A high school friend once told me that one of his football coaches told him that wind sprints were important and you were only as young as your legs. I think that this is a physiological insight on the same lofty level as Woody Allens' maxim (1). The time honored wind sprints have apparently morphed into high intensity interval training which is supposed to be good for things ranging from improving glucose tolerance to improving 02 uptake and exercise tolerance in heart failure with or without reduced ejection fraction.
The stylized story about muscle loss with aging can be told like this:
From age 40 to age 70 there is 8 % per decade decrease in muscle mass and a 10-15 % decrease in muscle strength. Around age 70 the rate of decline for both increases so that there is a 10-15% decrease in muscle mass per decade and a 25 to 40% decrease in strength.
There is a greater loss in the fast twitch or type 2 fibers with some of the fast twitch fibers being replaced by type 1.This observation helps explain the greater loss in strength relative to muscle size as type 2 fibers are larger and can generate more force and to some degree they are being replaced by smaller less forceful type 1 fibers.
Decreasing levels of exercise for various reasons accompany aging but not all muscle loss can be blamed on decreased activity. There is to varying degrees loss of the anterior horn cells in the spinal cord which seem to target horn cells that serve type 2 fibers. as well as multiple other changes in muscles that seem refractory to continuing exercise.
So why should we call the numbers stylized? The estimates of percentage per decade losses are of course averages which are largely derived from longitudinal studies and individuals have been noted to vary greatly. Further one study showed that various sample sites from the lateral thigh muscle which investigators prefer for biopsy studies on muscle fiber type often give rather discrepant fiber distribution counts.
1) As you get older everything that should be soft gets harder and everything that should be hard get softer. We can all think of examples.