Although loss of muscle mass is considered a cause of diminished muscle str
ength with aging, little is known regarding whether composition of aging mu
scle affects strength. The skeletal muscle attenuation coefficient, as dete
rmined by computed tomography, is a noninvasive measure of muscle density,
and lower values reflect increased muscle lipid content. This investigation
examined the hypothesis that lower values for muscle attenuation are assoc
iated with lower voluntary isokinetic knee extensor strength at 60 degrees
/s in 2,627 men and women aged 70-79 yr participating in baseline studies o
f the Health ABC Study, a longitudinal study of health, aging, and body com
position. Strength was higher in men than in women (132.3 +/- 34.5 vs. 81.4
+/- 22.0 N.m, P < 0.01). Men had greater muscle attenuation values (37.3 <
plus/minus> 6.5 vs. 34.7 +/- 7.0 Hounsfield units) and muscle cross-section
al area (CSA) at the midthigh than women (132.7 +/- 22.4 vs. 93.3 +/- 17.5
cm(2), P < 0.01 for both). The strength per muscle CSA (specific force) was
also higher in men (1.00 <plus/minus> 0.21 vs. 0.88 +/- 0.21 N.m.cm(-2)).
The attenuation coefficient was significantly lower for hamstrings than for
quadriceps (28.7 +/- 8.7 vs. 41.1 +/- 6.9 Hounsfield units, P < 0.01). Mid
thigh muscle attenuation values were lowest (P < 0.01) in the eldest men an
d women and were negatively associated with total body fat (r = -0.53, P <
0.01). Higher muscle attenuation values were also associated with greater s
pecific force production (r = 0.26, P < 0.01). Multivariate regression anal
ysis revealed that the attenuation coefficient of muscle was independently
associated with muscle strength after adjustment for muscle CSA and midthig
h adipose tissue in men and women. These results demonstrate that the atten
uation values of muscle on computed tomography in older persons can account
for differences in muscle strength not attributed to muscle quantity.