What is the recommendation from the 2008 Physical Activity Guidelines for Americans and still being used today regarding muscle strengthening activity?

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Volume 30, Issue 7, October 2020, Pages 407-412

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Importance: Approximately 80% of US adults and adolescents are insufficiently active. Physical activity fosters normal growth and development and can make people feel, function, and sleep better and reduce risk of many chronic diseases.

Objective: To summarize key guidelines in the Physical Activity Guidelines for Americans, 2nd edition (PAG).

Process and evidence synthesis: The 2018 Physical Activity Guidelines Advisory Committee conducted a systematic review of the science supporting physical activity and health. The committee addressed 38 questions and 104 subquestions and graded the evidence based on consistency and quality of the research. Evidence graded as strong or moderate was the basis of the key guidelines. The Department of Health and Human Services (HHS) based the PAG on the 2018 Physical Activity Guidelines Advisory Committee Scientific Report.

Recommendations: The PAG provides information and guidance on the types and amounts of physical activity to improve a variety of health outcomes for multiple population groups. Preschool-aged children (3 through 5 years) should be physically active throughout the day to enhance growth and development. Children and adolescents aged 6 through 17 years should do 60 minutes or more of moderate-to-vigorous physical activity daily. Adults should do at least 150 minutes to 300 minutes a week of moderate-intensity, or 75 minutes to 150 minutes a week of vigorous-intensity aerobic physical activity, or an equivalent combination of moderate- and vigorous-intensity aerobic activity. They should also do muscle-strengthening activities on 2 or more days a week. Older adults should do multicomponent physical activity that includes balance training as well as aerobic and muscle-strengthening activities. Pregnant and postpartum women should do at least 150 minutes of moderate-intensity aerobic activity a week. Adults with chronic conditions or disabilities, who are able, should follow the key guidelines for adults and do both aerobic and muscle-strengthening activities. Recommendations emphasize that moving more and sitting less will benefit nearly everyone. Individuals performing the least physical activity benefit most by even modest increases in moderate-to-vigorous physical activity. Additional benefits occur with more physical activity. Both aerobic and muscle-strengthening physical activity are beneficial.

Conclusions and relevance: The Physical Activity Guidelines for Americans, 2nd edition, provides information and guidance on the types and amounts of physical activity that provide substantial health benefits. Health professionals and policy makers should facilitate awareness of the guidelines and promote the health benefits of physical activity and support efforts to implement programs, practices, and policies to facilitate increased physical activity and to improve the health of the US population.

The department of health and human services of the United States has published guidelines for physical activity for Americans in 2018. It is the modified from one from the 2008 US Physical Activity Guidelines and the World Health Organization (WHO) Physical Activity Guidelines in 2010 [2]. The Ministry of Health and Welfare of Korea issued guidelines of physical activity for Koreans in 2013. The Korean and US guidelines are similar, and the revised guidelines have the same basic principles, although some guidelines have been revised and supplemented.

In all the physical activity guidelines, adults are encouraged to practice moderate aerobic physical activity for at least 150 minutes per week, or alternatively 75 minutes of high intensity exercise. High intensity exercise means vigorous intensity exercise. In this article, both words have been used as the same meaning. We can assume 1 minute of high intensity activity is equal to 2 minutes of moderate intensity activity. When combining high and moderate intensity activities, the guidelines of 150 minutes or more per week should be based on moderate intensity activity. For example, if you have 50 minutes of high intensity activity per week (100 minutes as moderate intensity) and 60 minutes of moderate intensity activity, it will be calculated as 160 minutes of moderate physical activity.

The latest US guidelines state that the moderate aerobic physical activity should be at least 150–300 minutes. This suggests that it is a minimum goal to increase to 300 minutes as much as possible, rather than just 150 minutes per week. It also stated that more than 300 minutes of moderate physical activity is linked to more health benefits.

Moderate physical intensity refers to the exercise intensity corresponding to 3 to 5.9 metabolic equivalents (METs), while high intensity refers to 6 METs and more. One MET is an oxygen uptake of 3.5 mL/kg/min when an adult is resting and corresponds to an energy consumption of 1 kcal/kg per hour. Moderate (3 to 5.9 METs) and high intensity (over 6 METs) physical activities are broad, but the high intensity is roughly double the intensity of moderate intensity.

During light-intensity activities most people are able to sing, during moderate-intensity they can talk but not sing, and during vigorous activities, even talking is difficult.

In the past, physical activity for more than 10 minutes at a time was recommended because evidence was based on the research that studied physical activities for longer than 10 minutes. But it is now known that exercising for as little as 10 minutes is effective, so exercise duration is no longer emphasized [3].

Since sparse exercise, defined as between 1–2 times per week, increases the risk of injury, it is recommended to exercise three or more days a week, for as many days as possible. Particularly, some papers showed that chronic disease patients should perform exercises for more than 3 days a week. Because increased insulin sensitivity by acute bout of exercise dissipate in 48–72 hours [4].

Many data sets define the intensity of exercise by the type of physical activity. The most extensive and accurate list is the compendium of physical activities from the University of South Carolina [5]. Intensities of daily activities, such as gardening, house chores, as well as sports activities, are clearly defined.

It is inappropriate to express intensity of exercise by the amount of sweat. Sweating ability varies from person to person and is affected by temperature and humidity. To be more precise, the intensity can be expressed as heart rate or maximum oxygen uptake, but this is too complicated to use in practice.

Muscle strengthening exercises or resistance exercises reduce the risk of injury, and help maintain weight. It is better to space muscle strengthening exercises to 2 to 3 times a week by each muscle groups rather than on consecutive days. This is because micro-tears in muscle fibers caused by muscle strengthening exercises need time to recover. Moreover, if exercise is performed frequently, the risk of injury may increase.

Muscle strengthening exercises include dumbbells, weight-lifting, push-ups, sit-ups, knee-flexion and extension, and moving objects. Most of the exercises using instruments at a gym are muscle strengthening exercises.

A typical method of determining the appropriate weight for an individual is as follows:

First, there is a way to express the load as a percentage of the maximum weight, or 1 repetition maximum (RM). Approximately 60%–70% of 1 RM (moderate to hard intensity) is for novice to intermediate exercisers to improve their strength. At 80% or higher of 1 RM (hard to very hard intensity) is when experienced strength trainers to improve strength. With 40%–50% of 1 RM (very light to light intensity), older persons and sedentary people initially beginning to exercise can start to improve strength.

The second is a method that estimates appropriate starting weight by what can be lifted. It is used largely because it is easy to measure. Generally, the weight recommended for resistance exercise is the one that can be lifted 8 to 15 times at a time.

Muscle strength is usually increased by the weighted exercise with eight repetitions, while muscular endurance is improved by the one 15 repetitions. Therefore, at the beginning, using a weight that can be lifted only 8 times at a time is recommended. If the designated weight can be lifted 15 times at a time, then it can be increased.

The easiest way is to breathe while doing muscle strengthening exercises is slowly without a pause while lifting a weight for 3 seconds, resting for 1 second, then lowering it for 3 seconds. This is repeated 8–15 times, which is called a set. It is appropriate to perform three sets of muscle strengthening exercise.

Guidelines recommend the elderly or 65 years of age or older should do same physical activities as adults. Especially for the elderly, prevention of falls is important, so balance, leg strength, and flexibility should be improved.

Physical activities for fall prevention should be performed more than 3 times a week, which can include the following: standing on one leg, moving the upper body while standing on one leg, standing or moving on a heel, standing with closed eyes, and standing on a soft, bumpy, or inclined floor. Physical activity should be modified according to the physical fitness and situation to prevent injury.

Children and adolescents should perform 60 minutes or more of physical activity daily which includes aerobic, muscle and bone strengthening activities.

Most of the 60 minutes or more per day should be either moderateor vigorous-intensity aerobic physical activity and should include vigorous-intensity physical activity for at least 3 days a week. They should also include muscle and bone-strengthening physical activity on at least 3 days a week. Jumping, hopping, skipping, and dancing are activities that are good examples of these.

Warming up and cooling down should be done pre- and post-exercise period. These are physical activities with an intensity of about 50% of the originally planned exercise and may include stretching [6]. For example, you can walk and stretch before and after jogging.

Although the evidence of warming up for prevention of injuries is weak, it is recommended to be carried out to prevent musculoskeletal injuries and heart attacks. Cooling down is also recommended to promote recovery from minute injuries and to prevent dizziness after exercise.

Physical activity should also be performed for chronic patients because regular physical activity is very important for general health maintenance and prevention of cerebrovascular disease. Primary care physicians should encourage the patients with chronic diseases to perform physical activity, educate them on how to exercise safely in accordance with their physical strength, and give them precautions for exercise [7].

Table 1 summarizes the recommendations of primary care physicians recommend physical activity for the patients based on the WHO, US, and Korea Physical Activity Guidelines.

Recommendations for physical activities for the patients in primary care

Exercise is one of the main treatment methods for some musculoskeletal disorders [10]. Chronic pain leads to muscle atrophy and joint contracture, fatigue and even depression, which exercise can then alleviate.

Muscle strengthening exercises and flexibility and balance training are the cornerstones for the treatment of musculoskeletal diseases.

Muscle strengthening exercise is recommended to be performed 2–3 times a week. It is necessary to leave more than one day of rest in between to relax and strengthen muscles.

Stretching is required for whole body muscles. Larger muscles such as the calves, thighs, hamstrings, lumbosacral spine, and anterior shoulder muscles particularly should be stretched to be flexible. For those with back pain, it is essential to stretch the muscles behind the waist and femur. Range of motion exercises are helpful for the arthritic joints.

A sense of balance prevents injuries caused by falls. Although balance improves to some extent with muscle strengthening or stretching exercises, it is preferable to have active balance training.

Exercise therapy is effective in the treatment of some musculoskeletal problems such as shoulder instability and/or impingement syndrome, myofascial pain syndromes (upper trapezius, sternocleidomastoid muscle, levator scapulae, and other such muscles), adhesive capsulitis, tennis and golf elbows, osteoarthritis, patellofemoral pain syndrome, and chronic low back pain.

In order to treat musculoskeletal pain of the shoulder, exercises for shoulder stability and rotator cuff strengthening are important. Muscle strengthening and stretching of the wrist are needed for tennis and/or golf elbow. To prevent and treat knee osteoarthritis, walking, cycling and swimming are recommended. Aerobic exercise and muscle strengthening training around the joints are needed for the treatment of osteoarthritis. Weight reduction of 5% of total body weight improves osteoarthritis if the patient is overweight. Overall, exercise reduces musculoskeletal pain and improve function, quality of life as well as mental health.

Exercise is not recommended for acute lower back pain although it is effective for chronic lower back pain (LBP). Core stabilization exercises are recommended for chronic LBP. Aerobic exercise is also good for chronic LBP.

Exercise should be modified according to the individual’s response. For example, if the patient feels pain during the musculoskeletal exercise, it is recommended to hold the exercise. If the patient feels pain next day as well, the amount of exercise should be reduced 50%. From there, they can gradually increase the amount.