The short answer:
[not(ActivityDone) and System.owner = '[%CurrentUser%]']
Since you probably want the same for WorkoutMaster, have Mendix do the work mostly for you: go to the domain model, right click, select ‘Generate Overview pages’ for master and tracker, openWorkoutMaster_Overview and modify it's Datasource to match this:
Then open WorkoutMaster_ and add a Datagrid to the Dataview (getting off topic now)
Open WorkoutTracker_Overview and change the Datasource to;
And you're done.
Here is the retrieve object…
Here is the full microflow needed
If you have a better more efficient way of counting records retrieved
Doing a loop for the list found using the retrieve object does work.