Sarcopenia, the decline in skeletal muscle mass and function without any underlying disease, is a major contributor to physical disability, poor quality of life, and death among the elderly […] 40%–50% of individuals over 80 years of age suffer from this loss of muscle mass and function […] The molecular mechanisms responsible for this age‐related condition remain unknown.

Source : Aging Cell, Volume 19, Issue 10

Despite its widespread prevalence (there are currently thought to be more than 50 million sufferers worldwide) , the syndrome was only named in 1989,

Many causes have been proposed (genetic, environmental, systemic etc ) but it's now thought that it's likely to be the result of multiple, as yet unidentified, interacting factors.

Note: Sarcopenia is a separate syndrome from Cachexiaplugin-autotooltip__plain plugin-autotooltip_bigCachexia

Cachexia (CC) is the name given to weight-loss (mostly muscle, but sometimes also including fat) associated with cancer and a long list of other severe diseases. It's thought to affect as many as 70 million people worldwide.

It's been found to be linked to abnormal increases in certain cytokines (cell-signalling peptide proteins) circulating in the bloodstream. These specific cytokines trigger protein breakdown of the muscle fibres. - which involves cytokine imbalances as its main mechanism. The symptoms can be substantially similar, and the two syndromes often occur together.