Whole muscle strength and cross-sectional area (WMCSA), and contractile properties of chemically skinned segments from single fibers of the quadriceps were studied in 7 young men (YM, 36.5 ± 3.0 yr), 12 older men (OM, 74.4 ± 5.9 yr), and 12 older women (OW, 72.1 ± 4.3 yr). WMCSA was smaller in OM compared with YM (56.1 ± 10.1 vs. 79.7 ± 13.1 cm 2 ; P = 0.031) and in OW (44.9 ± 7.5; P &lt; 0.003) compared with OM. Age-related, but not sex-related, differences in strength were eliminated after adjusting for WMCSA. Maximal force was measured in 552 type I and 230 type IIA fibers. Fibers from YM (type I = 725 ± 221; type IIA = 792 ± 271 μN) were stronger ( P &lt; 0.001) than fibers from OM (I = 505 ± 179; IIA = 577 ± 262 μN) even after correcting for size. Type IIA fibers were stronger ( P &lt; 0.005) than type I fibers in YM and OM but not in OW (I = 472 ± 154; IIA = 422 ± 97 μN). Sex-related differences in type I and IIA fibers were dependent on fiber size. In conclusion, differences in WMCSA explain age-related differences in strength. An intrinsic defect in contractile proteins could explain weakness in single fibers from OM. Sex-related differences exist at the whole muscle and single fiber levels.

Skeletal muscle fiber quality in older men and women