More advanced exercisers should shoot for at least 70 percent of their max heart rate, but don't go above 92 percent. At this point, most of us cross our aerobic threshold, according to a recent study by researchers at the University of Birmingham, England. Almost all your calorie burn comes from stored carbohydrates. After about an hour at that pace (depending on how many carbs you're storing), your muscles will run out of fuel, causing you to feel weak and fuzzy-headed – an experience athletes call "hitting the wall."
Body fat percentage
Without exercise, once you hit your 25th birthday you'll begin to lose lean muscle mass and replace it with fat at the rate of up to 3 percent per year. By age 60, an inactive woman might weigh the same as she did at age 20, but have twice as much body fat. Excessive body fat, especially in areas such as the abdomen, is increasingly recognized as an important risk factor for killers like heart disease and diabetes.
| Jun 26, 2009