Ultimate Plank Workout (com.workout.ultimate.plank) on Google Play
The ultimate plank app - a lot of default plank workouts to choose from - create your own workout combining exercises from a big list - counter for the time of each position with sounds to change it - you always know the next position so be prepared for it. How to use it - put the phone on the floor - stay in plank - stay with an eye on the phone to know when to change the position and how much time you need to hold