Muscle mass, function, and structure
Definition: Appendicular skeletal muscle mass (mass divided by height squared) more than 2 standard deviations below the mean for a young healthy reference population. Measured by dual x-ray absorptiometry (DXA)
Muscle fatigability increases with age:
Structure of the whole muscle
Increase in connective tissue and fat
The loss of lean body mass partially results from the loss of individual muscle fibers, or motor units.
Progressive loss of motor units:
The increase in connective tissue and fat mass and corresponding decrease in muscle fibers reduce the amount of muscle tissue available to perform the body's normal metabolic and locamotive functions.