As noted, within the past few decades, the prevalence of overweight and obesity has increased dramatically. In 1980, the percentage of obese and severely obese adults was 15.1% and 1.3% of the total population, respectively; by 1994, these numbers increased to 23.3% and 3.1% [1]. In 2002, 31.1% of the adult population was obese and 5.2% were severely obese [1]. Data collected in 2017–2020 show that 41.9% are obese, 9.2% are severely obese, and an additional 32.1% of adults are overweight [1]. This means that roughly 7 out of 10 Americans 20 years of age and older are above a healthy weight and may be at an increased risk for disease and early death.

With race/ethnic origin not factored, the prevalence of overweight and obesity is higher for men than for women; moderate and severe obesity are more common in women [1]. In the overall population, 39% of men and 37% of women are obese, and approximately 5.5% of men and 9.8% of women have severe obesity [1].

When considered as a single race, the prevalence of obesity among individuals of African (49.9%) and Hispanic (45.6%) descent is greater than that reported among White Americans [1]. American Indian/Alaska Natives and Native Hawaiian/Pacific Islanders have an obesity prevalence of 39.1% and 51.7%, respectively [17]. Asian Americans are an exception, with a prevalence of 16.1%, much lower than in the general population. Although the rate of obesity is higher in many American racial/ethnic groups compared to non-Hispanic whites, white individuals make up the majority of cases (55.2 million out of 70.7 million total cases) [17].

Numerous reasons for failure to exercise exist, including lack of interest, competing demands for limited leisure time, fear of injury or pain, no access to facilities, and lack of knowledge of proper technique. Clinicians should also bear in mind that segments of minority and low-income population groups may live in unsafe environments and are fearful of walking in the neighborhood. Barriers to safe walking, such as local crime and aggressive dogs, may prevent people from being physically active. The 2015 NHIS found that non-Hispanic Black and Hispanic Americans were twice as likely than White Americans to report crime and animals as barriers to safe walking [93]. With education, encouragement, and selective involvement of social services, many impediments can be resolved so that patients will be able to incorporate exercise into their daily lives.

There are two types of motor units in skeletal muscle, Type 1 and Type 2. Type 1 has a small cell diameter, with a high excitability and fast conduction velocity. It has an oxidative profile with moderate contraction velocity and low fatigability. There are few muscle fibers of this type. In contrast, Type 2 has a large cell diameter, with low excitability but a very fast conduction velocity. Type 2 fibers are numerous in quantity, with a glycolytic profile and high fatigability. The small motor units, with Type 1 (also known as "slow-twitch") fibers, are recruited first and are frequently active, while the large motor units, with Type 2 ("fast-twitch") fibers, are used infrequently, in forceful contractions. Maximal efforts, in which fast motor units are recruited, cannot be sustained because of the rapid depletion of glycogen.

When exercising, the "size principle" should be considered in developing muscle mass. This refers to the fact that slow-twitch muscle fibers are the first fibers recruited to do an activity, while fast-twitch fibers are recruited after the majority of slow-twitch fibers have been recruited. Therefore, if a small or moderate amount of force is needed to perform an activity, slow-twitch fibers will primarily be used. Fast-twitch fibers will only be recruited if the slow-twitch fibers cannot generate enough force to fully perform the exercise. In order to make continued progress, both slow-twitch and fast-twitch fibers must be recruited. If a muscle fiber is not recruited, it will make no adaptation, such as an increase in size. In general, these two types of fibers are not different in the amount of force they produce, but rather differ in rate of force production. The "overload principle" refers to the idea that one must increase the resistance, frequency, or duration of an activity beyond that which would normally be expected. Overload will result in strength development.

VO₂ max is the maximum volume of oxygen consumed by the body each minute during exercise. Oxygen consumption is equal to cardiac output multiplied by arterial-venous oxygen difference. It often is used as a measure of a person's maximal capacity to do aerobic exercise because oxygen consumption is linearly related to expenditure of energy. In general, exercise increases heart rate, which will then increase oxygen consumption. A rough correlation shows 65% of maximal heart rate correlates to 50% VO₂ max.

There is little doubt that exercise improves glycemic control in patients with diabetes and those with impaired glucose tolerance (i.e., persons who are at risk for diabetes). Exercise has been shown to reduce baroreflex sensitivity and heart rate variability in patients with type 2 diabetes and to reduce glycated hemoglobin (A1C) by approximately 1% [31,32]. A Cochrane Review concluded that exercise significantly improves glycemic control, lowers plasma triglycerides, and reduces visceral adipose tissue in people with type 2 diabetes. These improvements appear to occur independently of weight loss [33]. In addition, because nearly 75% of the risk of type 2 diabetes is attributable to overweight/obesity, the reduction in body weight and body fat through exercise also improves glycemic control and reduces the complications of diabetes.

Various studies suggest a possible link between excess body weight and cancer, including colon, breast, endometrial, and possibly other cancers. Possibly the largest study to date was a 16-year prospective study that enrolled nearly one million participants (405,000 men and 495,000 women) from the Cancer Prevention Study II. The results showed that there were positive linear trends in death with increasing BMI values for esophageal, stomach, colon, rectal, liver, gallbladder, pancreas, prostate, cervical, ovarian, and kidney cancers, as well as non-Hodgkin lymphoma, multiple myeloma, and leukemia; there was no significant association with melanoma or cancers of the brain or bladder [37]. Other studies have indicated a slightly increased risk of melanoma and non-melanoma skin cancers [76]. A 2018 study estimated that cancers attributable to excess weight ranged from 3.9% to 6% in men and 7.1% to 11.4% among women [72]. It is suspected that most, if not all, cancers, while perhaps not directly caused by obesity, can be linked to lifestyles associated with obesity, including a diet high in fat and sugar and a marked lack of exercise.

Anaerobic activities focus on muscular strength and muscular endurance. These activities, which involve major muscle groups, are typically known as "resistance training." Muscular strength relates to exerting maximum force for a brief time period with repeated contractions until the muscle becomes fatigued. Weightlifting is a good example. Muscular endurance involves sustaining repeated contractions or applying force against a fixed object for an extended period of time. Push-ups are an example of a muscular endurance exercise. Oxygen is not used, and the muscles produce lactic acid as a by-product.

Before beginning resistance training, patients should become familiar with two important concepts: correct form and breathing technique. Form is broken down into repetition rhythm, range of motion (ROM), and proper angle. The rhythm should be controlled and consistent throughout the entire set of repetitions. One should resist the temptation to jerk the weight up or use momentum to move the weight. ROM simply means to fully extend or flex the muscle being worked. One should allow the muscle targeted to fully stretch at the bottom of the movement. Each exercise has two phases: the eccentric and concentric motion. The concentric is the pushing part of the motion and the eccentric is the resistance portion. Contrary to popular opinion, muscles get stronger from the eccentric part of the movement, or the stretch, not the push. Therefore, patients must go through the full range of motion to benefit. Each motion should be fluid, with equal time (e.g., three seconds up, three seconds down) given to the eccentric and concentric motions.

With respect to breathing, patients should exhale during the push phase of each exercise and inhale during the resistance phase. For example, during a bench press exercise, one would inhale while lowering the weight to one's chest and exhale as one pushes it back up. Doing this allows the best possible flow of oxygen-rich blood to the working muscles. Patients should be cautioned not to hold their breath during any exercise because Valsalva maneuver, which increases intra-abdominal pressure, may occur. This happens when patients close the glottis and activate abdominal muscles. This increase in abdominal pressure causes an increase in blood pressure and should be avoided. One trick to encourage patients to breathe is to ask them to count the number of repetitions out loud.

The current recommendation by the Institute of Medicine is for adults to set a long-term goal of at least 60 minutes per day of moderate-intensity physical activity (e.g., brisk walking) or shorter periods of more intense daily activity [42].

The key criterion is for patients to elevate their heart rate. Patients should calculate their maximal heart rate as 220 - age. For example, a patient 45 years of age has a maximal heart rate of 175 bpm. For patients having difficulty finding their pulse, a pulse monitor, which is available at most gyms and health stores, can be used.

Low-intensity exercise is defined as exercising at <50% maximal heart rate; moderate-intensity is defined as exercising at 50% to 70% of maximal heart rate. Vigorous exercise generally involves a heart rate about 70% to 85% of a person's maximum [43].

It is estimated that approximately 50% of people who start an exercise program will have quit within six months. The people who are most successful demonstrate some of the following characteristics:

The AHA strongly recommends physical activity counseling as an important strategy for implementing primary and secondary prevention guidelines. The AHA believes that healthcare providers should deliver counseling systematically, including asking specific questions about the kinds of activity and how much activity each patient is getting [47].

The AHA specifically advises that in the patient-visit setting, physicians and their staff should discuss physical activity and provide exercise prescriptions for patients and their families. At times, implementing physical activity at the workplace should be discussed. The AHA recommends at least 30 minutes of moderate-intensity physical activity five days per week, or 25 minutes of vigorous exercise at least three days each week [81]. Moderate and vigorous activities can be combined to meet the goal, and moderate-intensity activities can be performed in 10-minute segments if necessary. They also recommend resistance training using free weights or gym equipment be done at least twice per week, with 8 to 10 exercises working different muscle groups repeated on non-consecutive days, starting with 10 to 15 repetitions and building to three sets of 15 repetitions for each body area [82]. For older adults, the AHA also advises stretching for flexibility, about 10 minutes at least twice per week [40,48].

The ADA has published guidelines for patients regarding exercise. The ADA recommends that patients with diabetes undergo a detailed medical exam prior to beginning an exercise program. Specifically, they recommend screening for macrovascular and microvascular diabetic complications that could be exacerbated by exercise. They also suggest considering an exercise test for patients at high risk of underlying cardiovascular disease. In general, however, the ADA recognizes that both type 1 and type 2 diabetics can and should participate in physical activity [51]. The ADA recommends at least 30 minutes of moderate-to-vigorous intensity activity at least five days every week for all adults, including adults with prediabetes or diabetes, and at least 60 minutes daily for children and teens [10]. It is important to not go more than two days in a row without exercise. Resistance exercise is recommended two to three days per week, aiming for three sets of 8 to 10 repetitions for each muscle group targeted. People with diabetes should check blood glucose before, after, and several hours following exercise, at least in the beginning of an exercise program.

Most patients with diabetes can safely exercise. Certainly, patients with uncontrolled diabetes should not begin an exercise program until glucose levels are stabilized. Patients with proliferative or severe nonproliferative retinopathy are advised to avoid vigorous exercise, because there is a potential risk of vitreous hemorrhage or retinal detachment. Patients with nephropathy or peripheral neuropathy have traditionally been told to avoid vigorous exercise. However, ADA recommendations state that both aerobic exercise and resistance training may actually be beneficial in patients with nephropathy. These patients do, however, need careful workup for cardiovascular risks, including a stress test. For patients with peripheral neuropathy, non-weight-bearing exercise is advised as a common-sense precaution, as limited sensation could increase the risk of injury. Autonomic neuropathy in diabetes is closely tied to cardiovascular disease and carries a risk of postural hypotension, impaired thermoregulation, and other serious problems; for these patients, a thorough cardiac investigation is recommended before starting an exercise program [51].

Although patients should include all types of exercises in a program, as discussed, it is important for them to perform resistance training to prevent and treat osteoporosis. This is because it is weight-bearing exercises—not aerobic or stretching—that increase bone mineral density (BMD). The increase in BMD subsequently reduces fracture risk. In order to increase BMD, there should be a physical stress on the muscles/bones. Brief, high-intensity periods of loading that generate a diversity of strain patterns on the bones provide an osteogenic response. Low-impact exercises do not create enough stress to increase muscle mass or BMD.

HIV/AIDS is a significant public health problem. For many years, patients with HIV/AIDS were counseled not to engage in exercise. This was due to the belief that intense or prolonged exercise could lead to decreased immune function in a population that was already immunocompromised and/or immunodepressed. This belief has not been supported in the literature. Physical activity may in fact offer substantial health benefits for persons with HIV. Data suggest that exercise can improve HIV patients' quality of life and improve overall strength.