Hi Rod,
If you want to calculate the max value per subject and muscle - it seems to me muscle is another dimension (not yet present in your model). It would also make sense to me to unpivot your data so each muscle/sensor would become an Attribute and then use DAX to calculate these new percentages.
Here are some items you might find interesting:
https://forum.enterprisedna.co/t/how-to-use-variables-in-dax-formula-with-power-bi/468
https://forum.enterprisedna.co/t/table-functions-filter/3291