I have downloaded about five-seven plank apps and this is the one I am sticking to. Main thing - you can choose the exercises and how long you do them. Four stars instead of five only because of a small bug - when you change the interval between exercises you can not go back to the app, you have to shut it down and load again. Another desired feature - for the app to be suggesting new lengths of app once I reach a certain level but this can be done manually, so its a minor issue.
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