STUDIES ON THE TENSION AND SHORTENING AS WELL AS THE WORK DONE DURING MUSCLE TWITCH AND TETANUS

Abstract

(1) The materials used were the sartorius muscles of the toads and frogs. The load, the moment of inertia of the inertia lever and the initial length of muscle were changed indepently of each other. Concerning a twitch, the tension or the velocity of contraction was a function of time, and the completely isotonic twitch was not realized as well as the completely isometric one.
(2) When the external resistance, i.e. load or inertia(velocity), is smaller than a certain limit, the initial tension develops over the load level till the muscle contracts as much as the early phase of initial shortening; then it turns to decrease and comes back to the load level at the moment when the muscle contracts to the critical point. After the initial tension, the maintenance tension follows which is almost equal to the load level and is the cause of the excess shortening. Therefore, the complete twitch is defined for a twitch performed under the condition that the muscle can contract to the critical point. On the contrary, when the external resistance is too large for the muscle to contract to the critical point, twitch becomes the incomplete one.
(3) When the initial length was not varied, the foot of the initial tension coincided with that of isometric twitch, independently of the external resistance.
(4) When the initial length was not varied, the lengths of the early and late phases of initial shortening were independent of the external resistance and the course of tension curve. The early phase is probably the necessary shortening for the formation of contractile substance and the late phase is caused by the transmutation of it.
(5) The initial shortening ratio was a linear function of the initial length.
(6) The mechanical work done during complete twitch was analysed into three components. The first is the work done for the load during the initial shortening and it is independent of equivalent mass(M). The second is the work done for inertia during the initial shortening and it is almost proportional to one third power of M. The third is the work done during the excess shortening and it is decreases exponentially, when M increases.
(7) The initial shortening (or the height of the critical point) was summated by two shocks, and the summation was complete when the interval was equal to the duration of the initial tension of the first twitch.
(8) The shortening of the complete tetanus was divided into two phases, the quick one which is caused by A band and the slow one which is caused by I band.
(9) When a muscle was stimulated repeatedly at shorter intervals than the contraction time in isotonic condition, the fluctuation of tension curve was minimized, particularly at lower temperature, except the first initial tension, and after the quick phase was over, the completely isotonic state was realized.