A decline in physical function and significant increases in the risk for disability and dependence are typical of advanced adult age [3]. In humans, a number of age-related changes in various biological systems contribute to this decline including, but not limited to, decreases in muscle and bone mass, increases in body fat, loss of brain volume and cognitive capacity, loss of cardiovascular and pulmonary reserves, impaired visual and hearing function, sleeping disorders, anxiety and depression, and changes in dietary habits leading to mal-/undernutrition. Some of these changes are directly associated with lower levels of habitual physical activity. Further, pain associated with musculoskeletal disorders is frequently reported by older men and women and is associated with a reduction in their habitual level of physical activity.

It is important to note that the prevalence of mobility limitations in elderly is high but it does not have to be considered a static condition. In other words, older people experience frequent changes in their level of independence transitioning between more functional to less functional states and vice versa. For example, several studies [4, 5] have shown that older men and women transition more frequently from intermittent to continuous mobility limitation than from no mobility limitation to intermittent mobility limitation. The dynamic nature of this condition is demonstrated by the fact that it is possible to recover mobility after being disabled for 3 months (40% recovery rate) or even for 6 months (30% recovery rate). These findings suggest that the road from impairment to activity limitation and restricted participation in elderly is a bidirectional process. This is precisely why rehabilitation, as a health and functionally oriented strategy, is so crucial to influence this process. Perhaps more importantly, the plastic nature of the process suggests specific opportunities to intervene, restore function, and recover independence [4–6]. The habitual level of physical activity, an increase in various types of physical activity including work-related and leisure activity, and more structured exercise training programs have been shown to be modifiers of this process.

A very important determinant of functionality and independence is skeletal muscle function. Muscle strength is a strong predictor of severe mobility limitation, slow gait speed, increased fall risk, risk of hospitalization, and high mortality rate. For example, older adults with low muscle strength have a 2.6 fold greater risk of severe mobility limitation, 4.3 fold greater risk for slow gait speed, and a 2.1 fold greater risk of mortality compared to older adults with high muscle strength [7]. The loss of muscle strength in elderly cannot be explained only by the characteristic presence of skeletal muscle wasting or atrophy. During the last two decades, several studies have shown that other factors such as changes in central nervous system drive and activation, peripheral nerve dysfunction, alterations in the structure and function of the neuromuscular junction, fat infiltration of muscle, and a number of complex cellular and molecular changes at the level of single muscle fibers and individual myofilaments such as myosin impair muscle force generation, velocity of movement, and power production [8]. It is interesting to note that the well-described loss of muscle strength in older men and women appears to be less in those that have maintained a higher level of physical activity throughout their lives [9, 10]. This is true even in the presence of obesity and highlights the importance of habitual physical activity in this process. Thus, it is reasonable to suggest that different levels of reduced activity including a sedentary lifestyle, inactivity, and/or immobilization contribute to age-related changes and that these changes do not have to be considered inevitable consequences of the biological process of aging. It is interesting to note that sedentary behaviors such as driving a car and watching television are being considered independent risk factors equivalent to the more traditional ones such as cigarette smoking. Finally, scientific evidence suggests that interventions such as those including physical activity and appropriate nutritional support are safe and beneficial and can enhance the functional capacity of even very old frail people. (Sarcopenia will be discussed in another chapter of this book.)

It should not come as a surprise that part of the solution to the functional decline and muscular dysfunction associated with advanced adult age is an increase in physical activity and/or exercise training. Many rehabilitation interventions are characterized by increases in one or the other and sometimes both. However, it is important to make a distinction between these two related but different concepts. For the purpose of this chapter, physical activity will be defined as any bodily movement produced by skeletal muscles that result in energy expenditure [11]. Furthermore, physical activity can be subdivided depending on associated characteristics, for example, into household (e.g., gardening), leisure-time or recreational (e.g., walking), occupational (e.g., climbing stairs on the job), or sports-related activity (a game of soccer with friends on a Saturday afternoon). In many research studies however, all types of physical activity are combined into a single estimate of daily energy expenditure making the interpretation of the results more difficult. Exercise, on the other hand, will be considered as subset of physical activity that is planned, structured, and repetitive and has as an objective, the improvement or maintenance of physical fitness. This is the subtype of physical activity used in many rehabilitation programs and clinical intervention studies. Finally, fitness is a set of attributes that are health related (e.g., muscle strength, cardiorespiratory endurance, body composition, flexibility) or skill related (agility, balance, coordination) and can be measured with specific tests. One example of a health-related fitness test is the duration of exercise on a treadmill test. Physical fitness is an attribute influenced, but not entirely determined, by behaviors such as the level of physical activity and exercise training. The distinction between physical activity, exercise, and fitness, although not always clear and/or possible, is of practical value when evaluating the scientific literature on this topic. This brief review is more focused on physical activity while other chapters in this volume discuss exercise in more detail.

Some brief comments about assessing physical activity are needed here. The measurement of the level of physical activity in humans requires valid and reliable instruments that are simple enough to be used in epidemiological studies and at the same time flexible enough to be adjusted to the challenges of impaired mobility that is prevalent in older adults. One of the more traditional methods is the questionnaire. Many studies have used a set of standard questions that examine the type, frequency, intensity, and duration of physical activity in a certain period of time. The results can be used to estimate average daily energy expenditure. These self-report instruments, although easy to administer and appropriate when a large number of individuals are included in a study, depend on the memory of the participant and can be subject to bias and influenced by the participant’s psychological status and the training of the interviewer (see reference [12] for review).

More objective measures of physical activity include double-labeled water (considered by many as the gold standard to measure energy expenditure over time), metabolic chambers (expensive facilities that are available only in very few specialized research centers), and accelerometers (devices that measure acceleration and estimate total amount of activity). The latter have received significant attention because they are wearable (usually at the level of the waist), easy to use, and relatively inexpensive, capture real-time data, can be used over multiple days (usually 7 days), and have been validated in several research studies and in various populations including older adults [13]. Because of recent advances in this technology, it is likely that this measurement instrument will continue to be used in research, in the clinic, and by those individuals who monitor their daily activity.

During the last 60 years, a large number of scientific studies have evaluated the association between the level of physical activity (leisure time, occupational, total) and the incidence and prevalence of several chronic diseases. These studies are of particular relevance for the aging population because the incidence of these chronic noncommunicable diseases is high in older men and women and increases with age. Some of these diseases are associated with a high mortality rate particularly in older people. Furthermore, the level of leisure-time aerobic physical activity among individuals with chronic disease has been shown to be very low [14]. This is particularly true in patients with comorbidities or more than one chronic illness. Taken together, the main conclusion supported by these studies is that a high level of habitual physical activity significantly reduces the risk and incidence of many chronic noncommunicable diseases.

The hypothesis that a higher level of daily physical activity of work is inversely related to coronary heart disease was tested for the first time by investigators in London, England [15]. Since that classical study by Morris and collaborators, hundreds of other studies have confirmed that same hypothesis in various age groups and in several countries. Furthermore, the same hypothesis has been tested with similar results in various studies looking at other chronic conditions. Many studies, although not all, have included both, men and women, and people older than 60 years of age. A higher level of physical activity has been shown to be associated with a reduction in the risk of developing heart disease, hypertension, obesity, type 2 diabetes, osteoporosis, several forms of cancer (including colon, breast, lung, esophageal adenocarcinoma, liver, gastric cardia, head and neck, and other types of cancer) [16], sarcopenia, and sarcopenic obesity. It is important to note that these studies have controlled for the potential confounding influence of other risk factors such as body mass index, smoking status, alcohol intake, education, race, and sex.