Basqiat

There's the Beast sensor (https://www.thisisbeast.com/en) You put the little sensor in a wrist band or chest rig, and as you do your exercise, the app measures the change and rate of change in distance between the sensor and your smartphone. 

So if doing a bench press, for example, you put the sensor in your wristband, put your phone on the bench, the floor, or your chest. Start the Beast app on the phone, choose your exercise (bench press), and the poundage, then commence with lifting. The sensor and app then use the speed of your lifts, the distance that you lift (the range of motion), and the poundage, to measure your effort. 

Or something like that  

If Apple wanted to make strength training accessories, they can simply license/ buy Beast.