. International Journal of Sports Physiology and Performance 12(Suppl 2):S2-161-S2-170. https://doi.org/10.1123/ijspp.2017-0208
- Calvert TW et al. (1976). A systems model of the effects of training on physical performance. IEEE Transactions on Systems, Man, and Cybernetics SMC-6(2):94-102. https://doi.org/10.1109/tsmc.1976.5409179
- Foster C. (1998). Monitoring training in athletes with reference to overtraining syndrome. Medicine & Science in Sports & Exercise 30(7):1164-1168. https://doi.org/10.1097/00005768-199807000-00023
- Garmin. Training Load. Garmin Technology. https://www.garmin.com/en-IE/garmin-technology/running-science/physiological-measurements/training-load/
- Halson SL. (2014). Monitoring training load to understand fatigue in athletes. Sports Medicine 44(Suppl 2):139-147. https://doi.org/10.1007/s40279-014-0253-z
- Impellizzeri FM et al. (2019). Internal and external training load, 15 years on. International Journal of Sports Physiology and Performance 14(2):270-273. https://doi.org/10.1123/ijspp.2018-0935
- Miller DJ et al. (2022). A validation of six wearable devices for estimating sleep, heart rate and heart rate variability in healthy adults. Sensors 22(16):6317. https://doi.org/10.3390/s22166317
- Saw AE et al. (2016). Monitoring the athlete training response, subjective self-reported measures trump commonly used objective measures, a systematic review. British Journal of Sports Medicine 50(5):281-291. https://doi.org/10.1136/bjsports-2015-094758
- TrainingPeaks. The science of the Performance Manager. https://www.trainingpeaks.com/learn/articles/the-science-of-the-performance-manager/
- TrainingPeaks. What is TSS (Training Stress Score)? https://www.trainingpeaks.com/learn/articles/what-is-tss/
- WHOOP. API reference. WHOOP for Developers. https://developer.whoop.com/api/
- WHOOP. Cycle. WHOOP for Developers. https://developer.whoop.com/docs/developing/user-data/cycle/
- WHOOP. WHOOP 101. WHOOP for Developers. https://developer.whoop.com/docs/whoop-101
