# Activity Units (AU)

Your daily activity dose from heart rate and movement, against your resting heart rate and an age-based maximum, every day whether or not you trained.

Type: score · Unit: points/day · Domain: Activities & Lifestyle · Accuracy: Calculated · Importance: Core

Also called: activity dose, daily activity, activity score, activity level, activity multiplier, PAL

## In one sentence

Your daily activity dose, worked out on your phone from minute-by-minute heart rate and movement against your resting heart rate and an age-based maximum, recorded every day whether or not you trained.

## Why VT does not use an activity multiplier

The population way of estimating what a person burns multiplies resting metabolism by one of five activity levels chosen from a self-description (sedentary, lightly active, moderately active and so on, from about 1.2 to 1.9). Three easy half-hour rides a week and four 90-minute weight sessions plus intervals both land in "moderately active", though they are different lives, and the category you picked when you signed up never changes when your life does. VT measures the dose instead.

## What it counts

Two things, from the device you wear through the day: total movement, and the minutes spent with your heart rate raised, weighted by how far above your own resting rate and toward your maximum it sat. A brisk walk to the station counts; a hard interval counts more per minute; a day on the sofa scores low without being penalized. It is recorded every day, trained or not, which is what makes it a dose rather than a training log.

## Why the ceiling

More activity does not add up to more expenditure forever. In 332 adults across five populations measured with doubly labeled water, total energy expenditure rose with physical activity at the lower end of the range and plateaued at the upper end as the body adapted to keep the total within a narrow band (Pontzer 2016). VT's energy model therefore maps your training load through a curve with a ceiling rather than a straight line, so that a very active week raises the activity term, but not without limit.

## What moves it

How much you moved and how hard; the device's heart-rate signal, which a loose strap or a cold wrist degrades; and the maximum heart rate that sets the scale, which VT takes from your age (208 − 0.7 × age) and raises a little when your watch sees you hold a higher rate; the maximum you record in your body measurements is not used here.

## How Vitality Tracker uses it

The load side of Capacity reads your logged training (Borg × minutes); when you have logged none in the last 14 days, it reads Activity Units instead. The TDEE engine's activity term is built from the session-RPE training load of the sessions you log; it does not read Activity Units. They are not scored in the Vitality Score; they are one of the signals it checks to decide whether an older measurement can still count.

## How far to trust the number

Calculated from a measured heart rate and movement, on your own scale; comparable with your own days on the same device, not with anyone else's. A day without the device is a gap, not a zero.

## Method

AU = movement + heart-rate stimulus. Movement = 8 × (1 − e^(−steps ÷ 7,000)). Stimulus = the sum over the day's minutes of a weight that rises with heart-rate reserve fraction, (HR − HRrest) ÷ (HRmax − HRrest): 0 up to 0.30, 1 at 0.50, 2 at 0.75, 3 at 0.95 and above, divided by 10; a minute counts only as part of three or more raised minutes in a row or of a recorded workout. Computed on your phone and shown as points, ten per unit. Resting heart rate from your device's daily figure; maximum 208 − 0.7 × age, raised a little when your watch sees you hold a higher rate.

## Sources

1. Pontzer H, et al. Constrained total energy expenditure and metabolic adaptation to physical activity in adult humans. Curr Biol 2016;26:410–7. [doi:10.1016/j.cub.2015.12.046](https://doi.org/10.1016/j.cub.2015.12.046)
2. Vitality Tracker, How Vitality Tracker estimates your energy expenditure, method note v1.0
3. Vitality Tracker, What VT measures (catalogue of 15 October 2026).

## Related

- [Capacity: how much you can take on today](https://vitalitytracker.app/knowledge/capacity)
- [The TDEE engine](https://vitalitytracker.app/knowledge/tdee-engine)
- [Perceived exertion: the Borg CR10 scale](https://vitalitytracker.app/knowledge/perceived-exertion-borg-cr10)
- [Maximum heart rate and heart-rate zones](https://vitalitytracker.app/knowledge/maximum-heart-rate-and-zones)
- [Daily steps](https://vitalitytracker.app/knowledge/daily-steps)
- [Training load and Cardio Load](https://vitalitytracker.app/knowledge/training-load-and-cardio-load)

Reviewed October 2026 · v1

Source: https://vitalitytracker.app/knowledge/activity-units