Suunto has had FusedSpeed in their Ambit, Spartan, and more recent watches for at least 8 years. Here is a video from 2012:
https://www.youtube.com/watch?v=qCpOK0L8o5E
Here is a summary from Suunto website:
FusedSpeedTM is a unique combination of GPS and wrist acceleration sensor readings for measuring your running speed more accurately. The GPS signal is adaptively filtered based on wrist acceleration, giving more accurate readings at steady running speeds and a quicker responses to changes in speed. FusedSpeed benefits you the most when you need highly reactive speed readings during training, for example, when running on uneven terrain or during interval training. If you temporarily lose the GPS signal, for example, Suunto 9 is able to continue showing accurate speed readings with the help of the GPS calibrated accelerometer.
The question is does Garmin has a similar technology implemented in their watches, and if it doesn't, why not?
