Physiological changes in body composition may influence physical performance in older age. Lean mass, and appendicular muscle mass in particular, undergoes quantitative and qualitative changes with aging. The reduction in skeletal muscle mass has been estimated at 8% every 10 years between 40 and 70 years of age, after which this phenomenon seems to speed up, with a loss of 15% or more per decade [2, 3]. This ultimately gives rise to a roughly 40% difference in muscle volume between individuals 20 and 60 years of age [4]. Some of the factors that may influence these changes include an inadequate dietary protein intake, oxidative stress, vitamin D deficiency, hormonal dysregulation (with low levels of GH, IGF-1, and androgens), and inflammatory states [5, 6]. The loss of muscle mass particularly concerns the fast-twitch glycolytic muscle fibers (type IIb), which may be lost or partially replaced by slow-twitch muscle fibers (type I). This conversion affects the muscle’s energy metabolism and physical performance because type II fibers are involved in short exercises demanding power and speed, not in long-endurance activities [7]. To describe the age-related reduction in muscle mass and the associated decline in muscle function, Rosenberg coined the term “sarcopenia,” a condition characterized by a progressive mobility impairment and loss of physical performance [8]. Like lean mass, fat also undergoes peculiar physiological changes with aging. The proportion of adipose tissue tends to increase in older people, who reported 10% more body fat than younger people [9]. Older people’s fat mass tends to deposit mainly in the visceral compartment, generating hormonal and inflammatory pathways associated with negative metabolic and cardiovascular outcomes, and thus limiting their endurance and physical performance [10]. In older age, adipose tissue also tends to infiltrate the muscle fibers, thereby accelerating muscle loss through encumbrance and lipotoxicity mechanisms [11]. Estimating changes in fat and lean mass with such noninvasive methods such as dual X-ray absorptiometry and bioelectrical impedance analysis [12, 13] thus represents the first step when planning tailored rehabilitation programs.

The resting metabolic rate (RMR) represents the energy needed to maintain basic body functions in a state of rest. Using a calorimetric method, it has been estimated at around 50–65% of the total daily energy expenditure. The RMR gradually decreases with aging, due mainly to changes in body composition (and especially the reduction in fat-free mass) but also to changes in tissue energy metabolism [14]. Skeletal muscle metabolism, in particular, has been recognized as one of the most relevant factors influencing the RMR [15]. However, regardless of the impact of body composition on the RMR decline over time, the effect of aging per se as a factor capable of influencing the RMR has been confirmed by several studies [14, 16], which identified a 4.6% lower RMR in the middle-aged than in younger adults, irrespective of body size, body composition, and physical activity [16]. Moreover, other factors can contribute to influencing resting energy expenditure in older people, as well as age and the proportion of lean mass. Patients undergoing rehabilitation, in fact, often suffer from various comorbidities and have a history of neurohormonal dysregulation, fasting, mobility problems and body temperature alterations—all variables that may affect their RMR. These factors therefore need to be considered before initiating any rehabilitation programs. The potential effect of physical exercise in increasing an individual’s RMR and daily energy requirement must be considered too, in order to ensure an adequate energy intake for patients under rehabilitation.

The maximal aerobic capacity (VO₂max) is the highest rate of oxygen uptake and distribution to the peripheral tissue that a cardiorespiratory system can cope with during exercise, beyond which there is no increase even if more effort is made. Like the RMR, VO₂max values may be influenced by numerous factors, including age, gender, body composition, and training. Aging has a strong impact on VO₂max, which can decline by 10% per decade beyond the age of 50 years [17], depending on cardiopulmonary fitness and exercising behavior but also due to age-related changes in skeletal muscle mass, with the related loss of muscle fibers and decline in oxidative function [18]. A more standardized index of cardiopulmonary performance has been introduced to express individual tolerance of physical activity for clinical purposes, namely, metabolic equivalents (METs), defined as the ratio between the work metabolic rate and the RMR. Under maximal workload, this measure expresses how many times an individual’s energy metabolism could be increased to cover the body’s needs in maximal effort. While younger adults achieve METs of around 11, elderly people generally reach values of 6–8 METs, compatible with a moderate physical activity level [19]. The decrease in METs in maximal effort begins from the age of 40 years and is accelerated by certain subclinical conditions or a sedentary lifestyle [20]. Although measuring METs in older people fails to take age-related changes in body composition and energy metabolism into account [19], their use can facilitate the estimation of an individual’s tolerance of exercise and cardiorespiratory reserves. A low capacity for exercise, judging from a low METs count in maximal oxygen uptake, is a significant predictor of cardiovascular disease and mortality, with each increase of 1 METs corresponding to a 10% lower mortality risk [21]. Aerobic capacity is therefore another factor to consider when planning safe rehabilitation programs for older people.

Age-related changes in body composition and energy metabolism need to be taken into account when planning rehabilitation programs for older people. Patients needing rehabilitation have often suffered from medical conditions that may have exacerbated the age-related reduction in their metabolic performance and accelerated the functional decline of skeletal muscle mass. Such physiological and pathological changes mainly affect the individual’s maximal aerobic capacity, giving rise to a lower exercise tolerance. This decline can be countered, however, by rehabilitation aiming to improve older patient’s physical performance, which often leads to an increase in their energy needs. Estimating body composition and maximal aerobic capacity could therefore represent fundamental steps in the design of rehabilitation programs tailored to each patient’s characteristics.