Ask HN: Do you know any iPhone app that can track inactivity time?

I have a watch as a fitness tracker that I wear all day long. It tracks my steps pretty well. But I can't find any app on iphone that can sum up my inactivity time in minutes, I mean the total time I am sitting or in stationary position during a day (aka sedentary time). Do you know any app that can do that ?

I can't figure out why any of the fitness apps have it, even if it is not 100% accurate.