We addressed whether muscle quality (force per unit muscle mass) changes wi
th age in cross-sectional and longitudinal analyses from three groups from
the Baltimore Longitudinal Study of Aging: (1) Isometric arm strength studi
ed cross-sectionally in 617 subjects with muscle mass estimated by cross-se
ctional area (CSA)from arm circumference and by 24-hour urinary creatinine
excretion (CREAT) (2) longitudinal study for 10 to 25 years in 412 men usin
g the same measures as the first group; and (3) isometric knee extensor str
ength studied cross-sectionally in 675 subjects; muscle mass estimated by G
REAT CSA from thigh circumference, and leg nonosseous fat free mass (FFM)fr
om dual energy x-ray absorptiometry. Muscle quality declined in both arm an
d leg with age in cross-sectional analyses using GSA and FFM, but not GREAT
No age-associated arm muscle quality declines were observed longitudinally
using GREAT or GSA. The relationship between muscle quality and age is dep
endent on how muscle mass is estimated and on whether subjects are studied
cross-sectionally or longitudinally. In addition, GREAT may measure a muscl
e property not accounted for by CSA or FFM.