Increase of resistance to stretch during the latent period in single muscle fibres of the frog.

The time from stimulation to the first change of the extensibility in an isolated skeletal muscle fibre was measured by subjecting the fibre to a rapid, small stretch at various times during the latent period. The experiments were performed at constant temperatures in the range 1-16 degrees C. Irrespective of the temperature, the first increase of the resistance to stretch occurred after the onset of the latency relaxation. At a temperature or 10 degrees C and a sarcomere length of 3 microgram the resistance started to increase 5.5 ms after stimulation, i.e. 1.5-2 ms after the onset of the drop in tension, and then increased nearly linearly with time. Corresponding to the end of the latent period, i.e. when the tension recrosses the resting level, the amplitude of the response was about 1.5 times its resting value. There was an interval between the onset of increase f the resistance to stretch and the time when the rate of drop in tension had passed its maximum value. The results are compatible with the hypothesis that the latency relaxation arises from a minute lengthening of the thin filaments as proposed by Haugen & Sten-Knudsen, and that attachment and generation of force take place at separate steps of the cross-bridge cycle.