This app was mentioned in 8 comments, with an average of 1.13 upvotes
There is one called Move It! which works fairly well, though the detection is a bit wonky at times. It uses Google Fit basically, and that's not quite perfect at it's current stage...
There's also this other app called MoveUp! Sitting Alert for Wear, which unfortunately crashes as soon as I launch it on my phone.
I hope someone else has discovered a better app...
Edit: LinkMe got it wrong...
I have tried couple of these apps and even attempted to make my own, but in the end I have settled for Move It, most simplistic solution with good visualization: https://play.google.com/store/apps/details?id=uk.me.rga.moveit
It is only phone app (not Wear), but both steps and notifications are synced between phone and watch, so it is the same thing in the end (unless you want warnings be completely standalone without phone in range).
Please ignore the first link, the playstore bot gets it wrong every time.
Correct link: https://play.google.com/store/apps/details?id=uk.me.rga.moveit&hl=en
Also recently started using "Move It", which will bug you if your steps (pulled from Fit) in the last hour are below a set amount.
https://play.google.com/store/apps/details?id=uk.me.rga.moveit
Check out "Move It", it has nice Google Fit integration.
https://play.google.com/store/apps/details?id=uk.me.rga.moveit
Not that I disagree but a lot of work today requires a lot of people to sit in front of their screens for whatever reason. It's a good thing OP is aware of the drawbacks and wants to do something about it. If we could all be out in the fields lolling about without worrying about the next pay-check, we likely would.
Edit: Check these out -
Option 1 - https://play.google.com/store/apps/details?id=uk.me.rga.moveit&hl=en Option 2 - https://play.google.com/store/apps/details?id=com.mtmurdock.dailyreminders&hl=en Stretching - https://play.google.com/store/apps/details?id=info.tom4sM.TakeBreakFree&hl=en