Single skeletal muscle fiber behavior after a quick stretch in young and older men: a possible explanation of the relative preservation of eccentric force in old age
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The origins of the smaller age-related decrease in eccentric force compared to isometric and concentric conditions in vivo remain unclear. Could this originate from contractile elements of muscle cells? The main intent of the current investigation was to assess the force behavior of muscle cells with aging, during lengthening. Chemically skinned single muscle fibers (n=235) from m. vastus lateralis of six young (mean age 31.6 years) and six older men (mean age 66.1 years) were maximally activated with pCa 4.5 at 15°C. Maximal isometric force and cross-sectional area were measured allowing the calculation of the tension (T0). A quick stretch (2 nm per half-sarcomere length) was applied and caused an immediate increase in tension followed by a decrease and a secondary delayed and transient rise in tension (phase 3); finally, the tension recovered a steady state value (phase 4). The tension enhancements during phase 3 (ΔT3) and phase 4 (ΔT4) were evaluated. The myosin heavy-chain isoform composition of each single fiber was determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. ΔT3 and ΔT4 were preserved in older men for both type I and IIa fibers despite a reduction in T0. Therefore, the age-related preservation of the tension increments after a quick stretch in single muscle fibers could explain in part the smaller decrease in force during eccentric contractions compared to isometric and concentric conditions in vivo with aging usually observed.
KeywordsAging Skinned single muscle fiber Mechanics Stretch response
The authors gratefully acknowledge Dr. Stefan Galler (Department of Cell Biology, University of Salzburg, Austria) for his valuable suggestions in refining the methods. The authors also wish to thank Gomathi Krishnan and Andreas Johansson for their valuable help with experiments. This work was supported by grants from the National Institute of Health (#AG18844-01).
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