Strain score
What strain means on a wearable, and how MyBodyAI reads a week's load and monotony.
How much did today cost your body?
Strain measures the total physiological demand placed on your body. It's not just about exercise: commuting, work stress, poor sleep the night before, and even a heated argument all add to the bill. The word is used in two different ways, and knowing which one a screen means saves a lot of confusion.
Two meanings of one word
Daily strain is the WHOOP sense, and the one most people mean. It is the cardiovascular load of a whole day, computed from how long your heart rate spent in each zone from waking to bedtime, on a scale that grows logarithmically so that the last few points are hard to earn. A day of meetings might score 6, a long ride 18, a race 20. The number describes the day that just happened; it does not predict anything on its own.
Training strain is the sports-science sense, from Carl Foster's 1998 work on monitoring athletes: weekly load multiplied by monotony, the ratio of the average daily load to how much it varied. The insight was that a week of seven identical medium days can carry less total work than a varied week and still be more dangerous, because the body never gets an easy day to adapt. High monotony, roughly above 2.0, preceded illness and overreaching in Foster's athletes even when the weekly totals looked ordinary.
The five levels of a day
In the daily sense, most platforms describe strain in bands rather than as a bare number:
- Rest: minimal demand. A true recovery day
- Light: easy movement. A walk, gentle stretching
- Moderate: a regular workout or an active day at work
- Strenuous: a hard training session. Your body will need extra recovery
- All out: race day, competition, or extreme effort. Full recovery takes 24 to 48 hours or more
Strain + Readiness = the full picture
Strain alone doesn't tell you much. What matters is the relationship between strain and readiness. High strain on a day when you're well-recovered? Productive training. High strain when your readiness is already low? That's the danger zone where injuries, illness, and burnout live. The simplest rule that works: earn a strenuous day with a green morning, and let a red morning cap the day at light.
What strain is not
- Not calories. Strain scales with heart-rate intensity and time, not with energy burned; a long easy walk can burn more than a short hard interval session and score lower
- Not comparable between people. It is read against your own maximum heart rate and your own habit, so a friend's 14 is not your 14
- Not a target to maximize. The productive pattern is a mix of levels across the week, not the highest possible number every day; that is exactly the monotony Foster warned about
In MyBodyAI
MyBodyAI works with the training-science sense. The training state behind the coach computes Foster's weekly monotony and strain from your recorded sessions and flags a flat, loaded week, the one with no easy day in it, alongside the acute:chronic ratio and yesterday's training effect. A flat week of easy walks is monotonous and perfectly healthy, and so is a flat deload, so the flag only fires when the week also carries training-class load and is not lighter than your own recent weeks. The Readiness score pairs with that on the Overview so you can make better daily decisions about training and recovery, and the Progress tab groups the Overtraining and Breathing indices under a Strain heading, because both describe capacity under load.
Frequently asked questions
What is a good daily strain score?
There is no good number, only a good pattern. On the WHOOP-style 0 to 21 scale, a rest day sits under 8, a normal training day around 10 to 14 and a race near 20. What matters is that the week mixes levels and that hard days land on mornings when readiness is high; seven moderate days in a row is the pattern most linked to illness and overreaching.
What is training monotony?
The average daily training load of a week divided by how much it varied day to day. A week where every day is roughly the same has high monotony, above about 2.0 in Foster's work, and that pattern preceded illness and overreaching in his athletes even when the weekly total was unremarkable. One clearly easy day and one clearly hard day each week keep it low.
Does MyBodyAI show a daily strain number?
Not as a single 0 to 21 figure. It reads the training-science version: the coach's training state computes weekly monotony and Foster strain from your sessions and warns about a loaded week with no easy day, and Readiness on the Overview tells you what today can carry. The Progress tab groups the Overtraining and Breathing indices under a Strain heading.