We all know physical fitness is crucial. But how many days weekly should you work out?

Though few would deny the importance of physical exercise or working out, less of us may understand how often we're supposed to hit the gym or run on the treadmill at home.

It's also useful to understand the benefit of rest days and of alternating the kinds of exercise we do each day - be it cardio, weightlifting or targeting different muscle groups with a variation of each workout. "Strength is not the only goal in working out for most individuals, and people's priorities vary," explains Loren Fishman, MD, a professor of physical medicine and rehabilitation at Columbia University. 

No matter how you choose to exercise, here's how much of it you should be doing.

How many days a week should I work out? 

The first step towards deciding how many day to plan to work out is to understand how many minutes per week you ought to be engaging in physical fitness. 

The Physical Activity Guidelines for Americans published by the U.S. Department of Health and Human Services (HHS), says that for optimal health benefits, adults should engage in at least 2 hours and 30 minutes to 5 hours of moderate-intensity fitness each week OR 1 hour and 15 minutes to 2 hours and 30 minutes of vigorous-intensity fitness each week. The guidelines add that "adults should also do muscle-strengthening activities of moderate or greater intensity that involve all major muscle groups on 2 or more days a week." 

The Centers for Disease Control and Prevention (CDC) provides 3 different examples of how you could meet these recommendations:

• Engage in a moderate aerobic activity such as brisk walking for 30 minutes a day for 5 days a week AND work all major muscle groups (legs, hips, back, abdomen, chest, shoulders and arms) 2 days a week. 
• Engage in a vigorous aerobic activity such as jogging or running for 75 minutes spread across 2 days a week AND work all major muscle groups 2 days a week. 
• Engage in an equivalent mix of moderate- and vigorous-intensity aerobic activity on 2 or more days a week AND work all major muscle groups 2 days a week.

For children ages 6-17, the recommendation is to get in at least 60 minutes of "moderate to vigorous" physical activity every day. 

The overall goal for children and adults alike is to be as active as possible. "Be active as many days a week as you can - at least 3 days or more," advises Barbara Olendzki, associate professor of population and quantitative health sciences at UMass Chan Medical School. "Even short periods of time during the day can add up and have health benefits," she adds. This includes decisions like taking the stairs instead of the elevator, parking further away from your destination, or getting in smaller bouts of exercise when you can. "Sitting less is also a great idea," she says.

What are the benefits of rest days? 

For people who choose the upper-level recommendation of getting in closer to 5 hours of exercise each week, or for those who are spending a lot of that time lifting weights, the experts say to remember the importance of rest days. "Resting for a day between workouts allows the muscles to undergo proper healing before putting them under stress again - this will maximize your ability to build muscle," says Trevor Delaney, PT, DPT, a certified primary spine practitioner and a physical therapy program director at the University of Pittsburgh Medical Center. 

And even if you are only engaging in cardio, "if you are new to exercise, giving your cardiovascular system rest is also crucial to avoid stressing the system too much," Delaney adds. Having rest days built in can also prevent burnout. "This is something I am sure we are all familiar with whether starting a new exercise program or a new diet," he says. "You start off with the best of intentions and before you know it, you stop the program completely. Rest days can decrease the likelihood of this." 

What types of exercise should I alternate between? 

One of the most attractive elements of the HHS and CDC physical activity recommendations is that it provides total flexibility for a variety of schedules and lets you decide between a multitude of workouts and exercises.

When hitting the gym, for instance, many people like to alternate between the muscle groups they target on different days. In a 2-day split, one might work on upper-body muscle groups like the back, abdomen, chest, shoulders, and arms on one day and then target lower-body muscle groups like the glutes, hips and legs on the second day. "The best ways to alternate different kinds of strength training is to have a system," suggests Fishman.

There are also many good options to choose from when it comes to cardio activity. Walking, running, cycling, swimming, or playing team sports like basketball, volleyball or pickleball can all be good ways of getting in some moderate to vigorous physical fitness. "Mixing in Pilates, yoga and high-intensity interval training (HIIT) can also be very beneficial," says Delaney. "In general, mixing different types of exercise is important to maximize your benefit and enjoyment." 

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