In Cthulhu - use CON rolls to determine ability to engage in strenuous activity. If failed, impose penalty die to skill roll. If failed again (slightly later), then two penalty dice. Longer period, change this to Hard CON, then Extreme CON. After two failures (not matter the difficulty), then the task becomes impossible and First Aid/Medicine is probably required.
The main question becomes, does this help and push forward the story or simply act to stop players doing anything? If the latter, I would consider reworking so it fits the former.