Effects of aging on shortening velocity and myosin isoform composition in single human skeletal muscle cells.

Maximum unloaded shortening velocity (V0) and maximum force normalized to cross-sectional fiber area (specific tension) were determined in 400 single quadriceps muscle cells of young (n = 4, 25-31 yr) and old (n = 4, 73-81 yr) men. Two of the old men were physically very active, and the subjects were divided into young control, old control, and old physically active groups. The expressions of types I, IIa, and IIb myosin heavy chains (MHC) and essential and regulatory myosin light chains were determined by 6 and 12% sodium dodecyl sulfate-polyacrylamide gel electrophoresis. V0 was highly dependent on the MHC isoform composition, and a gradient from slow to fast was observed in the sequence I, I-IIa, IIa, IIab, and IIb MHC cells. The V0 values and specific tensions of types I and IIa MHC fibers in the young control group were significantly higher than those in the old control and old physically active groups. Thus the present results provide evidence of qualitative changes in contractile properties of human skeletal muscle in old age, which probably play an important role in the age-related impairment of skeletal muscle function.