If you made a New Year's resolution to exercise more, it's helpful to know the role gender and age play in your success. For example, the amount and type of exercise that most benefits a woman changes through the decades because of physiologic changes and her exercise goals.

Women in their 20s typically focus on cardiovascular exercise, which is good. But they should also incorporate strength or weight training to help build muscle definition and bone density, which is crucial to helping prevent osteoporosis later in life.

Many women find that they gradually gain weight as they age. That is due, in part, to the fact that women start losing lean body mass (muscle) beginning in their 20s, which affects metabolism. The lower your lean body mass, the fewer calories you need, so strength training to build muscle also can be beneficial in helping keep off extra weight.

After a woman reaches menopause, her body starts losing estrogen, which causes the muscles to lose strength. Muscle loss can affect many things including posture, balance and mobility. Strength training continues to be important, as well as flexibility and balance exercises. Several studies have shown that women can build muscular strength even into their 90s, so it is truly never too late.

The many benefits of exercise also include fighting disease and helping maintain your overall quality of life. In addition to your physical activity, you should look at the amount of time you spend sitting. A study recently published in the American Journal of Epidemiology found that more leisure time spent sitting was associated with a higher risk of death, particularly in women, regardless of physical activity levels. Researchers found that women who sat for more than six hours a day had a 37 percent increased risk of premature death compared to 18 percent for men.

If you didn't already pledge to be more physically active this year, these are some good reasons to add it to your list of resolutions.

Dawn DuBois is an exercise physiologist and a certified health fitness specialist with St. Luke's Hospital. Visit stlukes-stl.com. XX Files, a women's health column, rotates each week with Aging Successfully, a column for older people by Dr. John Morley of St. Louis University.