frailty and sarcopenia are addressed in a different chapter. However, it is relevant that the frailty phenotype includes unintentional weight loss (usually associated with muscle wasting), weakness (defined by a low grip strength), and reduced physical performance (slow walking speed) [43], all of them part of the definition of sarcopenia. Both conditions are closely linked, sarcopenia being a player in a relevant portion of cases with physical frailty [44, 45]. International and Asian definitions of physical frailty exist [46, 47].
THE RESEARCH ARENA
The definition of sarcopenia is rapidly evolving, as is true for many other common conditions and specialties [48–50].
Among the most relevant areas of research and debate that are needed to further improve the definition of sarcopenia some may worth mentioning, in no particular order [25, 45]:
Muscle mass measurements need to be improved from the present estimations to real measures, in order to decide how this parameter is best included in the definitions of sarcopenia [9].
The role of physical performance (as part of the definition, measure of severity, or upstream outcome) should be clarified [51].
Cut‐off points that are ethnically appropriate need to be developed.
Epidemiological studies enriched with complex populations (i.e. those living in nursing homes) are still needed to define the best cut‐off points for each parameter and technique used to define sarcopenia in their capacity to predict outcomes.
A practical way to separate cachexia, sarcopenia, and malnutrition in clinical practice, in order to improve clinical management, is needed, but may not be feasible in many cases.
The definition of sarcopenia when it comes as a comorbidity of other major diseases (i.e. liver disease, renal diseases, cancer, major surgery) is currently being addressed by many studies, but still many use the muscle mass paradigm not including function.
Agreement on which of the many adverse outcomes are more relevant to address sarcopenia both in clinical practice and research would increase the number of patients with the diagnosis and foster research of a wide range of therapies.
The need and role of simple screening tools [52] compared with muscle mass and function measures need to be established.
Finally, the concept of sarcopenia within a life course approach needs further refinement. Is sarcopenia an old‐age condition, or should the threshold be moved and extended to younger populations? If so, are the same definitions valid across the life span?
SUMMARY
Sarcopenia was originally defined as age‐related muscle mass. Recent definitions have extended this to include muscle function and muscle quality using different approaches. Current definitions have confirmed the concept that sarcopenia is relevant, frequent, and linked with adverse outcomes, but have not yet been able to extend the diagnosis and management to current clinical practice. The definition of sarcopenia is still work in progress.
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