HIGH-FREQUENCY CHARACTERISTICS OF ELASTICITY OF SKELETAL-MUSCLE FIBERS KEPT IN RELAXED AND RIGOR STATE

The viscoelastic properties of crossbridges in rigor state are studied by means of application of small length changes, completed within 30 mu s, to isometric skinned fibre segments of the iliofibularis muscle of the frog in relaxed and rigor state and measurement of the tension response. Results are expressed as a complex Young's modulus, the real part of which denotes normalized stiffness, while the imaginary part denotes normalized viscous mechanical impedance. Young's modulus was examined over a wide frequency range varying from 5 Hz up to 50 kHz. Young's modulus can be interpreted in terms of stiffness and viscous friction of the half-sarcomere or in terms of elastic changes in tension and recovery upon a step length change. The viscoelastic properties of half-sarcomeres of muscle fibre segments in rigor state showed strong resemblance to those of activated fibres in that shortening a muscle fibre in rigor state resulted in an immediate drop in tension, after which half of the drop in tension was recovered. The following slower phases of tension recovery - a subsequent drop in tension and slow completion of tension recovery - as seen in the activated state, do not occur in rigor state. The magnitude of Young's moduli of fibres in rigor state generally decreased from a value of 3.12 x 10(7) N m(-2) at 40 kHz to 1.61 x 10(7) N m(-2) at about 100 Hz. Effects of increased viscosity of the incubation medium, decreased interfilament distance in the relaxed state and variation of rigor tension upon frequency dependence of complex Young's modulus have been investigated. Variation of tension of crossbridges in rigor state influenced to some extent the frequency dependence of the Young's modulus. Recovery in relaxed state is not dependent on the viscosity of the medium. Recovery in rigor is slowed down at raised viscosity of the incubation medium, but less than half the amount expected if viscosity of the medium would be the cause of internal friction of the half-sarcomere. Internal friction of the half-sarcomere in the relaxed fibre at the same interfilament distance as in rigor is different from internal friction in rigor. It will be concluded that time necessary for recovery in rigor cannot be explained by friction due to the incubation medium. Instead, recovery in rigor expressed by the frequency dependence of the Young's modulus has to be due to intrinsic properties of crossbridges. These intrinsic properties can be explained by the occurrence of state transitions of crossbridges in rigor. Similarity of Young's modulus of fibre segments in the activated state and in rigor in the frequency range above 5 kHz strongly suggests that the same state transitions occur in force generating crossbridges in the activated fibre.