My Apple Watch keeps pausing during my exercise routine.

You may be pushing in the crown and the side button at the same time when you flex your wrist.

Try reversing the watch and wearing it with the crown towards your elbow instead of towards your wrist.

Change the orientation from your iPhone>Watch>General>Watch Orientation>

Tighten helps it read the heart rate (otherwise its grey) but keeping your heart rate above the expected minimum (220-age)*.7 still gives the pause ? message. So there is another reason.

which did you buy

Tighten the watch band a touch and don't wear the watch on top of any tattoos.

As the other post in this thread suggested,you could try this;