Apple Watch training load and effort ratings compared with Titan

Finish a 60 minute session of heavy squats and Romanian deadlifts, turn the Digital Crown to 8, and Apple Watch files the workout as Hard. Run for 60 easy minutes the next morning and accept a rating of 3. Multiply each rating by its minutes, as Carl Foster's session RPE method does, and the lift carries 480 units against the run's 180. Titan scores the same two workouts from heart rate. For a runner with a max heart rate of 190, the lift spent 50 minutes below 140 bpm and 10 minutes just above it, a load of 70. The run spent 10 minutes in Zone 1 and 50 in Zone 2, a load of 110. One method ranks the lift 2.7 times harder than the run. The other ranks it at about two thirds of the run.

Both numbers follow their own rules correctly. Your rating records how hard the session felt, and for heavy lifting that includes muscle and joint strain that heart rate barely registers. Titan's load records how hard your heart worked, which is the better record of a run's aerobic cost. The two apps then feed those per-workout numbers into the same kind of comparison, recent training against a longer baseline, and report the result on different scales.

This article covers how Apple's Training Load works according to Apple's own documentation, the research its design comes from, how Titan's load is built in the current app, where each method gets a session wrong, and how to run both without being misled by either. The TRIMP history behind heart rate load is in how Titan measures training load, and this piece builds on it. The performance physiology guide connects load to the physiology it measures.

01How Apple Watch training load works

Apple introduced training load with watchOS 11 in 2024. It has two parts, an effort rating for each workout and a comparison of your last week of workouts with the weeks before it.

The effort rating

Every workout gets an effort rating on a scale from 1 to 10 (Apple, watchOS 11 newsroom). Apple sorts the ratings into four labels, Easy, Moderate, Hard and All Out, and invites you to change the number to match how the workout felt (Apple, About watchOS 11 Updates).

The rating appears when you finish. You swipe right in the Workout app, tap End, turn the Digital Crown to set your effort and confirm. To change it later, scroll the workout summary on the watch and tap Effort, or open the Fitness app on iPhone, tap Sessions, tap the workout, tap Effort, drag the slider and tap Update (Apple, Start a workout on Apple Watch). A workout you log by hand in the Fitness app can take a rating too, which Apple marks as requiring an Apple Watch (Apple, Log a workout in Fitness).

HealthKit stores the two kinds of rating separately. Since iOS 18 and watchOS 11 it has one quantity type for the watch's estimated effort and another for an effort score a person or app supplies, with a unit Apple calls the Apple effort score (Apple Developer Documentation). Third-party workout apps can attach an effort sample to a workout through the same API.

Which workouts get an automatic estimate

For popular cardio workouts, the watch proposes a rating on its own. Apple's announcement lists the inputs as age, height and weight, alongside workout data such as GPS, heart rate and elevation, and says you can adjust the estimate for factors such as stress or soreness (Apple, watchOS 11 newsroom). The current user guide names heart rate, VO2 max, age, height and weight as the factors in each rating (Apple, Start a workout on Apple Watch).

Apple's release notes name running, cycling, swimming and hiking "and more" (Apple, About watchOS 11 Updates). In October 2024 an Apple developer technical support engineer told developers that the system is supposed to create estimated scores for walking, running, hiking and cycling, and added that he had not yet seen those samples in his own Health data (Apple Developer Forums). During the 2024 beta, Ray Maker saw automatic ratings for indoor and outdoor running, walking, cycling and rowing, pool and open water swimming, hiking, elliptical, stair stepper, HIIT, dance, kickboxing and cross country skiing (Maker, 2024).

Strength training gets no estimate. Apple's announcement uses it as the example of a workout where you enter the rating yourself (Apple, watchOS 11 newsroom). By the lists above, strength, core, yoga and Pilates sessions all depend on your rating.

From ratings to a 7 day versus 28 day comparison

Apple describes a 28 day training load as "a weighted average taking into account both the effort ratings and duration of users' workouts over this period" (Apple, watchOS 11 newsroom). The user guide says 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 classifies the result "on a scale from well below to well above" (Apple, Track your training load). The five labels are well below, below, steady, above and well above.

On the watch, open the Activity app, tap the training load button and turn the Digital Crown to move through the last 7 days. Tap your training load below the graph, then a workout, to see its details and edit its effort. A filter button shows training load for one workout type at a time. On the current day, a link at the bottom opens your overnight readings in the Vitals app (Apple, Track your training load). Apple says the Fitness app on iPhone adds detail on the possible effect of continuing at your current level, "like potential changes in fitness or risk of injury" (Apple, watchOS 11 newsroom).

Apple has not published the rest of the model. It does not give the weighting inside its 28 day average, the cutoffs between the five bands, or whether the "previous 28 days" include the most recent 7. It does not say what a workout with no rating contributes. In the model it describes, the rating is the only measure of intensity for a strength session, so a lift you never rate has no intensity to add. During the beta, Maker reported that the automatic estimate did not learn from his corrections and appeared to follow average heart rate for the whole session, which pulled interval sessions down toward the easy minutes between repeats (Maker, 2024). Apple may have changed the estimator since.

Readiness uses the same ratings

watchOS 27 adds a Readiness app on Apple Watch Series 12 and Apple Watch Ultra 4. It gives a score from 0 to 10, grouped as Recover at 0 to 1, Pace Yourself at 2 to 4, Ready at 5 to 7 and Go For It at 8 to 10. Its Activity input includes active calories, workout effort ratings and training load. Its Vitals input compares overnight heart rate, respiratory rate, wrist temperature, HRV and blood oxygen, plus daytime heart rate, with a baseline that needs at least 7 nights of sleep with the watch in the past 49. The score also uses recent sleep scores and updates through the day with calories, naps and daytime heart rate (Apple, Use the Readiness app).

On those watches your effort ratings now reach a second score. A careless rating moves your training load, and it also moves the number that tells you whether to push. Titan keeps those two jobs apart. The Recovery score comes from overnight physiology, and training load comes from heart rate during workouts.

02The research Apple's design comes from

Session RPE

In 1998 Carl Foster described a method in which athletes rate each whole session and multiply the rating by its minutes. Among 25 experienced athletes, he tracked that load along with monotony, the daily mean load divided by its standard deviation, and strain, the load multiplied by monotony. A high share of illnesses followed crossings of individual thresholds, mostly thresholds for strain (Foster, 1998).

Foster's group tested the method in 2001 against a heart rate method that sums minutes in five zones. During steady and interval cycling, and during basketball practice, the two methods moved together consistently. The session RPE score was always significantly larger in absolute terms, and the authors proposed it as a way to measure training that works for any mode and intensity of exercise (Foster and colleagues, 2001).

The athlete answers one question, "How was your workout?", on Foster's modified version of Borg's category ratio scale. It runs from 0 for rest through 2 for easy, 3 for moderate, 5 for hard and 7 for very hard, to 10 for maximal, with some numbers left unlabeled. An 87 minute session rated 4 scores 348 arbitrary units (Haddad and colleagues, 2017). Apple's scale starts at 1 and uses four broad labels in place of Foster's anchors, so a number on one scale does not translate exactly to the other.

When to rate

Foster originally collected the rating about 30 minutes after a session so that a hard final interval or an easy cooldown would not dominate the answer. Christen and colleagues tested the timing in 2016 with 15 trained cyclists who rode 30 minutes steady and 30 minutes of intervals. Ratings taken anywhere from 5 minutes to 24 hours afterward did not differ significantly. The interval structure did change the answer. At 30 minutes after the ride, the steady session averaged 3.7, 1 minute intervals 3.9, 2 minute intervals 4.7 and 3 minute intervals 6.2, with each ride lasting the same 30 minutes (Christen and colleagues, 2016). A 25 year review by Foster's group describes session RPE as stable from 1 minute to 14 days after a session (Foster and colleagues, 2021).

Apple asks for the rating seconds after you tap End, which falls inside that evidence. So does correcting it the next day.

How well ratings agree with heart rate

Haddad and colleagues found 36 studies that tested the validity or reliability of session RPE, across endurance, team, combat and aquatic sports and several kinds of strength and conditioning work. They concluded that it is valid and reliable across sports, ages and levels, and could be used on its own (Haddad and colleagues, 2017). In 19 young soccer players, individual correlations between session RPE and heart rate loads ran from 0.54 to 0.78 against Edwards' zone method, 0.50 to 0.77 against Banister's TRIMP and 0.61 to 0.85 against Lucia's three-zone method, as tabulated in that review (Impellizzeri and colleagues, 2004).

Agreement breaks down in predictable places. Borresen and Lambert followed 33 people training as they pleased for two weeks. Session RPE correlated with Banister's TRIMP at r = 0.76 and with a summated heart rate zone score at r = 0.84. The methods disagreed most for people who spent a large share of time at high intensity, 46 percent of training time at 80 to 90 percent of max heart rate in one group, or at low intensity, 15 percent at 50 to 60 percent in another. The authors concluded that session RPE is reasonably accurate against heart rate methods and deviates when training is weighted toward either end (Borresen and Lambert, 2008).

Strength work is where the two methods part most. Day and colleagues found that session RPE separated heavy, moderate and light lifting sessions, with an intraclass correlation of 0.88 across repeated trials. One set of 4 to 5 repetitions at 90 percent of one-repetition maximum felt harder than one set of 15 at 50 percent (Day and colleagues, 2004). In female soccer players' resistance sessions, session RPE correlated with Edwards' heart rate load at only 0.52 and with Banister's TRIMP at 0.25, as tabulated by Haddad and colleagues from Alexiou and Coutts.

Ratings carry their own noise. Haddad's review lists environmental temperature, altitude, music, caffeine, feedback, personality traits such as anxiety, and the presence of other people among the factors known to change RPE. In one study it cites, fatigue, stress, muscle soreness and sleep did not contribute much to the rating of a 10 minute submaximal effort (Haddad and colleagues, 2017). A tired morning does not reliably push your number up, which makes the rating a poor stand-in for a recovery check.

The 7 day versus 28 day comparison started with session RPE

The acute to chronic comparison that Apple displays came out of session RPE data. Hulin and colleagues followed 28 elite cricket fast bowlers across 43 individual seasons and computed internal workload as session RPE multiplied by duration. They compared each bowler's one week load with his four week rolling average. Bowlers whose internal workload in the latest week exceeded 200 percent of their four week average had 4.5 times the injury risk of bowlers between 50 and 99 percent (Hulin and colleagues, 2014). Apple's feature pairs the same two ingredients, effort multiplied by time and 7 days against 28.

The acute to chronic workload ratio that grew from that paper has since been taken apart. Lolli and colleagues showed that including the recent week inside the chronic average creates spurious correlation between the two (Lolli and colleagues, 2019). Impellizzeri and colleagues concluded that no study had tried to estimate a causal effect, that the ratio fails to normalize the numerator by the denominator even when uncoupled, and that there is no evidence supporting its use in training recommendations aimed at reducing injury risk (Impellizzeri and colleagues, 2020). A year later the same group replaced players' real chronic loads with random numbers. The original data gave an odds ratio for injury of 2.45, and random chronic loads still gave a mean of 1.89 (Impellizzeri and colleagues, 2021). The only randomized trial, 482 elite youth soccer players on 34 teams planned around ratio principles for a 10 month season, found no difference in health problems, with a relative risk of 1.01 (95 percent CI 0.91 to 1.12) (Dalen-Lorentsen and colleagues, 2021).

Apple's user guide words the feature carefully, as a way to understand "the relative strain on your body and decide whether it's sustainable." That is a fair use. A band of above or well above says your recent training rose relative to your recent history. It does not forecast an injury, and neither does Titan's ratio.

03How Titan computes training load

Titan publishes every constant in its model, and the figures below match the current app. The training load help article covers the same ground for day-to-day use.

Heart rate minutes weighted by zone

For each workout, Titan takes the average heart rate for every minute and assigns that minute to one of five heart rate zones. A Zone 1 minute counts 1 and a Zone 5 minute counts 5, and the day's load is the sum across all workouts. Gaps of more than a few minutes without a heart rate reading add nothing. Heart rate below the start of Zone 1 still counts as Zone 1, so no recorded minute scores zero. Heart rate outside workouts adds nothing to load, so a tense afternoon meeting leaves it untouched.

The zones come from your max heart rate. Titan takes the higher of the top reading in Apple Health over the last 30 days and 220 minus your age, unless you set a custom value. The default zones start at 53, 74, 81, 87 and 93 percent of that max. Titan keeps its own zones. Apple Watch computes its Heart Rate Zones as percentages of max heart rate personalized from your health data, and you can edit zones 2 to 4 on the watch (Apple, View Heart Rate Zones). Those zones do not reach Titan, so the same minute can sit in Zone 2 on the watch and Zone 3 in Titan. To check Titan's numbers, tap your avatar on Today, then Your Training on the You screen. The Heart Rate section has Use Custom Max Heart Rate and Heart Rate Zones.

When no workout on a given day has heart rate, Titan estimates load from duration and activity type. Endurance workouts count 1.0 per minute, mixed-intensity 0.95, strength 0.9, mobility 0.65, other workouts with distance 0.95, other workouts without distance 0.8, and an Other workout renamed as a sauna or steam session 0.5. If even one workout that day has heart rate, the day's load comes from zone time alone.

Titan does not read Apple's effort ratings, estimated or entered. A rating you set on the watch changes Apple's training load and, on newer watches, Readiness. It does not change anything in Titan.

Two exponentially weighted averages

Titan turns daily load into two exponentially weighted averages. Each day, each average moves a fixed share of the way from yesterday's value toward today's load. That share is α = 1 − e^(−1/τ), where τ is the time constant in days. Short-term load uses 7 days, so α is about 0.133 and it moves about 13 percent of the way each day. Long-term load uses 42 days, so α is about 0.024 and it moves about 2.4 percent. A day with no workouts counts as a load of 0.

To give the averages a history, Titan starts the calculation 84 days, twice the long time constant, before the first day it shows, and the first day's load seeds both averages. The load ratio is short-term load divided by long-term load, shown to two decimals and left blank on any day long-term load is 0.

Apple's comparison and Titan's differ in two ways that matter. Titan's baseline reaches back further, with a 42 day time constant against Apple's 28 day window, so it moves more slowly. Titan's averages also fade each old day gradually, where a fixed 7 day window drops a session in full on its eighth day. Williams and colleagues proposed exponentially weighted averages for load ratios for that reason (Williams and colleagues, 2017).

How the ratio zones are set

Titan sorts the ratio into four zones with three fixed cutoffs written into the app.

The cutoffs are the same for every user and every sport. Titan does not fit them to your history, and no study has validated them for Titan's arithmetic. The lower bound matches the familiar 0.8 from the ratio literature, and the acute to chronic workload ratio entry traces where the upper numbers came from and why they do not transfer between methods. Under the chart, Titan prints the latest value with its zone, such as "Ratio 1.12 · Optimal." Optimal appears in green, and Low, High and Risk appear in red.

The personal reference sits behind the lines. A shaded band marks the middle 50 percent of your own long-term load across the selected period, using interpolated quartiles. It appears once the period holds at least 28 daily values covering at least 80 percent of its days. Titan's cutoffs describe the change in your load. The band describes where your long-term load usually sits.

Apple shows five bands and no number. Titan shows the two loads, the ratio and four zones, and it publishes the math. Neither set of cutoffs has been shown to prevent injury.

Exertion, the daily number

The same daily load drives the Exertion score, a 0 to 10 total that accumulates through the day and starts again the next. Workout points follow 10 × (1 − e^(−load / 100)), so a load of 70 earns 5.0 points, 110 earns 6.7 and 150 earns 7.8. Steps add up to 1.2 points, rising in a straight line from 700 to 12,000 steps. A day with 700 or more steps or 90 kcal or more of active energy scores at least 1.0. Titan labels Exertion Low under 2.5, Moderate from 2.5, High from 5.5 and Very High from 7.5, and it sets a daily range for it from your training goal, morning Recovery and recent Exertion, explained in Exertion target.

04Two scales from 1 to 10 that measure different things

Apple's effort rating and Titan's Exertion look alike, and they answer different questions. The effort rating describes the intensity of one workout, with duration left out. Exertion describes the whole day, with duration built in, on a curve that flattens near 10. The table uses illustrative sessions for the runner with a max of 190.

A 10 for the time trial and a 6.3 for the same session are both correct. The 10 says the effort was maximal. The 6.3 says 20 minutes of maximal effort is a moderate day's work compared with three hours in the saddle. Apple adds duration when it builds training load from your ratings. Titan adds it earlier, in the load itself. Compare Apple's rating with Titan's per-workout Load, and compare Apple's band with Titan's ratio.

05Apple and Titan side by side

06Where each approach works and where it goes wrong

Strength training

Heart rate underrates lifting, and Titan's load inherits that. Take the runner from the opening, who usually runs four times a week. Monday is a 45 minute easy run, Wednesday 50 minutes of intervals, Friday 40 easy minutes and Sunday a 90 minute long run. Those score 80, 123, 70 and 180 in Titan, 453 a week. Rated 3, 7, 3 and 5, they score 1,055 in session RPE units. Now add the 60 minute lifting session on Tuesday, Thursday and Saturday, each scoring 70 in Titan and rated 8.

The last column applies the same plain 7 day over 28 day calculation to both, so the gap comes from how each method scores a lift. Titan's own exponential ratio went from 1.07 on the Sunday before to 1.27 after the first lifting week, still Optimal. Apple's band for that week cannot be predicted from outside because Apple does not publish its cutoffs. An effort-based load sees the week as more than double the usual, and Titan's sees less than half again.

For a strength athlete, Apple's rating is the better record of what the lifting cost. Heart rate responds to the cardiovascular part of lifting, and much of the cost of a heavy set falls on muscle and connective tissue instead. Wrist sensors add error. In a study of eight devices during cycling and resistance exercise, every device underestimated heart rate more as intensity rose (Boudreaux and colleagues, 2018).

Titan still counts lifting, and a lift recorded with heart rate enters the load at its zone time. Two details matter. A lift recorded without heart rate on a day that also has a run with heart rate adds nothing to that day's load, because the day's total comes from zone time alone. The Load shown on that lift's own detail screen still applies the 0.9 per minute estimate, so the workout screen can show 54 while the day's total ignores it. Record every lift with the watch on your wrist and a workout running.

Intervals

Intervals are where the automatic estimate is weakest, if Maker's beta observation still holds. A rating built from average heart rate treats a warm-up, six hard repeats and the recoveries as one moderate block. Christen's cyclists show what a person reports instead. Three minute intervals at a 1 to 1 work to rest ratio rated 6.2 against 3.7 for a steady ride of the same length.

Titan scores intervals minute by minute, so each minute in Zone 4 or 5 counts 4 or 5 however easy the recoveries around it were. Heart rate takes time to climb, so a 15 second sprint can finish before heart rate reaches the zone its effort belongs to, and a session of short sprints scores lower in Titan than it felt. Your rating catches that. For intervals, correct Apple's estimate when it looks low.

Long, easy days

Long, low-intensity days show the opposite pattern. Maker's complaint during the beta was that load models on other watches score a 12 hour hike about like a tempo run. Titan counts every minute, so duration shows up in full. A 6 hour hike with 300 minutes in Zone 1 and 60 in Zone 2 scores 420, against 150 for a 50 minute tempo run with 10 minutes in Zone 2, 30 in Zone 3 and 10 in Zone 4. The hike's Exertion reaches 9.9. Session RPE ranks the hike even higher. At a rating of 6 across 360 minutes it scores 2,160, more than six times a tempo run rated 7.

Heat and heart rate drift

Heart rate rises during long exercise at a fixed workload. Coyle and González-Alonso attributed the fall in stroke volume after 10 to 20 minutes of exercise mainly to the rising heart rate (Coyle and González-Alonso, 2001). Wingo and colleagues had nine male cyclists ride at 60 percent of VO2max in 35 °C heat. Between minutes 15 and 45, heart rate rose 12 percent, from 151 to 169 bpm. VO2max fell 19 percent over the same half hour, so the same power output demanded 78 percent of the riders' reduced VO2max, up from 63 percent. The authors concluded the results support using heart rate changes to reflect changes in relative intensity during prolonged exercise in the heat (Wingo and colleagues, 2005).

Titan counts that drift as extra load, and Wingo's data say it should. Take the 60 minute easy run in summer. If drift moves the last 30 minutes from Zone 2 into Zone 3, the load goes from 110 to 140, and workout points from 6.7 to 7.5. Your rating is likely to rise too, since environmental temperature is one of the factors known to change RPE (Haddad and colleagues, 2017). Both methods register a hot day as more work. When you compare fitness across seasons, compare pace or power at a given heart rate as well, because heat changes the heart rate without any change in fitness.

Forgetting to rate, forgetting to record

The two systems fail differently when you forget something. Apple depends on your rating for every workout it cannot estimate. If you skip the rating, the strength session has no intensity for Apple to count, and Apple does not document how it treats that gap. The research on timing makes the fix easy. Session RPE stays stable for days, so rate the lift the next evening in the Fitness app.

Titan needs no rating and depends on a recorded workout. Heart rate outside a workout adds nothing, and a day with no workout counts as a load of 0. A session you forgot to start on the watch, or a week with the watch in a drawer, reads as rest and pulls short-term load down. A workout you add by hand in the Fitness app has no heart rate, so Titan scores it from its minutes and activity type, as long as no other workout that day has heart rate.

New users and new watches

Apple's comparison needs weeks of rated workouts before the baseline means anything. Maker saw no training load for the first 10 days after installing the watchOS 11 beta, and he noted that Apple did not bring in older workouts, possibly because their ratings had never been confirmed (Maker, 2024). Titan reads your Apple Health history and starts its averages 84 days back. With little history, long-term load starts near zero and the ratio reads high for weeks, a pattern the acute to chronic workload ratio entry works through.

07Apple Watch training load vs WHOOP strain

People comparing Apple's training load with WHOOP Strain are often comparing different kinds of number. WHOOP Strain is a daily total on a 0 to 21 scale that starts again with each sleep cycle. Apple's effort rating describes one workout, and Apple's training load compares two windows. The closest Apple counterpart to Strain is a day's workouts combined, and Apple does not show that as a single number. Titan's Exertion is the daily total closest to Strain, and Titan's ratio is the comparison closest to Apple's band. The full breakdown is in how WHOOP strain compares to Titan training load.

08How to use both together

Record every session as a workout. Both systems count workouts and ignore the rest of your day for load. A lift or a walk that never becomes a workout in Apple Health is invisible to both.

Rate every workout, and correct the estimates. Titan does not need your ratings, and Apple has nothing else for strength, yoga or mobility sessions. Rate after the cooldown, and adjust the automatic estimate for intervals, long days and anything that felt out of line with its heart rate. On a Series 12 or Ultra 4, those ratings also feed Readiness.

Read disagreement as information. When Apple reads above or well above and Titan's ratio stays in Optimal, the added load is probably something heart rate undercounts, such as lifting, jumping or eccentric work, or the sessions felt harder than your heart said. When Titan's ratio climbs into High while Apple reads steady, your heart worked harder at the same perceived effort. Heat, dehydration, illness, a new zone setting or a generous rating are the usual causes.

Check your max heart rate after any change in Titan's zones. Titan's zones follow its own max heart rate, the higher of your top reading in the last 30 days and 220 minus your age. Check the value under Your Training, and set a custom max if you know yours from a test or race.

Compare like with like. Match Apple's per-workout rating to Titan's per-workout Load, which you find by tapping the Workouts tile on Today and then a workout. Its detail screen shows Exertion, Load and time in each zone. Match Apple's band to Titan's ratio. Tap the Trends tile on Today, which shows your latest load ratio and a sparkline of the last 7 days of daily load. Trends opens on the Year period, and the period menu at the top also offers 30 Days, 60 Days and 6 Months. Tap or hold the Training load chart to read short-term load, long-term load and the ratio for any day.

Treat both bands as descriptions of change. Above, well above, High and Risk all say your recent training outran your recent history. That is the point of a build block and a warning sign only when you did not plan it. Pair either reading with how you slept and felt, and with your morning Recovery score.

09References

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