Elasticity, Viscosity and Plasticity in the Cross Striated Muscle Fibre.

Abstract

Summary. A new method is devised for continuous measurement of stiffness at rest and during contraction. Tension increase in tetanic contraction is limited by a plastic lengthening of equilibrium length in the contracted fibre (yielding), the occurrence of which is most marked in the first 0.5 see of tetanic contraction. Yielding is characterised by a change in the gradient of static length‐tension diagrams and dynamically by a sudden decrease in stiffness occurring at a critical tension value. This tension may be due either to contraction or to tension induced externally in the contracted fibre. Comparative determinations of stiffness with low and high frequencies of periodic length alterations show that stiffness proper is masked by yielding when low frequencies are applied while a frequency of 100 cycles per sec is well suited to measurements of actual stiffness. In physiological muscular activity yielding ensures constant contraction tension over a large range of stretch. Apart from yielding the increasing stiffness with increasing frequency is due to viscosity, not uniformly distributed over the fibre. In the resting fibre no measurable dependence of stiffness on temperature can be observed at different frequencies (3–25°C). During contraction stiffness decreases essentially with increasing temperature. Evidence is given that this frequency dependence of stiffness is due to “linkage modifications” in contractile elements and that intrinsic friction is quantitatively of minor importance. The difference in temperature dependence of stiffness at rest and during contraction is probably due to the chain reaction occurring during contraction, the propagation of which is highly dependent on temperature. At all degrees of stretch, tension in the resting muscle fibre increases with increasing temperature. The temperature dependence is highest at medium elongations and is always less than proportional with absolute temperature.