20-Minute Arm & Core Circuit
For interval workouts I use this very simple (and free!) Interval Timer app that allows you to set your work intervals, rest intervals, and the number of rounds. A buzzer (or whatever sounds effect you choose) signals each period, making it easy to track where you are in the workout. For this workout, you’ll set the work interval for :45 seconds, rest interval for :15 seconds, 5 rounds repeating 4 times.