High heart rate notification while exercising

I’m not supposed to get my heart rate above 120-130 while exercising. Apple’s own software will only notify me of a high heart rate only when resting, so that does not help me. I cannot find an app that lets me do one simple thing: set a BPM and notify me if I’m hitting it. Surely there has to be one out there?! Googling it only brings up instructions on how to set the built in version that is for resting only. It’s extremely frustrating as I’d like to get back to being active but know that I am within the safe limits my doctors have advised. I can’t believe there is no way to se this on the Apple Watch .