The Journal of General Physiology, Vol 70, 697-705, Copyright © 1977 by The Rockefeller University Press
Mechanical control of the rising phase of contraction of frog skeletal and cardiac muscle
E Bozler
The effect of shortening on contractile activity was studied in experiments
in which shortening during the rising phase of an isotonic contraction was
suddenly stopped. At the same muscle length and the same time after
stimulation the rise in tension was much faster, if preceded by shortening,
than during an isometric contraction, demonstrating an increase in
contractile activity. In this experiment the rate of tension rise
determined in various phases of contraction was proportional to the rate of
isotonic shortening at the same time after stimulation. Therefore, the time
course of the isotonic rising phase could be derived from the tension rise
after shortening. The rate of isotonic shortening was found to be unrelated
to the tension generated at various lengths and to correspond closely to
the activation process induced by shortening. The length response explains
differences between isotonic and isometric contractions with regard to
energy release (Fenn effect) and time relations. These results extend
previous work which showed that shortening during later phases of a twitch
prolongs, while lengthening abbreviates contraction. Thus the length
responses, which have been called shortening activation and lengthening
deactivation, control activity throughout an isotonic twitch.
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