Why is 90 minutes of exercise per week the ideal amount?

All types of exercise, including aerobic exercise, stretching and resistance training, are great for your heart. In a general way, they help keep your weight down and increase your fitness levels, which makes you healthier overall. But the best type of exercise for your heart is aerobic activity.

Aerobic exercise, in which you increase your heart rate (such as when you jog, swim laps or move your couch to the other side of the living room), has three main benefits: First, it exercises your heart muscle directly. Like any other muscle, the heart gets stronger when you push it to work harder. Second, increasing your heart rate makes your heart pump more blood through your body, promoting good circulation and getting more oxygen to all your cells, including those in your heart. Finally, aerobic exercise burns the most calories, and studies have shown a direct correlation between the number of calories you burn and how much you improve your heart health.

So how much is enough? And how much is too much?

Researchers have studied all sorts of exercise levels and variables, from low exertion (like walking, gardening or golfing) at short intervals to high exertion (sprinting or swimming) for hours at a time. The bottom line is, any amount and level of exercise for any amount of time is good for your heart, as long as your doctor says you can handle it. It's important not to overdo it -- pulling a muscle or pushing your heart too hard is not going to improve your health much. (If you have chest pain or feel dizzy, and especially if you notice swelling in any part of your body, stop and call your doctor.) But while there's a broad range of heart-healthy exercise regimes, there are certain guidelines you can use to attain your maximum benefit.

Twenty minutes of aerobic exercise, three or four times a week, at moderate exertion will get you significant benefits [source: MedicineNet]. Moderate exertion is when your pulse and breathing rate increase, but you can still talk comfortably -- you're not out of breath. Jogging and jumping rope are good examples. On the higher end, the Center for Disease Control (CDC) recommends 30 minutes a day with only a couple of days off each week, or about 150 minutes per week. For someone who doesn't love exercise but is looking to significantly benefit his or her heart, the ideal amount is probably somewhere in between: 30 minutes a day at least three days a week. At 90 minutes a week -- and it doesn't seem to matter whether you do 30 minutes at a time or two 15-minute sessions a day -- you can help your heart get stronger, provide more oxygen to your cells, lower your blood pressure and improve your cholesterol numbers. It's a manageable amount of activity, too, so it's easier for people to maintain, which is key to getting the heart benefits you're after.