Relationship Bewteen Viscoelastic Stress Relaxation and Passive Stiffness in Single Muscle Fibers of Human Skeletal Muscles

Abstract

Human performance requires muscles to store energy when deformed or stretched and to recoil after being stretched. The association of viscoelastic and elastic properties with stiffness or extensibility still remains unclear in human single muscle fibers. PURPOSE: To evaluate the viscoelastic stress relaxation and passive stiffness of human single skeletal muscle fibers and to find the relationship between stress relaxation and stiffness. METHODS: Chemically skinned single muscle fibers (n=107) were prepared from biopsy specimens obtained from vastus lateralis muscles of 6 healthy volunteers (3 females and 3 males; mean age: 24.3±4.1 yr). Slack tests were used to determine specific force (maximal force/cross-sectional area, CSA) and unloaded shortening velocity. Stress relaxation and passive force were measured at pCa 9.0 using a stepwise stretch with 0.2 μm increments of muscle length from 2.4 to 4.2 μm. Peak responses to each ramp stretch and steady state force at the end of stress relaxation (after 1 min) were measured to obtain passive tension (steady state force/CSA), passive stiffness (slope of length-tension curve), peak steady-state force ratio (PSSFR, peak force/steady state force) and viscoelastic absorption (tension decayxt1/2). The myosin heavy chain isoform of each fiber was identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. RESULTS: Specific force and unloaded shortening velocity were not significantly different between females and males for type I (n=29 for females and 20 for males) and IIa (n= 15 and 18) fibers. There were no sex and fiber type differences in passive tension and PSSFR. Passive stiffness and viscoelastic absorption were greater in type IIa, but not type I fibers of males vs. females. PSSFR at maximum stretch was 5.61±1.99 suggesting 82.2% loss of peak response during stress relaxation. PSSFR was negatively correlated with stiffness (r = -0.369, p = 0.001) but not passive tension. Viscoelastic absorption was correlated with stiffness (r = 0.432, p < 0.001). CONCLUSION: We found a sex difference in viscoelastic properties and stiffness only for type IIa fibers. PSSFR indicating relative force loss during stress relaxation can be useful to assess viscoelastic and elastic properties along with passive stiffness.