IntervalTimer
Fitness timer for android
Main features
You can create different timers with different work/rest times and names, to practice different exercises. For instance 4 sets by 45/30 for regular plank, and 6 sets by 30/30 for side plank.
Disclaimer
This is an educational project which I've done for my self, use at you'r own risk. It's not at Google Play Store, build and install it manually.