Can't receive continuous CoreMotion updates in background

I need to register accelerometer data from CoreMotion even when the app is in background. Reading other questions on this network I've found out that I have to:

1. Use CoreLocation to get location updates and requesting the authorization to run always (locationManager.requestAlwaysAuthorization())
2. Enable background modes in capabilities and check "location updates"
3. Set the NSLocationAlwaysUsageDescription key in the Info.plist file
4. Call locationManager.startMonitoringSignificantLocationChanges() when the app enters in background

But I still can't manager to get accelerometer data updates in background mode. I am sure of this because I save the data in an array (which lately stored in a database), and I tag every accelerometer data object with the timestamp. If I put the app in background by simply pushing the home screen and wait some seconds before going in foreground again, I can see that there is a time gap in the updates (e.g. skipping from 2.06 to 9.66). What am I doing wrong?