Cardiovascular exercise is considered doing anything from walking your dog to skating in a full speed ice hockey game. Targeted cardio is cardio done for the purpose of burning fat and retaining muscle. For most people, this can be done by taking a jog in the park or on the treadmill at your local gym. But questions normally arise as to how fast should I run and for how long? Should I eat before I run? Should I take breaks?
Using cardio to target fat loss and muscle preservation is more a matter of timing instead of intensity and duration. Targeted cardiovascular exercise for fat loss has strict guidelines for intensity depending on factors such as what time and if you do resistance training, what time you eat your meals and how many hours of sleep you get a night.
For maximum fat loss and muscle preservation, cardio is most effective when performed at 65-75% of your maximum heart rate (220-age) for 30-60 minutes. When performed at this medium intensity, your body is less likely to burn carbs (and protein) as energy, and will turn towards fat for energy. Performing cardio at higher intensity (75-90%) will switch your body to burn carbs (glycogen) for energy to compensate for the greater requirements by the muscles as well as the cardiovascular system. Doing short interval training (less than 10 minutes) at higher intensity (75%+) will lower the demand by the muscles and burn more fat, since your heart rate will drop to the 65-75% range during rest and including up to 2 hours after high intensity training is done. More on this later.
Since macronutrient processing is so important for energy, it is important to make sure that you are doing cardio at the right time in relation to your nutrition consumption and resistance exercise is extremely important. The body burns dietary macronutrients (calories we eat) in the order of carbohydrates, fats then protein. Doing cardio at 65-75% of MHR on an empty stomach will be most effective at burning fat. The medium intensity will have your body looking for fat as energy and on an empty stomach no dietary macronutrients will be present, so the body will turn to stored fat as its source of energy for the cardiovascular system. This environment mostly can be achieved right when you wake up and your body is in a fasted state, since you haven't eaten for around 6-8 hours (depending on sleep and nighttime meal consumption). Doing 30-45 minutes of cardio (depending on where you are in your diet/exercise routine), will burn fat stores as energy, not muscle protein.
Another good time to achieve the fasted fat burning state is after a weight training session. At this time, your body has used up all the dietary carbohydrates and fat from your pre-workout meal as energy during the resistance training of higher intensity. Now doing medium intensity cardio will keep your heart rate elevated, but will switch your body to target fat stores as energy since all carb stores are exhausted.
Word of Warning -
Doing cardio higher than 75% MHR will have your body looking to use carbs for energy. If this cardio is done while your body is in a fasted state (no dietary carbs to burn), the body will look for carbs in another form - glycogen. The only problem is that glycogen is found in muscles and burning muscle glycogen is a catabolic process, IT IS BURNING MUSCLE!!! This is not good, but there is some good news for people who don't have a choice but to exercise at high intensity (sports players, etc...) developing a defensive nutritional plan around your exercise can help keep muscle and while giving you energy during the high intensity exercise. Since you know your body will be craving carbs when your get your heart rate above 75% (normally when you start breathing heavy), it is good to supplement your body with extra carbs before and during the exercise. A pre-workout shake with plenty of complex (slow absorbing) carbs will provide your body with a constant stream of energy that your body can use instead of stored muscle glycogen. You won't necessarily be burning fat, but you will not be burning muscle! Actually, as stated above, when doing high intensity cardio exercise above 75% MHR, your heart rate will stay elevated up to 2 hours in the good fat burning zone, thus burning fat after the fact.
Another good way to prevent muscle breakdown during high intensity activities where you can control the duration is to do the exercise at intervals. This is why weight training can be considered a fat burning activity. During weight training, you elevate your hear rate for only a specific period of time (seconds). Then it lowers back into the good range while you rest. With weight training, you are stressing the muscles more than you would during a fast sprint or other high intensity interval activity, so the activity is, in itself catabolic. So it is important after doing any kind of high intensity cardio exercise, to implement post-workout supplementation (see Nutrition Tips for Anabolic Nutrient Timing Factor).
Depending on what time you work out, and your eating schedule, you could do cardio anytime during the day or night. Just plan out a schedule and listen to you body and know what's going on with your nutrition levels. Preparation is the first step towards success in Health and FitnessCardiovascular Exercise Timing Effectiveness