Passive force and viscoelastic properties of single fibers in human aging muscles

Changes in stiffness or extensibility of the muscle or muscle-tendon unit with aging could lead to impaired function and an increased vulnerability to injury. We aimed to investigate the passive force and viscoelastic properties of single muscle fibers in older adults.

Seven older adults (mean age 79.0 ± 3.8 years) and 10 young control (mean age 25.6 ± 4.5 years) were recruited. Biopsy specimens were obtained percutaneously from m. vastus lateralis and skinned single fibers were used for the experiments. Slack tests were performed to determine maximal force and maximal unloaded shortening velocity. Passive force was measured in pCa 9.0 solution using a stepwise stretch technique with increment of sarcomere length from 2.4 to 4.2 µm. Myosin heavy chain (MHC) isoform was determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Specific force was calculated as maximal force divided by cross-sectional area. Passive force, peak passive force, time to half stress relaxation (T_1/2) and force decay index (a force time integral under a stress relaxation curve) were measured.

No difference between the groups were found in specific force and shortening velocity. Passive force and peak passive force were greater in both MHC I and IIa fibers of older adults (p < 0.001, p = 0.012, respectively, at 4.2 mm SL). Force decay index was higher in older adults. (p = 0.001 at 4.2 µm SL). There were no significant differences in passive force and viscoelastic properties between fiber types.

We demonstrated greater passive force and viscoelastic properties at the level of single fibers in older adults.