(from the Greek meaning "poverty of flesh") is the degenerative loss of skeletal muscle mass and strength associated with aging (0.5-1% loss per year after the age of 25). Sarcopenia is a component of the frailty syndrome.
This is not a happy thing. BUT, in the good news department, in case you haven't already heard.....there is hope!
Take a look at these MRIs images of upper thighs...the little white things in the center with very dark black around are the hollow femur bones and the dark gray stuff is muscle. The skin is also white.
You'll notice (and the paper here has more comments about it) that the 70 yr old triathlete's thighs look the same as the 40 yr olds.
But avert your eyes from the sedentary thighs. Yow. All that white stuff is "adipose tissue", i.e muscles that wasted away to fat.
So SHUT UP AND RUN! (Or Bike, or Swim, or All of Them)