Elastic and inelastic behaviour of resting frog muscle fibres.

Dynamic and static elasticity of the resting skeletal muscle of the frog have been studied as a function of the sarcomere length. Isolated intact fibres as well as glycerol extracted fibres show a resting tension starting at 2.05 microns sarcomere length and increasing approximately exponentially to 2 . 10(4) N/m2 at 3.0 microns sarcomere length. Differences between the two types of preparation were seen in the dynamic experiments. The dynamic Young's modulus of intact fibres (recorded at 1 Hz and small amplitudes) increased from 2 . 10(5) N/m2 at 2.1 microns sarcomere length to 2.5 . 10(6) N/m2 at 2.9 microns while the static modulus varied from 5 . 10(3) N/M2 to 3 . 10(5) N/m2, the dynamic modulus at small amplitudes was equivalent to the modulus of the short range elasticity (SRE). The range of the SRE did not depend on the sarcomere length and amounted to about 5 nm per sarcomere. The dynamic modulus strongly depended on the amplitude: at large amplitudes the muscle became less stiff by a factor of 10 to 20. This tendency levelled off at about 10 Hz by a strain-induced relaxation process. The dynamic modulus of the glycerol extracted fibres were nearly of the same magnitude as the static modulus, there was neither evidence for an SRE nor for a significant amplitude-dependence of the dynamic modulus. For interpreting the results we propose to further develop the meander model of muscle (Pechhold et al. 1977 b).