Nonlinear mechanical behavior in striated muscle and its relationship to underlying crossbridge activity

Small muscles dissected from the frog (Rana pipien) toe were submersed in a cooled (0 degrees C) Ringers solution and attached at opposite ends to a tension transducer and a length regulating servo device. Muscle stiffness was measured by imposing a length disturbance (bandlimited (0-500 Hz) noise or single-frequency sinusoid) on an initially isometric muscle preparation and recording this disturbance and the concomitant tension response. If active muscle stiffness was to be measured, the muscle was electrically stimulated to produce a fused tetanus prior to imposing the length disturbance. The passive muscle was characterized as highly compliant, almost linear elasticity (elastic modulus 638 kN/m ³). Estimates of the muscle stiffness as a function of frequency were obtained using spectral density methods. The active muscle appears to ´soften´ in response to release and ´harden´ in response to stretch. The linear behavior of active muscle was modeled as a linear spring (elastic modulus 102040 kNm ³) in parallel with a linear dampener (viscous modulus 810 kN*s/m ³).