Work out app on my apple watch keeps pausing

my work out app keeps pausing on my apple watch when the screen shuts off after about 45 seconds. I'd like to find out how to fix. I can't use this watch for work outs at all if it keeps doing this. Only time I got longer then that I had to hold up my arm and touch the screen every 20 seconds to keep it from going off. That kind of makes a work out hard to do and shouldn't have to do this to keep it going.