How many parts of your body you should work out in a day depends on your workout goals, the kind of exercise you are doing, and even your daily schedule. Although full-body workouts can be effective, seasoned bodybuilders, including Arnold Schwarzenegger, and celebrity personal trainer Bill Phillips agree that rest between workouts is at least as important as the workouts themselves.
Many cardiovascular exercises, such as swimming and group aerobics, work all the muscles of the body. However, the muscle workout for cardio exercise isn't particularly intense. According to Oregon-based fitness coach Ben Cohn, it is alright to participate in cardiovascular full-body workouts on consecutive days, so long as you take a day or two off each week.
Resistance training builds muscle by targeting specific muscle groups for overload. Weight training is the most iconic example of this kind of workout. Unlike the impact of cardiovascular workouts, resistance exercise intensely impacts the muscles you work. Because of this factor, the number of muscles you train in a day is most important when scheduling resistance workouts.
How many muscles you train in a day is your choice, but how much rest you give muscle groups between training sessions is critical. According to Stuart McRoberts, author of the landmark body building guide "Brawn," you should avoid training any particular muscle group two days in a row. Muscle growth occurs as the muscles damaged during training rebuild and heal, which is a process that requires rest. You can work out all the muscles in your body -- this is, in fact, a common training technique. However, if you do so, you should take the next day off from resistance training altogether.
Fitness coaches and personal trainers suggest several training patterns to ensure adequate rest. In "Brawn," McRoberts recommends dividing your training into "A" and "B" days, alternating between them. On "A" days, work out half the muscles of your body, and on "B" days, work the other half. This means you hit the gym every day, but the alternating timing gives all muscles a full day of rest between workouts. Bill Phillips recommends getting three full-body resistance workouts each week, and doing light to moderate cardio on the days you don't do resistance training.