The heat production in isometric and isotonic twitches

Abstract

In a recent paper from Meyerhof's laboratory P. Rothschild (1930) has shown that in a series of isometric twitches of a frog's gastrocnemius or semi-membranosus considerably more lactic acid may be liberated than in the same number of isotonic twitches with the same initial load: while with sartorius there may be little or no difference. 90 to 150 shocks were applied, at intervals of 5 to 10 seconds, at room temperature (11° C. to 17° C.), and either directly or to the nerve: the intervals were sufficient to allow complete relaxation between twitches and the number of twitches was not so great as to cause appreciable fatigue. With semi-membranosus, with an initial load of 10 or 20 g. the lactic acid in the isotonic averaged about 35 per cent. less than in the isometric twitches: with gastrocnemius stimulated through its nerve following six results were obtained at 15° to 17° C. in 92 to 102 twitches: Initial load: g. ............ 25 20 20 20 10 10 After-load (isotonic): g. ............. 13 50 50 50 140 140 Deficit due to shortening: p. c. ............. 70 49 64 76 34 25 Thus in an isotonic contraction with considerable shortening the lactic acid may be ½ to ⅓ of the amount in an isometric contraction with the same initial load: while when the shortening is smaller (greater after-load) the difference may be less (last two experiments). With sartorius, however, in 110 to 150 contractions at 12° C. to 17° C., there was a slight excess of lactic acid in the isotonic contractions, averaging about 8 per cent. The smallness of the difference was regarded by Rothschild as insignificant, in view of the probable error of his estimations: in this, as will be seen, he unduly depreciated the accuracy of his own observations: his difference was probably genuine.