The other answers nailed it -- different types of muscles -- but even so, if you do cardio every day for 8 hours a day, your body may still break down. If you d

The other answers nailed it -- different types of muscles -- but even so, if you do cardio every day for 8 hours a day, your body may still break down. If you do it for 16 hours a day, your body WILL break down. Hard labor is continuous cardio. Hard labor isn't healthy. Athletes have specific training regiments with up times and recovery times. If I go on a 60 mile bike ride, I need at least a few days downtime to recover. If I'm biking 20-30 miles, I'll do that at most every other day. And if I lift weights, I'll alternate upper body and lower body days. And if I'm doing basic calisthenics and stretching, I can do that every day. (Not that I do any of those anymore, at my age)