Factors determining Tension Development in Skeletal Muscle.

Summary. The mechanical factors determining tension in contraction of the single cross striated muscle fibre are investigated by a continuous registration of stiffness and tension at rest and during contraction. The method applied is in principle the same as that described in the preceding paper and stiffness is determined with a measuring frequency of 100 cycles per sec. Dynamic stiffness varies proportionally with tension at rest and during contraction, the proportionality factor at the contraction maximum being from 30–90 per cent above that of the resting fibre. It depends on the initial length and decreases when the contracted fibre is subjected to higher tensions (yielding). From tension, dynamic stiffness and their mutual interdependence the dynamic shortening ability can be computed, and the influence of stiffness and equilibrium length on tension is investigated under different conditions. At all elongations the maximum in stiffness precedes that of tension. Changes in dynamic equilibrium length contribute relatively less to the initial stage of contraction than to the other phases and its influence on the resulting tension decreases with increasing stretch. At the beginning of contraction dynamic shortening may even attain negative values, this decrease being the more pronounced the higher the strength of stimulation and the degree of stretch. When increasing the strength of direct stimuli, tension, stiffness and dynamic shortening increase, the latter varying more with grading than does static equilibrium length. The contribution of stiffness to the resulting contraction tension increases considerably with falling temperature while that given by dynamic shortening correspondingly decreases. The continuously falling tension during fatigue is accompanied by a gradual decrease in stiffness, while equilibrium length has a lowered but rather constant value in different stages of fatigue. Increase in extra-tension after partial restitution is due to a higher dynamic shortening, stiffness still being essentially lower than in the fatigued and the non-fatigued fibre. The falling contraction tension during fatigue is not caused by a decrease in number of active elements but is propably due to gradual changes in equilibrium length occurring in the single minute structural eleients proper. Proportionality between stiffness and tension is lacking in the initial phase of contraction (10–20 ms), and within a period where extra-tension is slight, often so slight that it is difficult to decide whether tension is present or not, considerable changes occur in fiber stiffness which are inherent in the contraction process itself. On the basis of the molecular model previously described a structural interpretation of mechanical factors determining tension is attempted. Apart from extrinsic tension, stiffness is caused by properties of the molecular links and their modifications, by properties of the chains and by the mutual interaction of different chains. The initial difference in the developing rate of stiffness and tension is due to intrinsic tension caused by the interaction of contracting and shunting resting elements, before mechanical consolidation between the elements is established. This increase in stiffness can be so considerable that it inhibits shortening and causes a negative contribution of equilibrium length to the resulting tension.