Usual Exertion Doctrine?

Q

I was reading about something called the “usual exertion” principle in a workers compensation case that was before the Delaware Supreme Court a while ago. The case dealt with a roofer who injured his back while unloading a bundle of roof shingles. The roofer's petition for workers compensation benefits was denied by the Industrial Accident Board due to a finding that he had a preexisting, congenital back problem; upon appeal, the Supreme Court decided the roofer was entitled to compensation based on the usual exertion principle.