I might suggest an alternate. Allow a user to mark an area of their body as injured, and the app asks the user if they’d like to skip that body part entirely or substitute some PT for that body part instead 

I use an Apple Watch. Would be nice to be able to start and end my workout on my watch and log reps as I go.

it would also be nice if it was also collecting heart rate volume at the same time, and calculated an estimated calorie burn based on both heart rate data and total volume lifted.

that said - it would still need to sync in real time with a phone so that more details could be referenced on the phone (like previous 1RM, videos, etc)