More than half of the average person’s waking hours are spent sitting: watching television, working at a computer, commuting, or doing other physically inactive pursuits. But all that sitting could be sending some to an early grave. That’s the conclusion of a Canadian study published in this week’s Annals of Internal Medicine. People in the study who sat for prolonged periods of time had a higher risk of dying from all causes — even those who exercised regularly. The negative effects were even more pronounced in people who did little or no exercise. In addition to premature death, the study documented higher rates of type 2 diabetes, cancer, and cancer-related deaths in very sedentary people. If you sit for work, try standing or moving around for one to three minutes every half hour. Better yet, think about working at a standing desk. At home, stand when watching TV or talking on the phone.

Even if you don't swim, exercising in the water can improve your strength, balance, and cardiovascular fitness. And it's easier on your joints.

Quick-start strength-training program

Work out at home or in your office to improve muscle mass.

Muscle mass tends to diminish with age, so making strength training part of your overall exercise regimen is very important. And while many people assume that effective strength-training programs require joining expensive fitness clubs and lifting heavy weights, there are plenty of strength-building exercises you can do at home and at work with little or no special equipment. "You really can strength train without the big machines and get effective results, without spending a lot of money," says Elissa Huber-Anderson, a physical therapist at Harvard-a-ffiliated Massachusetts General Hospital.

Try some low-impact activities to ease into an exercise routine.

You know that regular exercise is good for your heart. But only about half of American adults manage the 2.5 hours per week of moderate exercise (such as brisk walking) recommended by the federal physical activity guidelines. Being too busy is a common excuse, but there are bona fide reasons for not exercising. If you're recovering from a heart attack, are overweight, or are simply out of shape, 30 minutes of exercise a day may feel out of reach.

Q. What is better for my heart—a fast run on the treadmill for 10 minutes or 30 minutes of light cycling on a machine?

A. The benefit of exercise on heart health depends on the dose; in other words, the more the better. So, my answer to your question is "Why not do both?" Exercise of virtually any kind prevents heart attacks, strokes, high blood pressure, and diabetes.

Strength or resistance training challenges your muscles with a stronger-than-usual counterforce, such as pushing against a wall or lifting a dumbbell or pulling on a resistance band. Using progressively heavier weights or increasing resistance makes muscles stronger. This kind of exercise increases muscle mass, tones muscles, and strengthens bones. It also helps you maintain the strength you need for everyday activities — lifting groceries, climbing stairs, rising from a chair, or rushing for the bus.

The current national guidelines for physical activity recommend strengthening exercises for all major muscle groups (legs, hips, back, chest, abdomen, shoulders, and arms) at least twice a week. One set — usually 8 to 12 repetitions of the same movement — per session is effective, though some evidence suggests that two to three sets may be better. Your muscles need at least 48 hours to recover between strength training sessions.