**1. Introduction**

### **1.1 Definition of sarcopenia**

The term sarcopenia comes from the Greek words "*sarco*," which means muscle, and "*penia*," which means loss, and was used for the first time by Dr. Irwin Rosenberg, director of the "Research Center on Aging" at Boston University (USA) which in 1989 stated: "*the most dramatic and significant age-related physical decline was the loss of lean body mass*" (**Figure 1**) [1, 2].

Thus, sarcopenia was initially defined as "*normal and involuntary loss of muscle mass due to aging*" (Rosenberg, 1989).

This definition was based on the conceptual framework that states that the decline in muscle strength due to aging was due to a parallel decline in muscle mass. However, as the field of sarcopenia progressed, studies showed that the loss of age-related muscle strength outweighed the loss of muscle mass [3], so a definition of sarcopenia based only on muscle mass was not sufficient [4]. It was

#### **Figure 1.**

*Loss of muscle mass and strength due to aging (modified from: ADAM©, atlas of human anatomy. Todd R. Olson, 1997).*

more precisely defined as [5]: "*Decrease in muscle mass and strength due to aging*" (Morley et al., 2001).

Since then, the number of scientific publications has increased. In those, among other findings, its possible causes and consequences were identified, and the concept of sarcopenia has evolved as different definitions emerged among researchers [6, 7]. Still, there was still a lack of a definition of sarcopenia that would be suitable both for use in research settings and in clinical practice, until significant progress was made in 2010 thanks to a joint publication by the European Working Group on Sarcopenia in Older People [8] (EWGSOP) in which sarcopenia was defined as: "*A syndrome characterized by a gradual and generalized loss of skeletal muscle mass and muscle strength with the risk of causing adverse outcomes such as physical disability, poor quality of life and even mortality*" (Cruz-Jentoft et al., 2010).

This new definition incorporated sarcopenia not only the loss of muscle mass and strength but also its consequences on physical performance [5, 9] and for many years, it was the definition that was used in most studies as a reference or "*gold standard*" for the diagnosis of sarcopenia [10].

In October 2016, the World Health Organization gave a new advance to this condition, since through the International Classification of Diseases in its 10th revision (ICD-10-CM) recognized sarcopenia not as a geriatric syndrome but as a disease (muscular), with the code M62.84 [11, 12]. This forced it to revise and update its definition again.

In the 10 years passed since the initial work of the European group in 2010, researchers and clinicians have explored many aspects of sarcopenia, and expert groups around the world have published complementary definitions of sarcopenia [13–15]. However, the more recent definition and the current one are the one proposed by this same group [16–21] who, in a review carried out in 2019, defined sarcopenia as "*A progressive and generalized skeletal muscle disorder that* 

*Prevalence of Sarcopenia According to the Method Used to Determine Physical Performance DOI: http://dx.doi.org/10.5772/intechopen.100467*

*occurs with aging and is associated with a greater probability of adverse outcomes such as falls, fractures, physical disability and even mortality*" (Cruz-Jentoft et al., 2019).

Despite all these advances, international expert groups from around the world still do not reach a consensus on a definition of sarcopenia that is widely accepted, although they have agreed on the mechanisms and clinical implications of sarcopenia [22, 23] and especially in the fact that muscle mass, muscle strength, and physical performance are important components for the diagnosis of this disease and that therefore, all these parameters must be measured [4].

### **1.2 Clinical consequences of sarcopenia**

The clinical consequences of sarcopenia are basically due to loss of strength and muscle mass, not only in terms of functional disabilities, fractures, hospitalizations, and increased mortality [24], but also in quality of life [25].

In terms of human health, sarcopenia increases the risk of falls and fractures [26, 27] and impairs the ability to carry out activities of daily living [28]; it is associated with heart disease [29], respiratory disease [15], and cognitive impairment [30]; and leads to mobility disorders [13]; it contributes to a decrease in the quality of life [31] and ultimately death [26].

#### **1.3 Sarcopenia categories**

Sarcopenia is a disease with many causes and variable outcomes. In some people, a clear and unique cause of sarcopenia can be identified, largely attributable to aging, but in other cases, other causes can be identified. In this way, defining the sarcopenia categories as primary and secondary can be useful in clinical practice [8, 16].

Sarcopenia can be considered "primary" (or age-related) when there is no obvious cause other than aging. Sarcopenia is considered "secondary" when there are one or more obvious causes other than aging [16].

Sarcopenia staging is a concept that can help to guide its clinical treatment, in this way it can be categorized according to its severity in the following states [16]:


Identifying the stages of sarcopenia helps in selecting treatments and setting appropriate recovery goals. Staging can also support the design of research studies that focus on a specific stage or changes in stages over time [8].

### **1.4 Parameters that define sarcopenia and variables that measure these parameters**

The parameters that define sarcopenia are the amount of muscle and its function. The measurable variables are muscle mass, muscle strength, and physical performance [8].

Muscle mass can be expressed as total body skeletal muscle mass or as appendicular skeletal muscle mass, which is the sum of skeletal muscle mass of arms and legs [16].

Muscle strength refers to the amount of force that a muscle can produce with a single maximal effort [35].

The concept of the physical performance was defined for the first time to evaluate objectively and from a clinical point of view, how an individual performed different activities of daily living or physical tasks, as opposed to scales based on asking questions about the ability to perform these task [35]. However, since then, the concept of physical performance has evolved, and today, it is mainly related to ambulation and transfers [35], forming part of the most current definitions of sarcopenia.

The most up-to-date definition of physical performance was provided by Beaudart et al. [35] in 2019, who defined it as: "*A function of the whole body objectively related to locomotion*" (Beaudart et al., 2019).

#### **1.5 Measurement of sarcopenia parameters**

Currently, there are a wide variety of tests and tools available to measure the parameters that define sarcopenia [22, 36, 37], cost, availability, and ease of use, determine whether they are better adapted for clinical practice or more useful for research [8].

The selection of tools may depend on the patient (disability, mobility), access to technical resources in the setting where the tests are performed (community, clinic, hospital, or research center), or the purpose of the tests (monitoring of progression or follow-up of rehabilitation and recovery) [16].

Accurate measurement of muscle mass is a fundamental step to detect cases of patients with sarcopenia, and various techniques can be used for its quantification, but choosing one of them is not easy since all existing methods have advantages and disadvantages [38].

Nuclear magnetic resonance, computerized axial tomography, dual-energy X-ray absorptiometry, bioelectrical impedance analysis, determination of urinary creatinine excretion, and anthropometry are available [19].

There are few well-validated techniques to measure muscle strength, some assess upper extremity strength, and others lower extremity, and although the latter are more important for gait and physical function, and the two have been shown to be highly correlated [35]. Again, cost, availability, and ease of use determine whether techniques are best suited for clinical practice or useful for research purposes [8].

For the assessment of physical performance, there are a wide variety of tests. Short-distance walking tests can be used, such as the 2.4, 4, 6 m, or up to 10 m, or long-distance walking tests such as the 400-meter walk test, or the 6-minute walk test.

Other tests of physical performance are also the time up and go test (TUG), and the short physical performance battery (SPPB) [23].
