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.