This is like the software side of an idea I had maybe a decade ago. My idea would be to modify workout machines with sensors. Think of those Nautilus type pulley machines. You would swipe your card (magstripe, RFID, barcode, etc) when you sit down (start time recorded). Then you select the weight you want to lift (weight setting recorded). As you lift, various sensors would record the time per rep, range of motion, speed, etc. The idea would be that you don't have to waste time writing in a log, and the computer could capture data that you couldn't (like the fact that your last 4 reps only had 85% extension, vs your first 2 which had 98%). With sensors on all the machines in a typical gym (weight machines, treadmills, elliptical, etc.), you could get a full picture of how your workout went. Plus you could see how you progress over time. The challenge would be to get a big gym to allow you to wire up all of their machines, or work with a manufacturer to get them to include these sensors.