Stretchly is described as 'Cross-platform open source app that reminds you to take microbreaks and longer breaks, with customizable intervals, durations, sounds, color schemes, and strict mode' and is a very popular Prevent RSI app in the office & productivity category. There are more than 50 alternatives to Stretchly for a variety of platforms, including Mac, Windows, Linux, Android and iPhone apps. The best Stretchly alternative is Workrave, which is both free and Open Source. Other great apps like Stretchly are Loook, Safe Eyes Linux, BreakTimer and ZenBreak.
Stretchly is an app to help you take breaks when working, not an exercise app