Training load

Four numbers, four questions#

All four are estimates. The ones that rest on heart rate need it: a session without an average heart rate adds nothing to Fatigue, Fitness or Load vs. usual. Everything here is about training, not diagnosis.

The impulse of one session#

Fatigue, Fitness, Form and Load vs. usual start from one number per session, the training impulse. We use Banister's TRIMP (Morton and colleagues, 1990):

impulse = minutes × HRr × a × e^(b × HRr)

where HRr = (average pulse - resting rate) / (maximum - resting rate), the share of your heart-rate reserve the session averaged at, kept between 0 and 1. For men a = 0.64 and b = 1.92; for women a = 0.86 and b = 1.67. If no sex is set, the men's numbers are used. The exponential is the point: an hour near threshold is worth several hours of walking, which a straight line would not say.

• Minutes are the moving time when the device recorded it, otherwise the elapsed time.
• Resting rate is the night's resting rate of the session's own day; for a day with none, the median of your resting rates over the last 365 days. Maximum is the one described in Heart-rate zones.
• If the reserve of that day is under 30 bpm (a resting rate that is wrong), the median is tried instead, and if that fails too, the session adds nothing rather than something made up.
• A day's impulse is the sum of its sessions. Sessions that overlap on the clock are one stretch of training: the impulse of an overlapping group is scaled down by the clock time it really covered, so the same run sent by a watch and by a phone is not counted twice.
• A rest day is 0.

If your device sends its own "training load" value for a session, the session sheet shows it ("from device"), but it is on a different scale and is not used here. One scale, ours, is used throughout.

Without a resting rate in any night of the last year, or without a maximum (no birth date, no entered maximum, no trustworthy peak), no impulse can be built. The cards then say what is missing: for example "Your sessions carry a heart rate, but we do not know your maximum. Add or correct your date of birth in Settings and the curve appears." Or: "Fitness, fatigue and form need a heart rate on each session. Your device does not send one, so there is nothing to weigh."

Fatigue, Fitness and Form#

Fatigue and Fitness are two running averages of the daily impulse that give the newest days the most weight (exponentially weighted averages):

• Fatigue uses a time constant of 7 days: each day moves it 13.3% of the way to that day's impulse. Without training it falls to about a third in a week.
• Fitness uses 42 days: each day moves it 2.35% of the way. Without training it falls to about a third in six weeks, and it rises slowly when you train regularly.
• The factor per day is 1 - e^(-1/time constant): 0.133 and 0.0235.
• Form is Fitness less the part of Fatigue that exceeds it: Form = Fitness - (Fatigue - Fitness) when Fatigue is above Fitness, otherwise Form = Fitness. It never goes above Fitness and never below 0. A month off the bike therefore does not read as "fresh": Form falls with Fitness.

The averages are warmed up over 210 days (five Fitness time constants) before the window you look at, and seeded at the average daily impulse of that stretch, so the number for a day does not depend on how wide a window you asked for. Fitness and Fatigue are shown as whole numbers; they are model points, not calories or minutes.

Walking and the like count here. Fatigue, Fitness and Form include every session with a heart rate, walks and yoga as well. Load vs. usual does not (below).

Where you see them.

• Activities tab, card "Training load": two gauges, "Fatigue" and "Fitness", each with the change against yesterday in per cent ("vs. yesterday"; "vs. day before" on a past day). A full arc is your highest Fatigue or Fitness of the last 90 days. A tap opens a sheet that explains the number.
• Performance tab: tiles "Fitness", "Fatigue" and "Form", and the "Performance curve" with the daily readiness. The word beside Fitness and Form is the direction over two weeks: "Rising" when it has gone up by more than 2 points against 14 days ago, "Falling" when it went down by more than 2, otherwise "Stable".
• The Adaptation card on the Performance tab can plot Fitness over time.

The curve is a Premium feature. If your plan does not include it, the card says "Fatigue and fitness from your workout heart rate are part of Premium."

Load vs. usual#

"Load vs. usual" compares the last week's training with your habit: the average daily load of the 7 days up to yesterday, divided by the average daily load of the 21 days before those 7. Rest days count as zero load. This is the acute to chronic workload ratio of the sports-science literature, in the "uncoupled" form: the week is not part of its own reference.

• Up to yesterday. The ratio is read in the morning, when today's load is still zero; counting today would pull the average down by about a seventh. Today's workout shows tomorrow. On a past day the ratio is the one as of that day.
• Training only. The ratio uses the impulse of training sessions, not of recovery-class movement or everyday work. Walking, yoga, pilates, mobility, breathwork, golf, sauna, cold exposure, yard work and housework are left out whatever their heart rate, because the bands below come from team-sport training and a week of walks is not that kind of load.
• Two decimals. The number is rounded to 2 decimals and then placed in a band.

The bands come from Gabbett (2016), who found fewest injuries in team-sport squads at a ratio of 0.8 to 1.3 and clearly more above 1.5. Gabbett divided the last 7 days by a 28-day average that included them. Our ratio divides by the 21 days before them, which gives a different number for the same weeks, so each edge is carried over with the formula U = 3C / (4 - C): 0.8 becomes 0.75, 1.3 becomes 1.44 and 1.5 becomes 1.80. It is an observed association, not a prediction of injury, and later analyses question how far it can be pushed (Impellizzeri and colleagues, 2021). So the app never says "you will get injured"; it says the week is above your usual and today's guidance is gentler.

When it is not rated. The ratio is rated only if the 21 days before the last week hold at least 6 days with training load, roughly two sessions a week. With fewer, the number may still be shown, but with "Not rated yet" and no band, and the coach does not quote the research. If those 21 days hold no load at all, there is no number, and the tile says "Building" with "too few sessions with heart rate yet".

A few days off at the start of the week. The 7-day mean falls even when the last days are at your usual. So the coach also looks at the newest 4 days against the same 21-day reference. If the week reads "Below usual" but the last 4 days are at least 0.8 of your usual, it says so: "Week below usual (0.71), the last few days on par."

Where it appears. The Activities tile "Load vs. usual" and its gauge (a sheet explains the scale from 0 to 2), the Overview card "Load & Recovery", and the coach. The coach (see Daily coach) takes the band into account: "Sharp rise" asks for a lighter day, "Elevated" says a moderate session is enough. Energy pacing mode (Energy pacing) hides the load tile and gauges on the Activities tab.

Overtraining#

"Overtraining" is a 0 to 100 index where higher is worse. It watches whether several signs get worse together after training: sleep, resting heart rate, morning energy, overnight recharge, stress and deep sleep. The training itself is a smaller part. It is not a diagnosis; it points at a pattern that often comes with too much training and too little recovery. In the readiness breakdown the same number appears turned round as "Freshness".

It is scored only when you have trained: at least 2 training sessions, or at least an hour of training, in the last 7 days (walks and other recovery-class movement do not count here), and at least 7 days of history. Otherwise there is no score, because without training the same signals are tiredness from something else. The parts and their weights:

The split compares the older and the newest days of the last 7. The weights are shares of the parts that could be computed, not of a fixed total. When only 2 parts exist the score is pulled 40% toward 50, with 3 parts 20% (fewer data, less certainty); with 4 or more it stands.

A score above 70 is one of the signals the Body Status alert reads (see Body Status); one signal alone does not turn the day red.

Training Balance#

"Training Balance" is a 0 to 100 index of how well your week is put together for health, not how much you trained. It uses your last 7 days (and 4 weeks for regularity):

Between the marks the time score is joined by straight lines. There is no penalty for a long week: above 300 minutes more adds nothing, and the WHO guideline has no upper limit. Walks count towards time, number of sessions and regularity. A day with only an ordinary walk is still a day without training, and a walk enters the hard-session share only when its heart rate puts it in zone 3 or higher. A day still running with no training yet is not counted as a rest day; the rest days are then counted over the seven finished days.

Intensity: low, moderate, high#

One classifier is used everywhere a session must be called low, moderate or high, so the same session is not "high" on one card and "moderate" on another:

1. Power first, if the session has an average power and you have a power anchor: under 55% of the anchor is low, 55 to 85% moderate, 85% and above high.
2. Heart rate next: the zone of the session's average pulse in the zones of its own day. Zone 3 is moderate; zone 4 and 5 are high; zone 1 and 2 are low.
3. Fixed numbers if there is no maximum: under 120 bpm low, 120 to 154 moderate, 155 and above high.
4. Nothing to go on reads as low.

More on the anchor and the zones: Heart-rate zones. In Training Balance and in the rest-day count a walk, a yoga class or yard work counts as training when its own heart rate reaches zone 3, and a sauna never does.

Hard sessions and what they change#

A hard session is one the body has to recover from: a device training effect of 4.0 or more (4.0 to 4.9 "highly improving", 5.0 "overreaching" on the Firstbeat scale that Garmin watches use), or a session of high intensity (an average pulse in zone 4 or 5) that lasted at least 20 minutes. Duration alone never makes a session hard; a long walk is the commonest session there is. A row that is not a counted session is never hard.

What reads this flag, over the last two days (today and yesterday):

• Recovery. After a hard session in the last two days the recovery score gets 5 points of tolerance. On top of that, yesterday's training takes points off recovery: 15 if it lasted over 45 minutes and was hard, 10 if over 30 minutes and moderate or high, 5 if over 30 minutes or any high session.
• The model behind Body Alert. The bar for "resting heart rate up" rises from 3 to 5 bpm, and the bar for "HRV down" from a 15% to a 20% drop, since a hard session can cause those very signs itself.
• Body Status. After a hard session the "very low morning energy" and the "rising resting rate with falling HRV" signals are not raised as warnings.

Yesterday as the coach sees it. Yesterday counts as a hard day for the coach when its training effect was 4.0 or more, or when its longest single session lasted at least 1.8 times your usual for that sport and at least 90 minutes. Your usual for a sport is the median length of your sessions of that sport in the last 90 days, if there are at least 3 of them; otherwise the median length of all your sessions in those 90 days. For walking, yoga and other recovery-class movement the floor is 180 minutes: ninety minutes of riding is a session, ninety minutes of walking is a Sunday. The coach then says "A hard session yesterday, so let your body absorb it."

A week without an easy day#

Foster's monotony is the mean of the week's daily loads divided by their standard deviation (sample, 7 days up to yesterday). Seven identical days give the highest value, capped at 4.0. At 2.0 or more the coach may say "Seven days without an easy one, so today is it." It does so only when all of this holds: there are enough loaded days behind you (the same 6 of 21), the week contains real training and not only walks, the week is not lighter than your median week of the last 6 (at least 3 such weeks must exist), and the ratio is not already in "Elevated" or "Sharp rise". A flat week of easy walks, or a flat deload week, is not flagged.

Frequently asked questions#

Can I get Load vs. usual by dividing Fatigue by Fitness?#

No. They are built on different windows (7 and 42 days, exponentially weighted, against 7 and 21 days, plain averages), and Fatigue and Fitness include walks and yoga while Load vs. usual does not. The two cards can legitimately say different things on the same day.

Why does my Load vs. usual say "Building" or "Not rated yet"?#

"Building" means the 21 days before your last week hold no training load that could be measured, or no heart rate. "Not rated yet" means there is a number, but fewer than 6 days in those 21 had training load, so we do not put it in a band. Train regularly for a few weeks with a device that records heart rate.

Why did today's workout not change the ratio?#

Because it counts the 7 days up to yesterday. Today's workout moves it tomorrow. This also means the morning reading is not dragged down by a day that has not happened yet.

Why does a walk show in Fatigue but not in Load vs. usual?#

Fatigue, Fitness and Form weigh every session that has a heart rate, so a long brisk walk adds a little. The ratio compares training load with training habit, and the bands behind it come from team-sport training, so recovery-class movement is left out. This is why a week of walks cannot give you a "Sharp rise".

Is "Sharp rise" a warning that I will get injured?#

No. It says this week is well above your usual. Research has linked such weeks with more injuries in team sports, but that is an observed association, not a prediction for you, and we never present it as one. Treat it as a reason to check how you feel and keep the next day lighter.

Why is my Fitness so low or zero?#

Fitness is a slow average of the heart-rate impulse of your sessions, in model points. It starts low and grows over weeks. It stays at zero or empty if your sessions have no average heart rate, if there is no night with a resting rate, or if we do not know your maximum.

Does this match my watch's training load?#

No, and it does not have to. A watch uses its own model on its own scale. The impulse here is the same for every device and every session, so your weeks are comparable. If your watch sends a training load for a session, we show it on the session sheet as "from device" and do not mix it in.

Why does Form never go below 0?#

Because Fitness starts from nothing, and Form is Fitness with the excess Fatigue taken off. After a first ride Fatigue can be more than twice a small Fitness; the form left is none, not a debt.