In 2012, I co-founded this startup in college and ultimately took a few years off to try to bring it to life.
We were using Machine Learning to determine the best workout based on the equipment you have at your disposal. We used a combination of the weight you used, the difficulty of each rep, your time and rep counts on ongoing exercises, and your macro-numbers like weight and body fat over time.
After working on the beta for over a year we launched and got on the front page of Yahoo! gaining over 15,000 users in the first month. We had a subscription service that we had to build manually before Apple even allowed subscription services in the app stores.