Is It Safe To Do Cardio Every Day?

Q. Conventional wisdom says I can’t work the same muscle group every day, so I don’t lift weights two days in a row. But how does this rule apply to cardio? Whether I run, bike, walk or use the elliptical trainer, I’m still using my legs—often quite strenuously. I change my cardio every day, but since it always uses the muscles in my legs, should I be doing cardio every other day instead of daily?

A. Good question. You’re probably not alone in wondering what the difference is between working your glutes and thighs while riding a bike or hiking a hill, and working them while doing a series of squats and lunges with a barbell. On the face of it, it may not make sense based on official exercise guidelines such as those from the American College of Sports Medicine or the USDA Dietary Guidelines. Both recommend that moderate-to-vigorous intensity physical activity (such as walking) be performed on “most days of the week,” while resistance training is recommended to be performed less frequently (once or twice a week for beginners, two or three days per week for experienced weight lifters, and up to four days per week for the highly-trained—but only if a split routine is used—resulting in the same muscle groups only getting exercised twice a week.)

The key difference here is the intensity of the muscle contractions used in cardio versus resistance exercise, and as a result, how hard the muscles are worked and the type of muscle fibers that are stimulated.

Cardiovascular activities, aka aerobic exercises, tend to move the whole body so that many muscle groups are worked and the activity is sustained for an extended period of time. The forces acting on the body tend to be light. As a result of these conditions, muscles generally are not overly challenged. Also, slow-twitch fibers are the predominant type of muscle fibers activated. Fat is the primary type of fuel used by this type of muscle fiber. Because of the low-to-moderate intensity of the exercise, and the fuel and muscle fiber types that are activated, you can walk, run or bike for hours (assuming you’re fit enough to do so).

On the other hand, resistance exercise, or weight lifting, tends to target a select group of muscles intensely. So, the muscle fibers (fast-twitch variety) are required to produce a high level of force. If resistance training is done properly (that is, if the weight used is heavy enough to make the muscle fatigue at the end of a set of repetitions), then a greater degree of glycogen depletion will occur in those target muscles. Glycogen is a stored carbohydrate used for fuel by muscle fibers. Because of the intensity of the exercise, the fuel used, and muscle fiber types that are activated, you can only perform resistance exercises for a few minutes. The muscles poop out because they are worked extra-hard, and the fast-twitch fibers aren’t meant to endure for long periods of time.

When muscles are challenged intensely (or “overloaded”) in this way, there is microscopic breakdown in the muscle fibers. During the subsequent 48 hours, the body mends these tissues and makes them more resilient. That’s how you get stronger. The weight-lifting exercise is the stimulus; the recovery is the strengthening period.

If you don’t give your body proper recovery time between bouts of weight lifting, the rebuilding processes may be hampered. (Other factors that influence recovery are getting enough sleep, drinking water and eating healthful foods.)

So, what if your cardio workout is super intense?

Running distances of 10 miles, or hiking up very steep hills, could,  theoretically, challenge the muscles to work much harder than they would during the normal cardio workout. A fit person may be able to handle this overload without becoming overly fatigued. But, an untrained person would have intensely challenged their muscles. In some instances, the activity may be extremely intense—such as when running a marathon or running a race at full speed or biking up hills. These types of intense workouts would definitely trigger more fast-twitch muscle fibers to work and result in greater glycogen depletion, similar to a resistance workout. And because of that, a tough cardio workout is probably self-limiting: You can’t run a marathon every day, plus perform super-intense cardio sessions. This would leave most people so sore that they’d naturally want to skip their workout the following day.

But, as I described earlier, the typical cardio workout is a different mix of machinery at play compared to weights workouts, and so muscle groups don’t typically get overworked. Therefore, it’s OK to do cardio every day. And, for a variety of reasons, including increasing one’s weekly calorie burn, it’s recommended that you do. It’s a good idea to cross-train, that is, vary the activities you do to minimize over-use of any particular muscles or joints. For more tips, read my book, Cross-training for Dummies.

Find all articles by Martica.

Do you have a fitness or weight-loss question for Martica? Send e-mail to experts@microsoft.com. Please include Ask Martica in the subject line. Each of our experts responds to one question each week and the responses are posted on Mondays on MSN Health. We regret that we cannot provide a personalized response to every submission.

Martica Heaner, Ph.D., M.A., M.Ed., is a Manhattan-based exercise physiologist and nutritionist, and an award-winning fitness instructor and health writer. She has a Ph.D. in behavioral nutrition and physical activity from Columbia University, and is also a NASM-certified personal trainer. She has written hundreds of articles for publications such as Self , Health , Prevention , The New York Times and others. Martica is the author of eight books, including her latest, Cross-Training for Dummies. (Read her full bio.)

More on MSN Health & Fitness:

• The Vending Machine Survival Guide
• Pump Up, Slim Down
• Why Skipping Exercise Can Be Deadly
• Bing Search: Best Cardio Exercises