The title says it all, I knwo for upper/lower body you should wait two days, or until soreness goes away to work on them again so you dont over use your muscles and deplete them. But what about core? can I do isometrics and crunches everyday without a loss?
It all depends. How much are you loading the muscles. Are you training for muscle endurance or strength. You can even train for maximum strength every day, but you will get better results by giving muscles a recovery day. The recovery day may not be a rest day. It may simply be a reatively light day for the muscles worked heavy the day before. If you are simply doing maintenance training, working out once a week is fine. Its quite common to stretch and do some pushups and situps each day and an easy run. But this would be a light work out. A number of pushups and situps that are rather easy to perform. This has more to do with maintaining general fitness and health, rather than athletic training. For health reasons its recommended to perform light exercise at least three times a week, but that's a minimum. Its heavy exercise of the same muscle groups that you don't want to repeat daily.