It's important to note that Nexercise must be the open app, but it does not prohibit you from running the other Apps in the background so you can get detailed run/cycling/etc tracking.
1) Open the other fitness app (Nike+, Runkeeper, MapMyRun, etc.)
2) Start the tracking in the other fitness app.
3) Hit the home button to push that app to the background
4) Open Nexercise
5) Tap Nexercise, and select your activity
6) Tap Start and do your activity!
7) Make sure to use the lock icon on the upper right corner of our app screen and not the power button!
It's really cool because you still will receive audio alerts from the other app running in background!
When your are finished, submit your Nexercise activity
Then you can toggle back to the other fitness app and stop it as well.
It's really that simple.
In our 1.6.4 release that's coming out soon, we've implemented Limited Multitasking, where during the activity you can double tap to switch out of Nexercise and go to the other fitness app to check data, such as your speed, where you are on the map, etc. Nexercise will continue to track for up to 90 seconds in the background.
This reply was removed on 2011-07-16.
see the change log