Fitness

Overview

If a friend told you that delaying the aging process, controlling your weight, feeling happier and less anxious, sleeping better and warding off illnesses like heart disease, some forms of cancer, high blood pressure and diabetes was as easy as walking briskly for 30 minutes each day, would you believe her?

It’s true. You can receive all these benefits simply by taking that 30-minute daily walk. According to the Centers for Disease Control and Prevention, inactive nonsmoking women have an estimated 12.7 years of active life expectancy at age 65, compared with 18.4 years for highly active, nonsmoking women. The American Heart Association reports that a sedentary lifestyle is a major risk factor for heart disease and stroke. Studies find that people who start a regular physical exercise program after a heart attack have better rates of survival and an improved quality of life compared with those who remain sedentary. And people who do not exercise have a risk of coronary heart disease double that of people who exercise regularly—an increased risk similar to that caused by high cholesterol, high blood pressure and cigarette smoking.

In addition, being sedentary has several negative health consequences. Your muscles, including your heart and lungs, become weak; your joints become stiff and easily injured; and you can develop high blood pressure, fatigue, obesity and osteoporosis. Lack of physical activity can also contribute to anxiety and depression. Being physically fit, on the other hand, reduces the risk of heart disease, some forms of cancer, high blood pressure, diabetes and other diseases. Exercise may also reduce bone loss after menopause.

What’s missing in this age of modern conveniences and desk jobs are reasons to get our bodies up and moving regularly.

That’s why the U.S. Department of Health and Human Services’ National Center for Health Statistics reports that about 40 percent of American adults report no leisure-time physical activity, and only 31 percent of adults say they engage in regular leisure-time activity (defined as either three sessions per week of vigorous physical activity lasting 20 minutes or more, or five sessions per week of light-to-moderate physical activity lasting 30 minutes or more).

The good news is that it’s never too late to take up exercise. At any age, at any level of health, even if you already suffer from a chronic disease, you can improve your level of fitness. Plus, if walking isn’t your cup of tea, there are endless options, all with similar results.

What, exactly, is fitness? Physical fitness has the following components:

  • Cardiovascular fitness. Your level of cardiovascular fitness determines your body’s ability to use oxygen to help provide energy. It provides the stamina or endurance to be active without gasping for breath.
  • Muscular strength. Muscular strength is the ability of your muscles to exert force during an activity.
  • Muscular endurance. Muscular endurance is the ability of the muscle to continue to perform without fatigue.
  • Flexibility. Flexibility refers to maintaining an optimal range of motion in the joint areas, making bending and stretching easy.
  • Body composition. Body composition refers to the ratio of lean muscle tissue to fat (also known as adipose tissue).

Diagnosis

You should discuss your fitness level with a health care professional before you try to improve it on your own, especially if you haven’t been active in awhile, have any chronic health conditions or are over 50 years of age.

During a fitness assessment, your health care professional should ask you about chest pain, faintness or dizziness, bone or joint pain and about any medications you’re taking. The health care professional should check the health of your heart and joints, measure your blood pressure and weight and determine if you have a hernia or diabetes. These issues may affect how vigorously you may exercise or the types of exercise you can safely do. If you have heart disease or any risk factors for heart disease, you may need to undergo an electrocardiogram while exercising, commonly called a stress test. During this test, you walk on a treadmill while the health care professional monitors your heart and blood pressure.

Sometimes, your health care professional is the one suggesting a fitness program. This is a good option if you have high blood pressure or high cholesterol, are overweight, have a high percentage of body fat or are losing bone density (a precursor to osteoporosis).

Your health care professional also may recommend a fitness regime if you have or are at risk of developing one of a variety of chronic conditions, such as diabetes.

Treatment

The first and easiest change to make on your journey to fitness is to add “lifestyle physical activity” to your day. This means being more physically active during your usual daily activities. You can:

  • park in a far-away spot and briskly walk to your destination
  • take the stairs instead of an elevator
  • rake leaves instead of using the blower
  • play tag with the kids instead of computer games
  • go golfing, bowling or dancing for fun
  • walk down the hall instead of using the phone or e-mail
  • take a walk during a morning or afternoon break.
  • do indoor chores such as window washing, tub scrubbing or reorganizing your closet
  • do active outdoor chores, such as mowing the grass, gardening or washing the car

Making these changes is an easy way to improve mood, heart and respiratory function and muscular fitness, as well as to reduce body fat.

However, for women who need to make more dramatic gains in fitness or need to lose weight, a more formal exercise program, in addition to lifestyle physical activity, may be necessary. Your program should address the five components of fitness by including:

  • Aerobic activities, which involve using the large muscles of your body in a rhythmical, continuous activity, improving cardiovascular conditioning and helping reduce body fat. Aerobic exercises include walking, jogging, bicycling and swimming, as well as aerobics or other exercise classes or videos.
  • Strength training, such as weight lifting. This improves muscular strength and endurance and helps maintain bone density. It also raises metabolism, helping you burn more calories.
  • Stretching exercises, which include slow, gentle movements that elongate your muscles and improve flexibility. These are often part of exercise classes or videos, as well as yoga and Pilates.

How Much Is Enough?

One of the most common questions is, “How much do I need to exercise?” The Centers for Disease Control and Prevention’s 2010 Physical Activity Guidelines for Americans recommend that adults do both aerobic and strength training. Specifically, the guidelines recommend the following:

  • A total of at least 150 minutes of moderate-intensity aerobic activity (such as brisk walking) per week, which translates to about 30 minutes per day, five days a week, coupled with muscle-strengthening activities that work all major muscle groups on two or more days a week or
  • A total of at least 75 minutes of vigorous-intensity aerobic activity (jogging or running) every week and muscle-strengthening activities that work all major muscle groups on two or more days a week or
  • An equivalent mix of moderate- and vigorous-intensity aerobic activity and muscle-strengthening activities that work all major muscle groups on two or more days a week.

The guidelines also recommend that adults perform muscle-strengthening activities that involve all major muscle groups at least two days a week.

The new guidelines note that adults can achieve additional health benefits, including the promotion of greater weight loss or the prevention of weight regain, by increasing to five hours (300 minutes) a week of moderate-intensity aerobic physical activity or two hours and 30 minutes a week of vigorous-intensity physical activity or an equivalent combination of both.

These minutes can be accumulated in increments of 10. For instance, 10 minutes of an aerobics video in the morning, 10 minutes of brisk walking at lunch and 10 minutes of brisk walking in the evening. Intermittent exercise (intermittently increasing the heart rate) can be part of a good weight-loss strategy because your metabolism is elevated following each bout of exercise.

If you have been inactive, you need to work up slowly to this amount. Start with five or 10 minutes—whatever you’re comfortable with—every other day and add one minute every other session. If you do too much too soon, you can become injured, fatigued and discouraged. You can know that you are not pushing yourself too hard if you feel recovered by the next day.

Similarly, don’t overdo strength training. Start slowly, with lighter weights, and work up to heavier weights. You don’t need to strength train more than a couple times per week. Finally, always wait at least 48 hours before exercising the same muscle group to give those muscles adequate time to recover between sessions.

Ideally, you should warm up the muscles that you plan to use for the activity. This can be done by starting your walk or activity slowly for two to three minutes and then increasing to a brisk walk or increasing the intensity of the activity. It is also helpful to stretch any muscles and joints that you routinely use at work or play a couple of times per week. Hold each stretch for about 30 seconds. Some lighter stretches can even be done at your desk or while you watch TV. Examples of stretching exercises include shoulder or arm circles. There are also a number of stretches specifically targeted to arm, back, chest, thigh and calf muscles.

How Hard Should You Work?

The second question is, “How hard do I need to exercise?”

As you work on increasing the length of your exercise sessions, you also need to work on increasing their intensity. Low-intensity aerobic exercise, like housework, gardening and walking the dog, provide many general health benefits, but to truly enhance fitness, especially if weight loss is one of your goals, you need to up the ante and exercise at a moderate or higher intensity with vigorous activities like brisk walking or jogging, singles tennis, aerobics classes or cycling.

In fact, results from a University of Pittsburgh study published in the Journal of the American Medical Association found that women trying to lose weight can benefit as much from moderate-intensity physical activity as from an intense workout. The exercise duration and intensity trial involved 201 overweight, healthy women ages 21 to 45 years. All received reduced-calorie meals and were assigned to one of four physical activity regimens.

The regimens consisted of either a moderate or vigorous-intensity physical activity performed for either a shorter (2.5 to 3.5 hours per week) or longer (3.5 to 5 hours per week) duration. The physical activity consisted primarily of brisk walking that burned between 1,000 and 2,000 calories a week.

Women in all four groups lost between 13 and 20 pounds, or 8 percent to 10 percent of their body weight, and maintained that weight loss for a year. They also improved their cardiorespiratory fitness. But, most importantly, the amount of weight lost and fitness improvement was essentially the same among the four groups.

The author concluded that an intervention program should initially target the adoption and maintenance of at least 150 minutes a week of moderate-intensity exercise, and, when appropriate, eventually progress to exercise levels of 60 minutes per day, most days of the week. This upper level is consistent with the Centers for Disease Control and Prevention’s 2010 Physical Activity Guidelines for Americans’ recommendation of a total of 300 minutes per week (60 minutes per day, most days of the week) for greater weight loss and the prevention of weight regain.

Because the goal of aerobic exercise is to work your heart, your exercise needs to increase your heart rate. One way to determine if you are exercising intensely enough is to measure your heart rate. Your heart rate should be about 50 percent to 85 percent of its maximum. Maximum heart rate for one minute is your age subtracted from 220.

After warming up and then sustaining an aerobic activity for about five minutes, take your pulse by placing two fingers on the radial artery on your wrist (it will be toward the thumb side of your wrist). Count the beats for 10 seconds. The number of beats you count should fall between the two numbers listed beside your age in the chart below. The following chart illustrates recommended 10-second heart rate counts.

Age Number of beats in 10 seconds
20 17 to 28
30 16 to 30
40 15 to 26
50 14 to 24
60 13 to 23

Older adults should exercise as often as others but aim for a lower number of beats per minute. To determine exactly what your heart rate should be during exercise, subtract your age from 220; divide that number by six for a 10-second heart rate count, then multiply that number by 0.5 for the lower end of the range and 0.85 for the higher end. For example, if you’re 70 years old:

220 – 70 = 150 (this would be your maximum heart rate for one minute)

150 / 6 = 25 (this would be your maximum heart rate for 10 seconds)

25 x 0.5 = 12.5 (this would be your target heart rate for 10 seconds at the lower end of the range; for one minute, the target would be 75 beats)

25 x 0.85 = 21 (this would be your target heart rate for 10 seconds at the higher end of the range; for one minute, the target would be 126 beats)

An easier way to judge intensity is the talk test. You shouldn’t be exercising so hard that you can’t talk with a friend or recite a poem. If you can’t talk without gasping for breath, slow down.

If you take medications for high blood pressure, your heart rate may be kept artificially low, and intensity should be monitored using the talk test.

The intensity of your strength training exercise will increase over time as well. Again, don’t strain to do more, but slowly work your way up to heavier weights or more repetitions. Choose weights that are heavy enough to tire your muscles after about 12 repetitions. The last two reps should be difficult to achieve, because the idea is that the muscle is challenged. When you can comfortably do 12 or more repetitions at a certain weight, increase the weight 5 percent to 10 percent. You will be amazed at how much more you can do after even a few weeks.

What kind of exercise?

The third question is, “What should I do?” The key to sticking with an exercise program is choosing activities you enjoy. There are many to choose from.

Strength training. The best way to start may be to hire a certified personal trainer for three or four sessions to develop a plan and show you how to use the equipment properly. You can use weight machines, free weights or resistance equipment like specially made rubber bands or a weighted vest, and you can strength train at a health club or at home.

Strength training videos that show you how to use common household items such as food cans and water bottles can save you money on weights or other fancy equipment. In any case, if you don’t use the proper form, you can injure yourself, so you do need to learn how to use the equipment, whether it’s from a personal trainer, a video or a book. Be sure any video or book you use is current, as some once-popular strength-training exercises have been found to be potentially harmful.

Strength training is important for women of all ages. In young women, it can set the stage for a lifetime of stronger bones. Research shows that women start to lose muscle strength as early as age 20. For these women, strength training can help slow or reverse the natural process of muscle degeneration. And studies also find that older women who strength train not only maintain bone density, but have a much lower risk of hip fractures, due in part to the improvement in dynamic balance that often accompanies stronger muscles.

Functional or core strength training. This type of training helps strengthen the muscles of the back, trunk, abdomen and pelvis. The idea is to strengthen these muscles first in the “movement chain” to prevent injury and to provide a solid, stable base so the muscles further down the chain—your legs and arms—have a stable base supporting them and can get stronger and more efficient. So, for instance, rather than strengthening your legs with hamstring curls and leg extensions—which don’t have much application in real life—you do squats, step-ups or walking lunges that challenge your entire body and improve dynamic balance while strengthening your legs and thigh muscles.

Aerobic exercise. The options are many and varied. Some of the more popular choices include:

  • Brisk walking burns almost as many calories as running or jogging for the same distance and poses less risk for injury. If you are a beginning walker, choose a level surface. Gradually increase your pace until you can do one mile in about 15 minutes. To intensify the exercise, add hills and varied terrain to your course.
  • Jogging burns more calories in less time and is as simple and convenient as walking, but it is too strenuous for some and may cause joint injuries. If you are a beginner, alternate walking and jogging for the first three or four weeks. Then gradually increase the jogging portion until you can comfortably run for the entire workout. Remember not to exceed your target heart rate; the talk test may be the best way to easily monitor your exertion level.
  • Aerobics classes or home videos offer variety, music and choreography. Some women prefer the extra motivation an instructor provides. Start with beginner classes or videos, and watch the instructor carefully for proper foot placement and body alignment to avoid injury, especially to your knees. There are a variety of types of aerobics classes, including:
    • Step classes, which incorporate a low bench that allows you to step up and down while performing various moves.
    • Boxing classes and Tae Bo, which have become a craze in some parts of the country. Boxing classes consist of aerobic moves combined with boxing moves such as punching and footwork. Tae Bo adds martial arts moves, including karate-type punches and kicks, to the mix. The feet and upper body move for most of the class, providing a total body workout.
    • Slide classes that involve a special mat and booties that slip over your shoes and allow you to slide back and forth on the mat. Great for toning the lower body and building strength in the inner thigh area but should be avoided by those with knee injuries.
    • Interval classes that combine step or floor aerobics with weight training using hand-held weights or special rubber bands.
    • Toning/sculpting classes that incorporate floor aerobics that concentrate on isometric exercises for specific body parts.
    • High-impact classes, which incorporate moves such as jumping, running and hopping. These are not recommended for women with joint problems in the lower extremities.
    • Low-impact classes, which incorporate moves in which one foot is always on the floor. They are not necessarily low-intensity exercises, though.

If you’re taking an aerobics class, take care of your feet! According to the American Podiatric Medical Association, proper shoes are crucial to successful, injury-free aerobics. Before you start an aerobics program, consider seeing a podiatrist for a biomechanical or gait analysis to estimate your risk of injury. Shoes should provide sufficient cushioning and shock absorption to compensate for pressure on the foot many times greater than found in walking.

Impact forces from aerobics can reach up to six times the force of gravity, which is transmitted to each of the 26 bones in the foot. Because of the many side-to-side motions, shoes need an arch design and upper leather or strap support to provide stability and prevent slippage of the foot.

Make sure shoes have a toe box that is high enough to prevent irritation of toes and nails. Major shoe companies today have designed special shoes for aerobics, which provide the necessary arch and side support; they also have soles that allow for the twisting and turning of an aerobics regimen.

  • Spinning is an exciting aerobic exercise developed in the 1980s. Participants use a specially designed stationary bike and the instructor leads the class on an imaginary ride accompanied by energizing music. During an average 45-minute class, you can burn 400 to 500 calories. Be sure to talk with the instructor before your first class to go over the type of clothing you might need (padded shorts), your target heart rate and your physical limitations.
  • Swimming is an ideal exercise for pregnant women and those with physical limitations such as musculoskeletal problems and asthma. However, swimming does not raise the heart rate quite as much as other aerobic exercises because humans are equipped with a reflex that causes the heart to slow when immersed in water. It is also not the best activity for losing weight because the body tends to conserve body fat as insulation in cold environments. For those whose only option is swimming, however, it is certainly better than remaining inactive. If you have arthritis, try to find a facility with a warm-water pool that conforms to Arthritis Foundation Guidelines.
  • Flexibility training is important because it helps prevent cramps, stiffness and injuries. These exercises also ensure a wide range of motion, particularly important as women age. Two flexibility/stretching regimes are popular enough now that you should be able to find a class for either that fits your needs and schedule:
    • T’ai chi, an ancient Chinese practice, is becoming popular for older adults. Tai chi incorporates slow, graceful movements with relaxation and breathing techniques. It can improve strength, flexibility, balance, coordination and posture and is recommended by the National Institute on Aging because it may reduce older adults’ risks of falling. Traditionally performed on land, tai chi can also be done in chest-deep water for added resistance and support (called water tai chi).
    • Yoga has been practiced for more than 5,000 years around the world, and according to results of a 2008 survey conducted by the Harris Interactive Service Bureau on behalf of Yoga Journal, 15.8 million Americans, or 6.9 percent of U.S. adults, are believed to practice yoga. Yoga increases flexibility, strength, balance and range of motion. It also reduces stress and increases feelings of well-being. Everyone from high-powered executives to stay-at-home moms to people coping with illness or injuries can practice yoga. A typical yoga class involves breathing, warm-up, yoga postures that consist of specific ways of stretching and moving the body, and relaxation and visualization. Be sure to find a certified yoga teacher and begin slowly.
    • Pilates is a more than 80-year-old, low-impact exercise technique that was developed by German immigrant Joseph Pilates. Some Pilates programs use machines with pulleys and ropes that gently stretch all parts of your body with mild resistance; others use a series of floor exercises more akin to yoga. Much of the focus of Pilates is on strengthening back and abdominal muscles, increasing flexibility and building core strength.

You can buy a video to show you how to do stretching exercises in the privacy of your own home or you can have a personal trainer at a gym show you how to incorporate the exercises after your cool-down period.

Special considerations: exercising when pregnant

Exercising when you’re pregnant can help you achieve better posture, less back pain, less stress, better digestion, more energy, an easier delivery and smaller “postpartum belly.” It can also help prevent or control gestational diabetes and reduce the risk of complications during delivery. If you’ve exercised throughout your pregnancy, you will be rewarded with increased strength, flexibility and stamina during labor and delivery, as well as a faster recovery.

Be sure to consult with your doctor about your exercise routine. If you were active before becoming pregnant, you should be able to continue, within reason. If you are new to exercise, be sure to start slowly and do not overdo. Low-intensity or low-impact cardiovascular exercise like walking, swimming, low-impact aerobics classes or special exercise classes for pregnant women are best. You can engage in these activities most days or every day of the week for about 30 minutes per session. Ask your health care professional about specific exercise recommendations. It’s critical that you keep your body cool and well-hydrated (drink lots of water) during exercise. Don’t forget to warm up and cool down.

Strength training during pregnancy can also help build your stamina and strengthen your muscles and bones. Use lighter weights or resistance because heavier weights increase your chances of injury. Remember to breathe normally and follow these pointers:

  • Don’t lie on your back to exercise once you pass your 20th week of pregnancy.
  • Avoid deep knee bends, abdominal exercises while lying down, double leg raises and straight-leg toe touches because your ligaments are more prone to injury during pregnancy.
  • Don’t exercise in hot, humid weather or wear excessive clothing because getting overheated could harm your baby.
  • Always drink plenty of liquids and stop and consult your health care professional if any unusual symptoms appear, including pain, bleeding, dizziness, shortness of breath, irregular heartbeat or difficulty walking.

Special considerations: women with chronic conditions

Today, exercise is often recommended as a management strategy for many chronic medical conditions. Of course, a thorough discussion of exercise with your health care professional is imperative prior to beginning any kind of program.

For example, exercise is highly recommended for women with osteoporosis, a bone disease that causes bones to thin and weaken. A carefully designed exercise program can help protect your bones and retard development of the disease. Weight training, in particular, helps counter the effects of osteoporosis by stimulating bone formation.

Choose weights heavy enough to tire your muscles after about 12 repetitions. The last two reps should be difficult to achieve, because the idea is that your muscles are challenged. When you can comfortably do 12 or more repetitions of a certain exercise, increase the resistance by 5 percent to 10 percent. Walking, jogging and aerobics classes also help build bone. Bicycling and swimming, however, don’t stimulate bone formation in the hips because you do not bear your full body weight on your feet. Flexibility exercises enhance your posture and increase your balance, making you less susceptible to dangerous falls.

Exercise is also extremely helpful if you have diabetes. Studies find that people with diabetes who are physically active have fewer complications. Exercise can reduce blood sugar levels and enable your muscles to use glucose more efficiently, reducing or eliminating the need for insulin.

The American Diabetes Association recommends aerobic activity at least five days a week for about 30 minutes, as well as strength training and stretching exercises several times a week. Your health care professional should oversee the design of your fitness program.

And always check your blood sugar level prior to exercise and make sure it’s not low. Exercise increases the ability of glucose to get into cells, reducing the need for insulin. Always have a fast-acting sugar source with you in case you do have a reaction, and wear a medical alert identification bracelet or necklace.

For women at risk of developing heart disease, exercise is crucial. In fact, according to the American Heart Association, lack of physical activity is now clearly shown to be a risk factor for cardiovascular disease, the number one killer in America.

Studies find that people who are physically inactive are about twice as likely to develop heart disease—a risk factor as significant as high cholesterol, high blood pressure and cigarette smoking. Even low-to-moderate-intensity activities such as pleasure walking, climbing stairs, gardening, yard work, moderate-to-heavy housework or dancing can bring some benefits when performed for as little as 30 minutes a day. More vigorous aerobic activities such as brisk walking, running, group fitness classes, swimming, bicycling, roller skating and jumping rope done for a total of at least 150 minutes a week are best for improving the fitness of the heart and lungs.

If you already have heart disease, you can exercise safely as long as you work out under medical supervision and carefully monitor warning symptoms. Check with your local hospital or university for monitored cardiac rehabilitation exercise programs.

Strenuous physical exertion, however, is never recommended for people who suffer from congestive heart failure, unstable angina, chest pain, significant aortic valve disease or aortic aneurysm, although some may benefit from mild or moderate exercise under controlled situations.

Exercise also is beneficial for and can help control obesity, high cholesterol, high blood pressure and back pain and may improve the symptoms of some neurological and emotional disorders. It also has been shown to help prevent certain types of cancer.

Facts to Know

  1. The American Heart Association reports that physical inactivity is a major risk factor for heart disease and stroke, and studies find that people who start a regular physical exercise program after a heart attack have better rates of survival and an improved quality of life compared with those who remain sedentary.
  2. According to the U.S. Department of Health and Human Services National Center for Health Statistics, about 40 percent of American adults report no leisure-time physical activity, and only 31 percent of adults say they engage in regular leisure-time physical activity (defined as either three sessions per week of vigorous physical activity lasting 20 minutes or more or five sessions per week of light-to-moderate physical activity lasting 30 minutes or more).
  3. No matter how poor your current fitness level, you can start an exercise routine and become fitter and healthier. Even 90-year-old women who use walkers have been shown to benefit from light weight training.
  4. Women with heart disease or arthritis actually experience improved daily function from involvement in various modes of physical activity.
  5. Fitness consists of five components: your body’s ability to use oxygen as a source of energy, which translates into cardiovascular fitness; muscular strength; endurance; flexibility; and body composition.
  6. To address all the components of fitness, an exercise program needs to include aerobic exercise, which is continuous repetitive movement of large muscle groups that raises your heart rate; weight lifting or strength training; and flexibility exercises or stretching.
  7. Walking at a brisk pace (a 15-minute mile or 4 mph) burns almost as many calories as jogging the same distance, and both walking and jogging benefit the bones. The advantage of jogging is that it takes less time to cover the same distance; however, it may be too strenuous for some.
  8. It takes about 12 weeks after starting an exercise program to see measurable changes in your body. Before 12 weeks, you will notice an increase in your strength and endurance.
  9. To reduce the risk of chronic disease, the Centers for Disease Control and Prevention’s 2010 Physical Activity Guidelines for Americans and other professional groups recommend that healthy women do some sort of moderate-intensity aerobic exercise for a total of at least 150 minutes a week (equivalent to about 30 minutes a day, five days a week) or a total of at least 75 minutes of vigorous-intensity aerobic activity per week. The guidelines note that adults can achieve additional health benefits, including the promotion of greater weight loss or the prevention of weight regain, by increasing to five hours (300 minutes) a week of moderate-intensity aerobic physical activity, or 150 minutes a week of vigorous-intensity physical activity, or an equivalent combination of both.
  10. The Center for Disease Control and Prevention’s 2010 Physical Activity Guidelines for Americans recommend that adults perform muscle-strengthening activities that involve all major muscle groups at least two days a week.