Tension responses of frog sartorius muscle to quick ramp-shaped shortenings and some effects of metabolic inhibition.

The dynamic properties of the contractile mechanism of skeletal muscle were investigated in electrically stimulated sartorius muscle of the frog at 0 degrees C by analysis of the force responses to quick changes in length. 1. The tension responses to ramp-shaped shortenings at different rates of shortening have been recorded. The tension course during the ramp is described in terms of a damped elastic element. In addition a maximum value for an additional series elastic element is estimated. 2. The tension reponses to pulse-shaped shortenings have been recorded. At least 15 ms after the pulse a minimum in tension is found, which depends on amplitude and duration of the pulse. This effect is optimal at a pulse duration of about 6 ms. It is concluded that within 15 ms after the pulse detachment of crossbridges occurs. 3. Muscles were incubated with the metabolic inhibitors IAA and FAA or IAA and FDNB. After repeated stimulation, but before the muscle passes into the rigor state, the fast recovery and the plateau in the responses to quick shortenings are either diminished or abolished, depending on the amplitude of the shortening. This can be explained by assuming that in the unpoisened muscle, ATP enables the crossbridges to exert temporarily a larger force.