> ## Documentation Index
> Fetch the complete documentation index at: https://docs.junction.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Heart Rate Zones

> Learn how Junction calculates heart rate zones using percentage brackets of maximum heart rate with configurable fallback birth dates.

## Heart Rate Zones Calculation

Heart rate zones are segments in a workout session. They are based on the percentage brackets of the user's maximum heart rate: under 50%, 50-60%, 60-70%, 70-80%, 80-90%, and above 90%.

The maximum heart rate for a person is calculated with a straightforward formula: `220 - age`.

## Fallback Birth Date

Some providers may not share the user's birth date, making it impossible for Junction to know the user's age. To solve this, users can provide a Fallback Birth Date as an alternative option.

You can specify the Fallback Birth Date when:

* [Creating a new user](/api-reference/user/create-user); or

* [Patching an existing user](/api-reference/user/patch-user)

You will also get information about the source (by slug) and the last updated time of the Fallback Birth Date [when getting an existing user](/api-reference/user/get-user).

Fallback Birth Date supplied via the REST API would always have a source slug of `manual`.

<Info>
  If no birth date information is available, an assumed age of 30 is used in the calculations.
</Info>

```jsonc Example theme={null}
{
  "user_id": "409d9870-21fb-443a-8daa-c5222659f40e",
  "team_id": "3aac677c-557f-40b7-9251-0315c1f48e77",
  "client_user_id": "d734e32e-dd43-4b77-ab56-692524279531",
  /** snipped **/
  "fallback_birth_date": {
    "value": "1980-09-12",
    "source_slug": "manual",
    "updated_at": "2022-09-11T13:45:56+00:00"
  }
}
```