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Healthy women with normal pregnancies should exercise at least three times a week, being careful to warm up, cool down, and drink plenty of liquids. Experts advise, in general, that when exercising, the expectant mother's pulse rate should not exceed 70% to 75% of the maximum heart rate or more than 150 beats per minute. Fit women who have exercised regularly before pregnancy, however, may work out more intensively as long as no discomfort occurs. According to a new study, vigorous exercise may improve the chances for a timely delivery. During exercise, women should monitor their temperature to avoid overheating -- a side effect that can damage the fetus. (No pregnant women should use hot tubs or steam baths, which can cause fetal damage and miscarriage.) And all pregnant women should avoid high-impact, jerky, and jarring exercises, such as aerobic dancing, which can weaken the pelvic floor muscles that support the uterus. Walking is also highly beneficial. To strengthen pelvic muscles, women should perform Kegel exercises at least 6 times a day, which involve contracting the muscles around the vagina and urethra for 3 seconds 12 to 15 times in a row. Swimming may be the best option for most pregnant women. It involves no impact, overheating is unlikely, and swimming face down promotes optimum blood flow to the uterus. Overly strenuous exercise during pregnancy is not advocated, however, for women who did not exercise intensely before becoming pregnant. Many prenatal calisthenics programs are available. Effects on Colds and Flu.

People should avoid strenuous physical activity when they have high fevers or widespread viral illnesses, however. High-intensity or endurance exercises might actually suppress the immune system while they are performed. Although offering no evidence of improved immunity from exercise, one study reported that people who exercised as little as once a week in employee fitness programs averaged nearly five fewer sick days annually than those who did not participate in such programs. The immediate effect of exercise on the immune system is uncertain. Some highly trained athletes, for instance, report being susceptible to colds after strenuous events. A recent study suggested that in people who already have colds, exercise has no effect on the illness -- severity or duration of the infection. Gastrointestinal Problems.

Older people who exercise moderately may have a lower risk for severe gastrointestinal bleeding. Experts suggest that moderate exercise might even reduce the risk for some intestinal disorders, including ulcers, irritable bowel syndrome, indigestion, and diverticulosis.

Weight Loss. Exercise burns calories and can help individuals fight obesity. Nonetheless, a fit body will look more toned and be healthier. If caloric intake remains constant, regular workouts lead to weight loss. Effective weight loss means a long-term commitment to a regular program of vigorous exercise. Be forewarned, however, that the pounds won't melt off magically. It takes 35 miles of walking or jogging to consume the calories in one pound of fat. One recent study indicated that for obese patients, a few daily sessions for as short as 10 minutes each was effective in helping the patients adhere to an exercise program. Abdominal fat is a particular danger to the heart, although it is unknown whether doing crunches will specifically protect against heart disease. Abdominal crunches may help replace abdominal fat with muscle. To perform this exercise, the individual lies on the back with the head and shoulders raised; he or she contracts the stomach muscles, curling the torso slightly forward. Swimming is less effective than walking or cycling in reducing body fat, but overall regular aerobic exercise is a good way to shed pounds. People should be warned that without dieting, weight loss may be minimal with exercise alone, because dense muscle mass replaces fat as the body gets more fit. Contrary to popular belief, exercise does not increase appetite in people who want to lose weight; oddly enough, however, exercise improves appetite in people who are already lean.

Psychological and Emotional Benefits. Exercising may even enhance creativity and imagination. According to one study, older people who are physically fit respond to mental challenges just as quickly as unfit young adults. (Stretching and weight training appear to have no such effects.) Both aerobic and nonaerobic workouts have been shown to reduce depression. Aerobic exercise is linked with improved mental vigor, including reaction time, acuity, and math skills. According to one study, exercise was as effective for improving mood in people with clinical depression as some common forms of psychotherapy. One study found that teenagers who were active in sports have a much better sense of well being than their sedentary peers; the more vigorously they exercised, the better was their emotional health. Either brief periods of intense training or prolonged aerobic workouts can raise levels of important chemicals in the brain, such as endorphins, adrenaline, serotonin, and dopamine, that produce feelings of pleasure, causing the so-called runner's high. In one study, regular brisk walking cut in half the incidence of sleep disturbances in people who suffer from them. It should be noted that exercise in the evening, however, can cause sleep disturbances. Rhythmic aerobic and yoga exercises may be particularly helpful for combating stress, anxiety, and sleeplessness.

• What Are the Specific Benefits of Exercise?
• Longevity and Aging. Exercise, even after age 50, can add healthy and active years to one's life. Adding workouts that focus on speed and agility may be even more protective for older people. Flexibility exercises help reduce the stiffness and loss of balance that accompanies aging. Simply walking regularly can prolong life in the elderly.
• Moderately fit people, even if they smoke or have high blood pressure, have a lower mortality rate than the least fit.
• Studies continue to show that it is never too late to start exercising and that even small improvements in physical fitness can significantly lower the risk of death. Resistance training is important for the elderly, because it is the only form of exercise that can slow and even reverse the decline in muscle mass, bone density, and strength.

Cardiovascular Health (Heart Disease and Stroke)

Heart Failure. In one study, patients between the ages of 61 and 91 increased their oxygen consumption by 20% after six months by engaging in supervised treadmill and stationary bicycle exercises. Performing daily hand grip exercises may improve blood flow through the arteries of patients with heart failure. Traditionally, heart failure patients have been discouraged from exercising. Now, exercise is proving to be helpful for many of these patients and, when performed under medical supervision, does not pose a risk for a heart attack.

• Stroke. According to one analysis, a group of 11,000 men, men who burned between 2,000 and 3,000 calories a week (about an hour of brisk walking five days a week) cut their risk of stroke in half. The benefits of exercise on stroke are uncertain. Groups who burned between 1,000 and 2,000 calories or more than 3,000 calories per week also gained some protection against stroke but to a lesser degree. In the same study, exercise that involved recreation was more protective than exercise routines consisting simply of walking or climbing.
• General Guidelines. Inactivity is one of the four major risk factors for heart disease, on par with smoking, unhealthy cholesterol, and even high blood pressure. Children should be especially encouraged to exercise every day to prevent heart disease later in life. Exercise does not increase the maximum heart rate, but a fit heart can pump more blood at this maximum level and can sustain it longer with less strain. Exercise even helps reverse some of the effects of smoking. Children should be especially encouraged to exercise every day to prevent heart disease later in life. Like all muscles, the heart becomes stronger and larger as a result of exercise so it can pump more blood through the body with every beat. For preventing heart disease frequency of exercises may be more important than duration. The resting heart rate of those who exercise is also slower, because less effort is needed to pump blood. Effect on Coronary Artery Disease and Cholesterol Levels. People who maintain an active lifestyle have a 45% lower risk of developing coronary heart disease than do sedentary people. A recent study reported that moderate dietary changes improve cholesterol levels and so lower the risk for coronary artery disease only when an aerobic exercise program is also followed. Regular aerobic exercises -- brisk walking, jogging, swimming, biking, aerobic dance, and racquet sports -- are the best forms of exercise for lowering LDL and raising HDL cholesterol levels. It may take up to a year of sustained exercise for HDL levels to show significant improvement. Burning at least 250 calories a day (the equivalent of about 45 minutes of brisk walking or 25 minutes of jogging) seems to confer the greatest protection against coronary artery disease. Even moderate exercise, however, reduces the risk of heart attack, but in terms of raising HDL levels, more is better. Resistance (weight) training offers a complementary benefit by reducing LDL levels. Triglycerides, which rise after a high-fat meal, can be lowered either with a single, prolonged (about 90 minutes) aerobic session or by several shorter sessions during the day. One study indicates, however, that short-bursts of exercise actually increase LDL oxidation -- the process that makes LDL dangerous to the heart -- so individuals should always aim for a consistency in their exercise program. Before engaging in any strenuous exercise, it is advisable to consult a physician.

High Blood Pressure. No person with high blood pressure should start an exercise program without consulting a physician. Before exercising, people with hypertension should avoid caffeinated beverages, which increase heart rate, the workload of the heart, and blood pressure during physical activity. In one study, for example, moderate exercise (jogging two miles a day) controlled hypertension so well that more than half the patients who had been taking drugs for high blood pressure were able to discontinue their medication. Studies have shown that high-intensity exercise may not lower blood pressure as effectively as moderate intensity exercise. Sedentary people have a 35% greater risk of developing hypertension than athletes do. Studies indicate that regular exercise helps keep arteries elastic, even in older people, which in turn keeps blood flowing and blood pressure low. Studies have indicated that T'ai Chi, an ancient Chinese exercise involving slow, relaxing movements may lower blood pressure almost as well as moderate-intensity aerobic exercises.