Some people pride themselves on doing their pushups and crunches every day. Bad idea.
You don’t get strong during your workout. You get strong during the recovery period afterwards. Without rest, your body cannot recover, so without rest you don’t get stronger.
One sign of overtraining is lack of progress. The more you do, the worse your results. This is your body’s way of saying take a rest.
You should not do strength training on the same muscle group on consecutive days. Depending on the intensity of your workouts and your own body, you might need 2 or 3 days of rest.
You should also take a day off from cardio. Spend one day a week resting your body. And it will get stronger as a result.
Personal Trainer, Fitness Instructor since 1998.