Energetics of shortening depend on stimulation frequency in single muscle fibres from Xenopus laevis at 20 degrees C.

Single intact slow-twitch (type 3) muscle fibres from the iliofibularis muscle of Xenopus laevis were shortened at a constant velocity (0.4 L0/S, where L0 is the initial length at different levels of activation (40, 15, 12.5, and 10 Hz). A stimulation frequency of 40 Hz gave fused tetanic records. At this frequency the mean heat production rate during shortening (0.38 +/- 0.05 W/g dry weight) was slightly higher than the isometric heat production rate (0.33 +/- 0.03 W/g dry weight). The lower stimulation frequencies gave unfused tetanic contractions, the average isometric force of which was 40 +/- 3% of the isometric force at 40 Hz. In these unfused tetani during shortening the heat production rate (0.18 +/- 0.02 W/g dry weight) significantly decreased below the isometric heat production rate (0.25 +/- 0.02 W/g dry weight). At full activation the rate of total energy production (mechanical power plus heat production rate) during shortening was 1.88 +/- 0.32 times the isometric total energy production rate. This effect, i.e. an increase in energy turnover with shortening, is known as the Fenn effect. At sub-maximal stimulation the energy output during shortening was only 1.07 +/- 0.08 times the isometric value. These results show that the Fenn effect is dependent on the level of activation. The efficiency (ratio of mechanical power to total energy output) was independent of the stimulation frequency (0.37 +/- 0.06).