Strain score

1

What does the Strain score show?

How hard your heart worked today and for how long, as a number with a word beside it.

At the top of the Strain page you see today's number with a word beside it: Quiet, Light, Moderate, Hard or Very hard. The number tops out at 98, never 100. Home and the widgets use the same five words once the day has a score.

Once the day has a score, a card under it gives today's room, the amount of Strain that suits the day, as one of the same words. It then says how far you are from it: To reach it, Room left or Past it by, with a count of points. Below that the page draws the day as a climb, with each workout marked where it ended.

The last 7 days sit below that, each against its own room. A day past its room is drawn in amber, and an easy day is not drawn as a worse one.

2

What goes into it?

Your workouts, and the walking you do outside them.

A workout counts for its effort, worked out in the next chapter. Walks, hikes, yoga and similar sessions count at half. That half is Aera's own choice, not taken from a study.

Steps outside your workouts add to the day. First Aera takes off the steps your workouts probably held, about 150 a minute for runs and 100 for the rest. What is left becomes walking time, up to 6 hours, weighed by your walking heart rate.

Walking time counts 100 steps a minute. That is Aera's own pick, a pace between casual and brisk walking, not a figure from a study. A lab study of adults aged 21 to 40 put the start of moderate walking near the same rate, but it asked a different question (Steps tells it). Aera uses one rate for everyone.¹

With no walking heart rate, active energy stands in when it is over 50 kilocalories, then steps beyond 3,000. The exercise minutes that Apple Health counts are not used.

3

How is a workout's effort worked out?

From its minutes and how far your heart rate rose between rest and your top heart rate.

Aera takes a workout's average heart rate and asks how far it sits between your resting heart rate and your top heart rate. That share is your heart rate reserve in use. The workout's minutes are weighted by it, and hard minutes count far more than easy ones. With no heart rate, the workout's energy or its type of activity stands in.

Why the reserve and not a plain share of top heart rate? One review notes that the same share does not necessarily put people at the same intensity above their resting level, and some authors recommend the reserve instead. It adds that the reserve varies between people too (Intensity tells the thresholds).²

A heart-rate training score of this family has been tested. In eight field hockey players, a modified version, averaged each week, went with the change in their aerobic fitness from the start of a season to its middle. It was a small study of men in one sport.³

The weights Aera uses come from a method worked out on men, and Aera applies them to everyone. That is a known limit.

Your top heart rate is the maximum you typed under Heart rate zones in Settings. With none typed, Aera takes the higher of two numbers. One is 220 minus your age, or 190 when it knows no age. The other is the highest rate a workout recorded in the past year. A recorded peak counts only if a second workout came within 5 beats of it, so one stray wrist reading cannot set it. Aera learns that peak when you open a workout from the last week, or the Heart rate zones page.

A single workout can lift it too: when that workout's own peak is higher, the higher number is used for it. A peak above 220 is ignored as a sensor error.

No age formula is exact: age alone leaves a typical error of about 11 beats a minute (220 minus your age compares them). Taking the higher of the two numbers is Aera's own choice, and Aera knows of no published rule for how to pick.⁴

When your top heart rate changes, your past days are scored again so that they sit on one scale. That happens when you type a new maximum, or when the estimate moves 3 beats or more. Aera reads back 190 days, about six months.

4

How do efforts become one number?

They add up, then pass through a curve that flattens, so each extra bit of effort adds a little less.

Aera adds up the day: workouts, half of any walks and the like, and your movement outside workouts. The total goes through one saturating curve. It rises fast at first and flattens as the day grows. On the starting curve, a session that scores 39 lifts the day to only 63 when done twice, not 78.

The curve starts the same for everyone. Once at least half of your recent days hold a workout, Aera stretches it to 2.1 times your median workout day, never below the start. So the same session scores lower for someone who trains a lot.

The starting curve and the 2.1 are Aera's own choices, not taken from a study. Aera fitted the 2.1 on one person's year of data, and that data no longer tests it.

The word comes from where the number sits among your last 60 days. About the lowest quarter of days read Quiet, the next quarter Light, and only the top 5 percent Very hard, with Moderate and Hard between. Until 30 days are scored, fixed starting cut-offs stand in.

These cut-offs are Aera's choice, not taken from a study. They make the words relative: Very hard marks the top of your own days, not a fixed amount of effort.

5

What is today's room?

How much Strain suits today, set from your Recovery and your usual days.

Aera sets the room from two things: this morning's Recovery, and the middle of your last 28 days of Strain. If today's Recovery has not come in, it uses yesterday's. With neither, it counts Recovery as a neutral 55.

A higher Recovery lifts the top of the room, and so does a heavier usual day. A point of usual day lifts it about twice as much as a point of Recovery. The bottom never sits under 20, and the room is at least 8 points wide.

Aera fitted these weights on one person's year of data, not from a study.

Mark a day on Home as sick, injured or on a break and Aera sets no room, and the word reads Rest day. Tell Home you are run down and the room moves down by its own width, never below zero.

6

When does it show nothing?

Before anything has moved today, and in your first weeks, when it has no room to show.

Early today, before anything has moved, the page shows the words Plenty of room and no number. A past day with nothing recorded shows one line instead: Strain fills in from your steps and workouts.

In your first weeks there is no room yet. Until 14 days of Strain are behind you, the page says Aera is learning your usual and counts the days to go.

If Aera has seen no workout and no Recovery yet, a quiet line says: A watch adds your workouts and sets this room by your recovery each morning.

7

What can't it tell you?

How a day felt, or work that never lifts your heart rate. It counts one day at a time.

Strain reads your heart rate and your movement. It cannot see how a day felt, and work that does not lift your heart rate adds little to it. A wrist also reads heart rate less well in motion: in a test of wearables, error during activity was on average 30% higher than at rest. (How a watch measures you tells why.)⁵

A workout's effort comes from its average heart rate. Because of that, a session of hard bursts with rests between can score lower than counting its minutes one by one would.

Strain is not Cardio load. Cardio load starts every workout from a flat top heart rate of 190, not your own, so the two can disagree about the same session (Cardio load tells it).

The number says how much you did, not whether it was the right amount. If you feel unwell, how you feel matters more than any score.