Very high tension with very little ATP breakdown by active skeletal muscle.

The tension developed and the chemical change occurring during stretching at velocities up to 2l0 sec-1 and during isometric contraction of dinitrofluorobenzene-treated and untreated frog sartorius muscle have been measured. At all velocities of stretching the tension was greater than during shortening at the same velocity and the mean rate of chemical change was less than or equal to the rates found in similar studies of shortening. For velocities 0.18, 0.33 and 0.66l0 sec-1, the mean rate of chemical change was significantly less (P less than 0.05) than during shortening at the same velocity. At the highest velocity of stretching the tension at l0 was about 0.8 P0 and the mean rate of chemical change was about the same as that for isometric contraction. At low velocity (0.1 l0 sec-1) the tension was as high as 1.4 P0 and the mean rate of chemical change during the stretches at the three slowest velocities was significantly less (P less than 0.05) than during isometric contraction; the lowest mean rate of chemical change amounted to only about 25% of that for isometric contraction. The fact that the chemical change was so low suggests that during stretching crossbridges can form links and develop tension without breaking down ATP.