I am going to do an activity that i cannot wear my apple watch for. Is there another way I can keep my 'Activity Rings' accurate?

I often play football as part of my weekly activities, however cannot wear the apple watch during this time as it could easily get damaged/ cause an injury. Is there a heart rate monitor that would be compatible with the watch, that would keep the activity rings more accurate? Currently, football isn't registered as i'm not wearing the watch, so it looks as though i have done no workout that day.