In Industry we focus on repetitive motion stress. I cast 10,000 nine mm bullets for a friend using a 4 cavity cast iron saeco mold....pretty sure it messed up my physical structure doing so. But if you are casting cores there is no reason you cannot do like I do and cast some cores, then go over and do some swage work, then go back to cores, rotate around. that is what we do at work, job rotation every 2 hours within an area, seems to help a lot. When casting bullets I will often cast some, size some, go back and cast more...not even 2 hours at each task, maybe more like 30 minutes.