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Official Description

Stravomatic starts (and stops) Strava rides and runs when it detects you are running or bicycling, and you forgot to start it.

No more missing traces from your Strava Activity feed!

It uses the accelerometer instead of GPS to avoid draining the battery, and to distinguish between running and cycling.

The detection parameters, and the various states of sleep that the phone can be in, will delay the activity start and your activity may be shortened by a few seconds to a few minutes at the start, depending on the settings you choose.

Frequently asked questions: https://www.colino.net/wordpress/en/stravomatic-an-app-to-start-strava-automatically/