It was suggested to add a large "Done" button, which would have the same effect as checking the checkbox of the first non-completed set of that exercise.
A couple of reasons:
  • Checking the first empty box is the most common action on that screen.
  • Users are used to finding a big button to complete the current set in other apps, so that's a familiar pattern.
There might be confusion about having two different elements on the same page that lead to the exact same result (the checkbox and the new button). One way to mitigate that could be to have a visual link between the first uncompleted set and the large "Done" button, so that we understand that the button is gonna affect that set before even clicking on it. For example there could be a color element that is repeated in both places.